| 1. |
- Revelli, A., et al.
(author)
-
Quasimolecular electronic structure of the spin-liquid candidate Ba3 InIr2 O9
- 2022
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 106:15
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Journal article (peer-reviewed)abstract
- The mixed-valent iridate Ba3InIr2O9 has been discussed as a promising candidate for quantum spin-liquid behavior. The compound exhibits Ir4.5+ ions in face-sharing IrO6 octahedra forming Ir2O9 dimers with three t2g holes per dimer. Our results establish Ba3InIr2O9 as a cluster Mott insulator. Strong intradimer hopping delocalizes the three t2g holes in quasimolecular dimer states while interdimer charge fluctuations are suppressed by Coulomb repulsion. The magnetism of Ba3InIr2O9 emerges from spin-orbit entangled quasimolecular moments with yet unexplored interactions, opening up a new route to unconventional magnetic properties of 5d compounds. Using single-crystal x-ray diffraction we find the monoclinic space group C2/c already at room temperature. Dielectric spectroscopy shows insulating behavior. Resonant inelastic x-ray scattering reveals a rich excitation spectrum below 1.5 eV with a sinusoidal dynamical structure factor that unambiguously demonstrates the quasimolecular character of the electronic states. Below 0.3 eV, we observe a series of excitations. According to exact diagonalization calculations, such low-energy excitations reflect the proximity of Ba3InIr2O9 to a hopping-induced phase transition based on the condensation of a quasimolecular spin-orbit exciton. The dimer ground state roughly hosts two holes in a bonding j=1/2 orbital and the third hole in a bonding j=3/2 orbital.
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| 2. |
- Balabanov, Oleksandr, et al.
(author)
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Quantization of topological indices in critical chains at low temperatures
- 2022
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In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 106:4
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Journal article (peer-reviewed)abstract
- Various types of topological phenomena at criticality are currently under active research. In this paper we suggest to generalize the known topological quantities to finite temperatures, allowing us to consider gapped and critical (gapless) systems on the same footing. It is then discussed that the quantization of the topological indices, also at critically, is retrieved by taking the low-temperature limit. This idea is explicitly illustrated on a simple case study of chiral critical chains where the quantization is shown analytically and verified numerically. The formalism is also applied for studying robustness of the topological indices to various types of disordering perturbations.
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| 3. |
- Bergholtz, Emil Johansson, et al.
(author)
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Microscopic theory of the quantum Hall hierarchy
- 2007
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 99:25, s. 256803-1-256803-4
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Journal article (peer-reviewed)abstract
- We solve the quantum Hall problem exactly in a limit and show that the ground states can be organized in a fractal pattern consistent with the Haldane-Halperin hierarchy, and with the global phase diagram. We present wave functions for a large family of states, including those of Laughlin and Jain and also for states recently observed by Pan et al., and show that they coincide with the exact ones in the solvable limit. We submit that they establish an adiabatic continuation of our exact results to the experimentally accessible regime, thus providing a unified approach to the hierarchy states.
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| 4. |
- Bergholtz, Emil Johansson, et al.
(author)
-
Quantum Hall hierarchy wave functions: From conformal correlators to Tao-Thouless states
- 2008
-
In: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 77:16, s. 165325-1-165325-9
-
Journal article (peer-reviewed)abstract
- Laughlin’s wave functions, which describe the fractional quantum Hall effect at filling factorsν=1/(2k+1), can be obtained as correlation functions in a conformal field theory, and recently, this construction was extended to Jain’s composite fermion wave functions at filling factors ν=n/(2kn+1). Here, we generalize this latter construction and present ground state wave functions for all quantum Hall hierarchy states that are obtained by successive condensation of quasielectrons (as opposed to quasiholes) in the original hierarchy construction. By considering these wave functions on a cylinder, we show that they approach the exact ground states, which are the Tao-Thouless states, when the cylinder becomes thin. We also present wave functions for the multihole states, make the connection to Wen’s general classification of Abelian quantum Hall fluids, and discuss whether the fractional statistics of the quasiparticles can be analytically determined. Finally, we discuss to what extent our wave functions can be described in the language of composite fermions.
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| 5. |
- Eschmann, Tim, et al.
(author)
-
Thermodynamic classification of three-dimensional Kitaev spin liquids
- 2020
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In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 102:7
-
Journal article (peer-reviewed)abstract
- In the field of frustrated magnetism, Kitaev models provide a unique framework to study the phenomena of spin fractionalization and emergent lattice gauge theories in two and three spatial dimensions. Their ground states are quantum spin liquids, which can typically be described in terms of a Majorana band structure and an ordering of the underlying Z(2) gauge structure. Here we provide a comprehensive classification of the "gauge physics" of a family of elementary three-dimensional Kitaev models, discussing how their thermodynamics and ground state order depends on the underlying lattice geometry. Using large-scale, sign-free quantum Monte Carlo simulations we show that the ground-state gauge order can generally be understood in terms of the length of elementary plaquettes-a result which extends the applicability of Lieb's theorem to lattice geometries beyond its original scope. At finite temperatures, the proliferation of (gapped) vison excitations destroys the gauge order at a critical temperature scale, which we show to correlate with the size of vison gap for the family of three-dimensional Kitaev models. We also discuss two notable exceptions where the lattice structure gives rise to "gauge frustration" or intertwines the gauge ordering with time-reversal symmetry breaking In a more general context, the thermodynamic gauge transitions in such 3D Kitaev models are one of the most natural settings for phase transitions beyond the standard Landau-Ginzburg-Wilson paradigm.
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| 6. |
- Eschmann, T., et al.
(author)
-
Thermodynamics of a gauge-frustrated Kitaev spin liquid
- 2019
-
In: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 1:3
-
Journal article (peer-reviewed)abstract
- Two- and three-dimensional Kitaev magnets are prototypical frustrated quantum spin systems, in which the original spin degrees of freedom fractionalize into Majorana fermions and a Z2 gauge field—a purely local phenomenon that reveals itself as a thermodynamic crossover at a temperature scale set by the strength of the bond-directional interactions. For conventional Kitaev magnets, the low-temperature thermodynamics reveals a second transition at which the Z2 gauge field orders and the system enters a spin-liquid ground state. Here, we discuss an explicit example that goes beyond this paradigmatic scenario—the Z2 gauge field is found to be subject to geometric frustration, the thermal ordering transition is suppressed, and an extensive residual entropy arises. Deep in the quantum regime, at temperatures of the order of one per mil of the interaction strength, the degeneracy in the gauge sector is lifted by a subtle interplay between the gauge field and the Majorana fermions, resulting in the formation of a Majorana metal. We discuss the thermodynamic signatures of this physics obtained from large-scale, sign-free quantum Monte Carlo simulations.
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| 7. |
- Eschmann, T., et al.
(author)
-
Thermodynamics of a gauge-frustrated Kitaev spin liquid
- 2019
-
In: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 1:3
-
Journal article (peer-reviewed)abstract
- Two- and three-dimensional Kitaev magnets are prototypical frustrated quantum spin systems, in which the original spin degrees of freedom fractionalize into Majorana fermions and a Z2 gauge field - a purely local phenomenon that reveals itself as a thermodynamic crossover at a temperature scale set by the strength of the bond-directional interactions. For conventional Kitaev magnets, the low-temperature thermodynamics reveals a second transition at which the Z2 gauge field orders and the system enters a spin-liquid ground state. Here, we discuss an explicit example that goes beyond this paradigmatic scenario - the Z2 gauge field is found to be subject to geometric frustration, the thermal ordering transition is suppressed, and an extensive residual entropy arises. Deep in the quantum regime, at temperatures of the order of one per mil of the interaction strength, the degeneracy in the gauge sector is lifted by a subtle interplay between the gauge field and the Majorana fermions, resulting in the formation of a Majorana metal. We discuss the thermodynamic signatures of this physics obtained from large-scale, sign-free quantum Monte Carlo simulations.
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| 8. |
- Hansson, Thors Hans, et al.
(author)
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Conformal Field Theory Approach to Abelian and Non-Abelian Quantum Hall Quasielectrons
- 2009
-
In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 102:16, s. 166805-1-166805-4
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Journal article (peer-reviewed)abstract
- The quasiparticles in quantum Hall liquids carry fractional charge and obey fractional quantum statistics. Of particular recent interest are those with non-Abelian statistics, since their braiding properties could, in principle, be used for robust coding of quantum information. There is already a good theoretical understanding of quasiholes in both Abelian and non-Abelian quantum Hall states. Here we develop conformal field theory methods that allow for an equally precise description of quasielectrons and explicitly construct two- and four-quasielectron excitations of the non-Abelian Moore-Read state.
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| 9. |
- Hansson, Thors Hans, et al.
(author)
-
Quantum Hall quasielectron operators in conformal field theory
- 2009
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In: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 80:16, s. 165330-1-165330-22
-
Journal article (peer-reviewed)abstract
- In the conformal field theory (CFT) approach to the quantum Hall effect, the multielectron wave functions are expressed as correlation functions in certain rational CFTs. While this approach has led to a well-understood description of the fractionally charged quasihole excitations, the quasielectrons have turned out to be much harder to handle. In particular, forming quasielectron states requires nonlocal operators, in sharp contrast to quasiholes that can be created by local chiral vertex operators. In both cases, the operators are strongly constrained by general requirements of symmetry, braiding, and fusion. Here we construct a quasielectron operator satisfying these demands and show that it reproduces known good quasiparticle wave functions, as well as predicts additional ones. In particular, we propose explicit wave functions for quasielectron excitations of the Moore-Read Pfaffian state. Further, this operator allows us to explicitly express the composite fermion wave functions in the positive Jain series in hierarchical form, thus settling a long-time controversy. We also critically discuss the status of the fractional statistics of quasiparticles in the Abelian hierarchical quantum Hall states and argue that our construction of localized quasielectron states sheds new light on their statistics. At the technical level we introduce a generalized normal ordering that allows us to “fuse” an electron operator with the inverse of an hole operator and also an alternative approach to the background charge needed to neutralize CFT correlators. As a result we get a fully holomorphic CFT representation of a large set of quantum Hall wave functions.
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| 10. |
- Hermanns, Maria
(author)
-
Condensing Non-Abelian Quasiparticles
- 2010
-
In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 104:5, s. 056803-1-056803-4
-
Journal article (peer-reviewed)abstract
- A most interesting feature of certain fractional quantum Hall states is that their quasiparticles obey non-Abelian fractional statistics. So far, candidate non-Abelian wave functions have been constructed from conformal blocks in cleverly chosen conformal field theories. In this work we present a hierarchy scheme by which we can construct daughter states by condensing non-Abelian quasiparticles (as opposed to quasiholes) in a parent state, and show that the daughters have a non-Abelian statistics that differs from the parent. In particular, we discuss the daughter of the bosonic, spin-polarized Moore-Read state at ν=4/3 as an explicit example.
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| 11. |
- Hermanns, Maria
(author)
-
Conformal field theory construction for non-Abelian hierarchy wave functions
- 2017
-
In: Physical Review B. - 2469-9950. ; 96:24
-
Journal article (peer-reviewed)abstract
- The fractional quantum Hall effect is the paradigmatic example of topologically ordered phases. One of its most fascinating aspects is the large variety of different topological orders that may be realized, in particular non-Abelian ones. Here we analyze a class of non-Abelian fractional quantum Hall model states which are generalizations of the Abelian Haldane-Halperin hierarchy. We derive their topological properties and show that the quasiparticles obey non-Abelian fusion rules of type su(q)(k). For a subset of these states we are able to derive the conformal field theory description that makes the topological properties-in particular braiding-of the state manifest. The model states we study provide explicit wave functions for a large variety of interesting topological orders, which may be relevant for certain fractional quantum Hall states observed in the first excited Landau level.
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| 12. |
- Hermanns, Maria, et al.
(author)
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Quantum Hall wave functions on the torus
- 2008
-
In: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 77:12, s. 125321-1-125321-16
-
Journal article (peer-reviewed)abstract
- We present explicit expressions for a large set of hierarchy wave functions on the torus. Included are the Laughlin states, the states in the positive Jain series, and recently observed states at, e.g., ν=4∕11. The techniques we use constitute a nontrivial extension of the conformal field theory methods developed earlier to construct the corresponding wave functions in disk geometry.
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| 13. |
- Hermanns, Maria, 1980-
(author)
-
Quasielectrons in Abelian and non-Abelian Quantum Hall States
- 2010
-
Doctoral thesis (other academic/artistic)abstract
- Strongly correlated electron systems continue to attract a lot of interest. Especially two-dimensional electron systems have shown many surprising behaviours. A fascinating example is the quantum Hall effect, which arises when electrons are confined to two dimensions, subjected to a very large magnetic field, and cooled to very low temperatures. Under these conditions, the electrons form new states of matter - the strongly correlated quantum Hall liquids. A hallmark of these quantum liquids is the precise quantization of the Hall conductance, but in recent years more attention has been focused on their exotic excitations. These have fractional electric charge, and fractional exchange statistics. The latter implies that the wave function is multiplied by a phase factor containing a fractional phase when two quasiparticles are moved around each other. Thus, they are neither bosons nor fermions, but so-called anyons. In recent years, there has accumulated theoretical and experimental evidence for some of the quantum Hall liquids having even more exotic excitations with not only fractional but non-Abelian exchange statistics. The quasiparticles of such systems could be used to build topologically protected quantum bits, which are much more robust than presently available quantum bits; they would be the ideal building blocks of a quantum computer. We use conformal field theory to describe the quantum Hall liquids, as well as their excitations. In particular, we represent the electrons and quasiparticles by conformal field theory operators. Even though the operator description for quasiholes is very well understood, it was for a long time unclear how to describe their antiparticles, the quasielectrons. We found an operator that describes quasielectron excitations correctly and shares many of the useful properties of the corresponding quasihole operator. For instance, many of the topological properties of the particles are manifest in the operator. This not only adds a missing piece to the quantum Hall puzzle, but it also opens up new and exciting possibilities. For instance, we were able to extend this construction to the non-Abelian states. A highly non-trivial application of our approach is the condensation of non-Abelian quasielectrons, which yields new non-Abelian quantum Hall states with non-Abelian properties that differ from those of their parent states.
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| 14. |
- Kjäll, Jonas, et al.
(author)
-
Matrix product state representation of quasielectron wave functions
- 2018
-
In: Journal of Statistical Mechanics-Theory and Experiment. - : IOP Publishing. - 1742-5468.
-
Journal article (peer-reviewed)abstract
- Matrix product state techniques provide a very efficient way to numerically evaluate certain classes of quantum Hall wave functions that can be written as correlators in two-dimensional conformal field theories. Important examples are the Laughlin and Moore-Read ground states and their quasihole excitations. In this paper, we extend the matrix product state techniques to evaluate quasielectron wave functions, a more complex task because the corresponding conformal field theory operator is not local. We use our method to obtain density profiles for states with multiple quasielectrons and quasiholes, and to calculate the (mutual) statistical phases of the excitations with high precision. The wave functions we study are subject to a known difficulty: the position of a quasielectron depends on the presence of other quasiparticles, even when their separation is large compared to the magnetic length. Quasielectron wave functions constructed using the composite fermion picture, which are topologically equivalent to the quasielectrons we study, have the same problem. This flaw is serious in that it gives wrong results for the statistical phases obtained by braiding distant quasiparticles. We analyze this problem in detail and show that it originates from an incomplete screening of the topological charges, which invalidates the plasma analogy. We demonstrate that this can be remedied in the case when the separation between the quasiparticles is large, which allows us to obtain the correct statistical phases. Finally, we propose that a modification of the Laughlin state, that allows for local quasielectron operators, should have good topological properties for arbitrary configurations of excitations.
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| 15. |
- Magnaterra, M., et al.
(author)
-
RIXS interferometry and the role of disorder in the quantum magnet Ba3 Ti3-x Irx O9
- 2023
-
In: Physical Review Research. - : American Physical Society. - 2643-1564. ; 5:1
-
Journal article (peer-reviewed)abstract
- Motivated by several claims of spin-orbit-driven spin-liquid physics in hexagonal Ba3Ti3-xIrxO9 hosting Ir2O9 dimers, we report on resonant inelastic x-ray scattering (RIXS) at the Ir L3 edge for different x. We demonstrate that magnetism in Ba3Ti3-xIrxO9 is governed by an unconventional realization of strong disorder, where cation disorder affects the character of the local moments. RIXS interferometry, studying the RIXS intensity over a broad range of transferred momentum q, is ideally suited to assign different excitations to different Ir sites. We find pronounced Ir-Ti site mixing. Both ions are distributed over two crystallographically inequivalent sites, giving rise to a coexistence of quasimolecular singlet states on Ir2O9 dimers and spin-orbit-entangled j=1/2 moments of 5d5Ir4+ ions. RIXS reveals different kinds of strong magnetic couplings for different bonding geometries, highlighting the role of cation disorder for the suppression of long-range magnetic order in this family of compounds.
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| 16. |
- Matern, S., et al.
(author)
-
Entanglement in 3D Kitaev spin liquids
- 2018
-
In: Journal of Statistical Mechanics-Theory and Experiment. - : IOP Publishing. - 1742-5468.
-
Journal article (peer-reviewed)abstract
- Quantum spin liquids are highly fascinating quantum liquids in which the spin degrees of freedom fractionalize. An interesting class of spin liquids axe the exactly solvable, three-dimensional Kitaev spin liquids. Their fractionalized excitations are Majonara fermions, which may exhibit a variety of topological band structures-ranging from topologically protected Weyl semimetals over nodal semi-metals to systems with Majorana Fermi surfaces. We study the entanglement spectrum of such Kitaev spin liquids and verify that it is closely related to the topologically protected edge spectrum. Moreover, we find that in some cases the entanglement spectrum contains even more information about the topological features than the surface spectrum, and thus provides a simple and reliable tool to probe the topology of a system.
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| 17. |
- Mishchenko, Petr A., et al.
(author)
-
Chiral spin liquids with crystalline Z(2) gauge order in a three-dimensional Kitaev model
- 2020
-
In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 101:4
-
Journal article (peer-reviewed)abstract
- Chiral spin liquids (CSLs) are time-reversal-symmetry-breaking ground states of frustrated quantum magnets that show no long-range magnetic ordering but instead exhibit topological order and fractional excitations. Their realization in simple and tractable microscopic models has, however, remained an open challenge for almost two decades until it was realized that Kitaev models on lattices with odd-length loops are natural hosts for such states, even in the absence of a time-reversal-symmetry-breaking magnetic field. Here we report on the formation of CSLs in a three-dimensional Kitaev model on a hypernonagon lattice composed of nine-site loops, which differ from their widely studied two-dimensional counterparts; namely, they exhibit a crystalline ordering of the Z(2) gauge fluxes and thereby break some of the underlying lattice symmetries. We study the formation of these unconventional CSLs via extensive quantum Monte Carlo simulations and demonstrate that they are separated from the featureless paramagnet at high temperatures by a single first-order phase transition at which both time-reversal and lattice symmetries are simultaneously broken. Using variational approaches for the ground state, we explore the effect of varying the Kitaev couplings and find at least five distinct CSL phases, all of which possess crystalline ordering of the Z(2) gauge fluxes. For some of these phases, the complementary itinerant Majorana fermions exhibit gapless band structures with topological features such as Weyl nodes or nodal lines in the bulk and Fermi arc or drumhead surface states.
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| 18. |
- Ortega-Taberner, Carlos, 1993-
(author)
-
Entanglement spectrum and the bulk polarization
- 2020
-
Licentiate thesis (other academic/artistic)abstract
- In this Licentiate thesis we give a brief review on the topics of topologicalinsulator and superconductor phases, the modern theory of polarization andthe entanglement spectrum, with a focus on one- and two-dimensional systems.In the context of symmetry protected topological systems the bulk polarizationcan be a topological invariant which characterizes the topological phase. Bythe bulk-boundary correspondence the bulk polarization is known to be relatedto the number of topological edge states, which is encoded in the entanglementspectrum.We study the general relation between the bulk polarization and the entanglementspectrum and show how the bulk polarization can always be decodedfrom the entanglement spectrum, even in the absence of symmetries that quantizeit. Applied to the topological case the known relation between the bulkpolarization and the number of topological edge states is recovered. Since thebulk polarization is a geometric phase, we use it to compute Chern numbersin one- and two-dimensional systems. The computation of these Chern numbersis simplied by using an alternative bulk polarization constructed usingthe entanglement spectrum. This alternative bulk polarization can also providemore information about the topological features of the boundary than theconventional bulk polarization.
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| 19. |
- Ortega-Taberner, Carlos, 1993-, et al.
(author)
-
From Hermitian critical to non-Hermitian point-gapped phases
-
Other publication (other academic/artistic)abstract
- Recent years have seen a growing interest in topological phases beyond the standard paradigm of gapped, isolated systems. One recent direction is to explore topological features in non-hermitian systems that are commonly used as effective descriptions of open systems. Another direction explores the fate of topology at critical points, where the bulk gap collapses. One interesting observation is that both systems, though very different, share certain topological features. For instance, both systems can host half-integer quantized winding numbers and have very similar entanglement spectra. Here, we make this similarity explicit by showing the equivalence of topological invariants in critical systems with non-hermitian point-gap phases, in the presence of sublattice symmetry. This correspondence may carry over to other features beyond topological invariants, and may even be helpful to deepen our understanding of non-hermitian systems using our knowledge of critical systems, and vice versa.
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| 20. |
- Ortega-Taberner, Carlos, et al.
(author)
-
From Hermitian critical to non-Hermitian point-gapped phases
- 2023
-
In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:23
-
Journal article (peer-reviewed)abstract
- Recent years have seen a growing interest in topological phases beyond the standard paradigm of gapped isolated systems. One recent direction is to explore topological features in non-Hermitian systems that are commonly used as effective descriptions of open systems. Another direction explores the fate of topology at critical points, where the bulk gap collapses. One interesting observation is that both systems, though very different, share certain topological features. For instance, both systems can host half-integer quantized winding numbers and have very similar entanglement spectra. Here we make this similarity explicit by showing the equivalence of topological invariants in critical systems with non-Hermitian point-gap phases, in the presence of sublattice symmetry. Also, the corresponding entanglement spectra show the same topological features. This correspondence may carry over to other features and even be helpful to deepen our understanding of non-Hermitian systems using our knowledge of critical systems and vice versa.
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| 21. |
- Ortega-Taberner, Carlos, et al.
(author)
-
Polarization and entanglement spectrum in non-Hermitian systems
- 2022
-
In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 105:7
-
Journal article (peer-reviewed)abstract
- The entanglement spectrum is a useful tool to study topological phases of matter, and contains valuable information about the ground state of the system. Here, we study its properties for free non-Hermitian systems for both point-gapped and line-gapped phases. While the entanglement spectrum only retains part of the topological information in the former case, it is very similar to Hermitian systems in the latter. In particular, it not only mimics the topological edge modes, but also contains all the information about the polarization, even in systems that are not topological. Furthermore, we show that the Wilson loop is equivalent to the many-body polarization and that it reproduces the phase diagram for the system with open boundaries, despite being computed for a periodic system.
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| 22. |
- Ortega-Taberner, Carlos, et al.
(author)
-
Relation of the entanglement spectrum to the bulk polarization
- 2021
-
In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 103:19
-
Journal article (peer-reviewed)abstract
- The bulk polarization is a Z(2) topological invariant characterizing noninteracting systems in one dimension with chiral or particle-hole symmetries. We show that the bulk polarization can always be determined from the single-particle entanglement spectrum, even in the absence of symmetries that quantize it. In the symmetric case, the known relation between the bulk polarization and the number of virtual topological edge modes is recovered. We use the bulk polarization to compute Chern numbers in one and two dimensions, which illuminates their known relation to the entanglement spectrum. Furthermore, we discuss an alternative bulk polarization that can carry more information about the surface spectrum than the conventional one and can simplify the calculation of Chern numbers.
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| 23. |
- Ortega-Taberner, Carlos, 1993-
(author)
-
Topology off the beaten path : From critical to non-Hermitian systems
- 2023
-
Doctoral thesis (other academic/artistic)abstract
- A central topic in condensed matter research during the last decades has been the study and classification of topological phases of matter. Topological insulators in particular, a subset of symmetry protected topological phases, have been investigated for over a decade. In recent years, several extensions to this formalism have been proposed to study more unconventional systems.In this thesis we explore two of these extensions, where key assumptions in the original formalism are removed. The first case is critical systems, which have no energy gap. Conventional topological invariants are discontinuous at topological transitions, and therefore not well-defined for critical systems. We propose a method for generalizing conventional topological invariants to critical systems and show robustness to disorder that preserves criticality. The second case involves non-Hermitian systems, which appear in effective descriptions of dissipation, where we study the entanglement spectrum and its connection to topological invariants. Furthermore, by introducing non-Hermiticity to critical systems we show how the winding numbers that characterize some topological phases of the non-Hermitian system, as well as topological signatures in the entanglement spectrum, can be obtained from the related critical model.
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| 24. |
- Revelli, A., et al.
(author)
-
Resonant inelastic x-ray incarnation of Young's double-slit experiment
- 2019
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In: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 5:1
-
Journal article (peer-reviewed)abstract
- Young's archetypal double-slit experiment forms the basis for modern diffraction techniques: The elastic scattering of waves yields an interference pattern that captures the real-space structure. Here, we report on an inelastic incarnation of Young's experiment and demonstrate that resonant inelastic x-ray scattering (RIXS) measures interference patterns, which reveal the symmetry and character of electronic excited states in the same way as elastic scattering does for the ground state. A prototypical example is provided by the quasi-molecular electronic structure of insulating Ba3CeIr2O9 with structural Ir dimers and strong spin-orbit coupling. The double "slits" in this resonant experiment are the highly localized core levels of the two Ir atoms within a dimer. The clear double-slit-type sinusoidal interference patterns that we observe allow us to characterize the electronic excitations, demonstrating the power of RIXS interferometry to unravel the electronic structure of solids containing, e.g., dimers, trimers, ladders, or other superstructures.
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| 25. |
- Revelli, A., et al.
(author)
-
Spin-orbit entangled j=1/2 moments in Ba(2)CWeIrO(6) : A frustrated fcc quantum magnet
- 2019
-
In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 100:8
-
Journal article (peer-reviewed)abstract
- We establish the double perovskite Ba2CeIrO6 as a nearly ideal model system for j = 1/2 moments, with resonant inelastic x-ray scattering indicating that the ideal j = 1/2 state contributes by more than 99% to the ground-state wave function. The local j = 1/2 moments form an fcc lattice and are found to order antiferromagnetically at T-N = 14 K, more than an order of magnitude below the Curie-Weiss temperature. Model calculations show that the geometric frustration of the fcc Heisenberg antiferromagnet is further enhanced by a next-nearest neighbor exchange, and a significant size of the latter is indicated by ab initio theory. Our theoretical analysis shows that magnetic order is driven by a bond-directional Kitaev exchange and by local distortions via a strong magnetoelastic effect. Both, the suppression of frustration by Kitaev exchange and the strong magnetoelastic effect are typically not expected for j = 1/2 compounds making Ba2CeIrO6 a riveting example for the rich physics of spin-orbit entangled Mott insulators.
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| 26. |
- Revelli, A., et al.
(author)
-
Spin-orbit entangled j=1/2 moments in Ba2CeIrO6 : A frustrated fcc quantum magnet
- 2019
-
In: Physical Review B. - 2469-9950 .- 2469-9969. ; 100:8
-
Journal article (peer-reviewed)abstract
- We establish the double perovskite Ba2CeIrO6 as a nearly ideal model system for j = 1/2 moments, with resonant inelastic x-ray scattering indicating that the ideal j = 1/2 state contributes by more than 99% to the ground-state wave function. The local j = 1/2 moments form an fcc lattice and are found to order antiferromagnetically at T-N = 14 K, more than an order of magnitude below the Curie-Weiss temperature. Model calculations show that the geometric frustration of the fcc Heisenberg antiferromagnet is further enhanced by a next-nearest neighbor exchange, and a significant size of the latter is indicated by ab initio theory. Our theoretical analysis shows that magnetic order is driven by a bond-directional Kitaev exchange and by local distortions via a strong magnetoelastic effect. Both, the suppression of frustration by Kitaev exchange and the strong magnetoelastic effect are typically not expected for j = 1/2 compounds making Ba2CeIrO6 a riveting example for the rich physics of spin-orbit entangled Mott insulators.
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| 27. |
- Rødland, Lukas
(author)
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Topological Phases of Non-Hermitian physics
- 2023
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Licentiate thesis (other academic/artistic)abstract
- Non-Hermitian physics has introduced phenomena like the skin effect and exceptional points, challenging traditional views of topological phases. This thesis contrasts two classification frameworks for non-Hermitian systems. The first approach employs K-theory classifications based on line-gaps and point-gaps, which explain the topological origins of the skin effect but fail to adequately describe exceptional points. The second, more nuanced approach uses homotopy theory, where the braiding of complex energies around exceptional points is integral to the classification. This method provides a detailed account of the behaviour of exceptional points, such as the splitting and merging of these points. We also study systems restricted by PT-symmetry, a symmetry that ensures that each eigenvalue is real or has a complex conjugate pair, which leads to the formation of an exceptional cone in the parameter space of two-band models. This cone features prominently in the accompanying papers, especially when introducing the concept of non-defective exceptional points and in the homotopy classification of PT-symmetric models.
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| 28. |
- Snoek, Frank J., et al.
(author)
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Roles and competencies of the clinical psychologist in adult diabetes care : A consensus report
- 2024
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In: Diabetic Medicine. - 0742-3071 .- 1464-5491. ; 41:5
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Research review (peer-reviewed)abstract
- Aims: Psychological care is recognised as an integral part of quality diabetes care. We set out to describe the roles and competencies of the clinical psychologist as a member of the multidisciplinary adult diabetes care team, focused on secondary care.Methods: The authors are clinically experienced psychologists involved in adult diabetes care, from Australia, Europe and North America, and active members of the international psychosocial aspects of diabetes study group. Consensus was reached as a group on the roles and competencies of the clinical psychologist working in adult diabetes secondary care, building both on expert opinion and a selective review and discussion of the literature on psychological care in diabetes, clinical guidelines and competency frameworks.Results: The clinical psychologist fulfils multiple roles: (1) as a clinician (psychological assessment and therapy), (2) as advisor to the healthcare team (training, consulting), (3) as a communicator and promotor of person-centred care initiatives and (4) as a researcher. Four competencies that are key to successfully fulfilling the above-mentioned roles in a diabetes setting are as follows: (a) specialised knowledge, (b) teamwork and advice, (c) assessment, (d) psychotherapy (referred to as STAP framework).Conclusions: The roles and competencies of clinical psychologists working in diabetes extend beyond the requirements of most university and post-graduate curricula. There is a need for a comprehensive, accredited specialist post-graduate training for clinical psychologists working in diabetes care, building on the proposed STAP framework. This calls for a collaborative effort involving diabetes organisations, clinical psychology societies and diabetes psychology interest groups.
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| 29. |
- Tournois, Yoran, 1991-
(author)
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Abelian and non-abelian quantum Hall hierarchies
- 2020
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Doctoral thesis (other academic/artistic)abstract
- A core tenet of condensed matter physics has been that different phases of matter can be classified according to Landau's symmetry breaking paradigm. It has become clear, however, that phases of matter exist that are not distinguished by symmetry, but rather by topology. A paradigmatic example of this topological order are the fractional quantum Hall phases, which are the topic of this dissertation. Such phases exhibit the fractional quantum Hall effect, which occurs when electrons confined to two dimensions are subjected to a strong perpendicular magnetic field at very low temperatures. Characterized by a precise quantization of the Hall resistance and a concomitant vanishing of the longitudinal resistance, the fractional quantum Hall effect results from the formation of a strongly correlated quantum liquid of electrons. This quantum liquid supports remarkable quasiparticle excitations, which carry a fractional charge and are thought to obey fractional statistics beyond the familiar Bose-Einstein and Fermi-Dirac statistics.The theoretical understanding of the topological orders realized by the fractional quantum Hall states has progressed by the proposal of explicit trial wave functions as well as various types of effective field theories. This dissertation focuses on two series of trial wave functions, abelian and non-abelian hierarchy wave functions. We study the non-abelian hierarchy wave functions using conformal field theory techniques, by means of which the associated topological properties are studied. These include the fractional charges of the quasiparticles and their non-abelian fractional statistics. In addition, we study abelian hierarchy wave functions using effective Ginzburg-Landau theories in a way that connects to their known conformal field theory description.
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| 30. |
- Tournois, Yoran, et al.
(author)
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Conformal field theory construction for non-Abelian hierarchy wave functions
- 2017
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In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 96:24
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Journal article (peer-reviewed)abstract
- The fractional quantum Hall effect is the paradigmatic example of topologically ordered phases. One of its most fascinating aspects is the large variety of different topological orders that may be realized, in particular non-Abelian ones. Here we analyze a class of non-Abelian fractional quantum Hall model states which are generalizations of the Abelian Haldane-Halperin hierarchy. We derive their topological properties and show that the quasiparticles obey non-Abelian fusion rules of type su(q)k. For a subset of these states we are able to derive the conformal field theory description that makes the topological properties—in particular braiding—of the state manifest. The model states we study provide explicit wave functions for a large variety of interesting topological orders, which may be relevant for certain fractional quantum Hall states observed in the first excited Landau level.
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| 31. |
- Tournois, Yoran, et al.
(author)
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Microscopic derivation of Ginzburg-Landau theories for hierarchical quantum Hall states
- 2020
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In: SciPost Physics. - : SCIPOST FOUNDATION. - 2542-4653. ; 8:5
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Journal article (peer-reviewed)abstract
- We propose a Ginzburg-Landau theory for a large and important part of the abelian quantum Hall hierarchy, including the prominently observed Jain sequences. By a generalized "flux attachment" construction we extend the Ginzburg-Landau-Chern-Simons composite boson theory to states obtained by both quasielectron and quasihole condensation, and express the corresponding wave functions as correlators in conformal field theories. This yields a precise identification of the relativistic scalar fields entering these correlators in terms of the original electron field.
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| 32. |
- Vergara, I., et al.
(author)
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Spin-orbit coupling and crystal-field splitting in Ti-doped Ca2RuO4 studied by ellipsometry
- 2022
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In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 106:8
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Journal article (peer-reviewed)abstract
- In Ca2RuO4, the competition of spin-orbit coupling ζ and tetragonal crystal-field splitting ΔCF has been discussed controversially for many years. The orbital occupation depends on ΔCF/ζ, which allows us to address this ratio via the optical spectral weights of the lowest intersite Mott-Hubbard excitations. We study the optical conductivity of Ca2Ru0.99Ti0.01O4 in the range of 0.75-5 eV by ellipsometry, using the large single crystals that can be grown for small Ti concentrations. Based on a local multiplet calculation, our analysis results in 2.4≤ΔCF/ζ≲4 at 15 K. The dominant crystal field yields a ground state close to xy orbital order but spin-orbit coupling is essential for a quantitative description of the properties. Furthermore, we observe a pronounced decrease of ΔCF with increasing temperature, as expected based on the reduction of octahedral distortions.
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| 33. |
- Warzanowski, P., et al.
(author)
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Multiple spin-orbit excitons and the electronic structure of alpha-RuCl3
- 2020
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In: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 2:4
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Journal article (peer-reviewed)abstract
- The honeycomb compound alpha-RuCl3 is widely discussed as a proximate Kitaev spin-liquid material. This scenario builds on spin-orbit entangled j = 1/2 moments arising for a t(2g)(5) electron configuration with strong spin-orbit coupling lambda and a large cubic crystal field. The actual low-energy electronic structure of alpha-RuCl3, however, is still puzzling. In particular, infrared absorption features at 0.30, 0.53, and 0.75 eV seem to be at odds with a j = 1/2 scenario. Also the energy of the spin-orbit exciton, the excitation from j = 1/2 to 3/2, and thus the value of lambda, are controversial. Combining infrared and Raman data, we show that the infrared features can be attributed to single, double, and triple spin-orbit excitons. We find lambda = 0.16 eV and Delta = 42(4) meV for the observed noncubic crystal-field splitting, supporting the validity of the j = 1/2 picture for alpha-RuCl3. The unusual strength of the double excitation is related to the underlying hopping interactions, which form the basis for dominant Kitaev exchange.
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| 34. |
- Wawrzik, D., et al.
(author)
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Topological semimetals and insulators in three-dimensional honeycomb materials
- 2018
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In: Physical Review B. - 2469-9950. ; 98:11
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Journal article (peer-reviewed)abstract
- Semimetals, in which conduction and valence bands touch but do not form Fermi surfaces, have attracted considerable interest for their anomalous properties starting with the discovery of Dirac matter in graphene and other two-dimensional honeycomb materials. Here we introduce a family of three-dimensional honeycomb systems whose electronic band structures exhibit a variety of topological semimetals with Dirac nodal lines. We show that these nodal lines appear in varying numbers and mutual geometries, depending on the underlying lattice structure. They are stabilized, in most cases, by a combination of time-reversal and inversion symmetries and are accompanied by topologically protected "drumhead" surface states. In the bulk, these nodal line systems exhibit Landau level quantization and flat bands upon applying a magnetic field. In the presence of spin-orbit coupling, these topological semimetals are found to generically form (strong) topological insulators. This comprehensive classification of the electronic band structures of three-dimensional honeycomb systems might serve as guidance for future material synthesis.
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