| 1. |
- Abouelkomsan, Ahmed, 1995-
(författare)
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Strongly Correlated Moiré Materials
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Recent advances in materials science have established Moiré materials as a new highly tunable and versatile form of quantum matter. When two dimensional atomic layers are brought into proximity, a tiny relative twist or a slight lattice mismatch produces Moiré patterns manifested in a superlattice structure with a lattice constant that is much larger than the lattice constants of the constituent layers. The new length scale has dramatic consequences for the underlying properties. A particular distinctive feature of Moiré materials is the emergence of nearly flat bands upon tuning external parameters such as the twist angle or the applied gate voltage. In a flat band, the kinetic energy is quenched, and interactions are enhanced bringing us to the realm of strongly correlated systems. A prime example of Moiré materials is twisted bilayer graphene, formed by taking two graphene layers and twisting them relative to each other.On the other hand, a famous class of interaction-induced phases of matter are fractional quantum Hall states and their lattice analogues known as fractional Chern insulators. These topologically ordered phases represent a departure from the conventional Landau symmetry breaking classification of matter, seen in the absence of local order parameters and the presence of global topological properties insensitive to local perturbations. Identifying and manufacturing materials that could host fractional Chern insulator states has a great potential for technological use.In this thesis, we provide the necessary background required for understanding the results of the accompanying papers [Phys. Rev. Lett. 124, 106803 & Phys. Rev. Lett. 126, 026801]. The theory of fractional Chern insulators is discussed followed by an introduction to the Moiré models used. In the two accompanying papers, we theoretically study a number of flat band Moiré materials aiming at identifying the possible phases that occur at fractional band fillings using a combination of analytical and numerical techniques. By reformulating the problem in terms of holes instead of electrons, it's possible to identify a variety of emergent weakly interacting Fermi liquids from an initial strongly interacting problem. In addition, our findings also include several high temperature fractional Chern insulator states at different fillings without external magnetic field.
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| 2. |
- Dong, Junkai, et al.
(författare)
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Topolectric circuits : Theory and construction
- 2021
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Ingår i: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 3:2
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Tidskriftsartikel (refereegranskat)abstract
- We highlight a general theory to engineer arbitrary Hermitian tight-binding lattice models in electrical LC circuits, where the lattice sites are replaced by the electrical nodes, connected to its neighbors and to the ground by capacitors and inductors. In particular, by supplementing each node with n subnodes, where the phases of the current and voltage are the n distinct roots of unity, one can in principle realize arbitrary hopping amplitude between the sites or nodes via the shift capacitor coupling between them. This general principle is then implemented to construct a plethora of topological models in electrical circuits, topolectric circuits, where the robust zero-energy topological boundary modes manifest through a large boundary impedance, when the circuit is tuned to the resonance frequency. The simplicity of our circuit constructions is based on the fact that the existence of the boundary modes relies only on the Clifford algebra of the corresponding Hermitian matrices entering the Hamiltonian and not on their particular representation. This in turn enables us to implement a wide class of topological models through rather simple topolectric circuits with nodes consisting of only two subnodes. We anchor these outcomes from the numerical computation of the on-resonance impedance in circuit realizations of first-order (m = 1), such as Chern and quantum spin Hall insulators, and second- (m = 2) and third- (m = 3) order topological insulators in different dimensions, featuring sharp localization on boundaries of codimensionality d(c) = m. Finally, we subscribe to the stacked topolectric circuit construction to engineer three-dimensional Weyl, nodal-loop, quadrupolar Dirac, and Weyl semimetals, respectively, displaying surface- and hinge-localized impedance.
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| 3. |
- Geilhufe, Matthias, et al.
(författare)
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Dynamically induced magnetism in KTaO3
- 2021
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Ingår i: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 3:2
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Tidskriftsartikel (refereegranskat)abstract
- Dynamical multiferroicity features entangled dynamic orders: fluctuating electric dipoles induce magnetization. Hence, the material with paraelectric fluctuations can develop magnetic signatures if dynamically driven. We identify the paraelectric KTaO3 (KTO) as a prime candidate for the observation of the dynamical multiferroicity. We show that when a KTO sample is exposed to a circularly polarized laser pulse, the dynamically induced ionic magnetic moments are of the order of 5% of the nuclear magneton per unit cell. We determine the phonon spectrum using ab initio methods, and we identify T-1u as relevant phonon modes that couple to the external field and induce magnetic polarization. We also predict a corresponding electron effect for the dynamically induced magnetic moment, which is enhanced by several orders of magnitude due to the significant mass difference between electron and ionic nucleus.
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| 4. |
- Grandi, Nicolas, et al.
(författare)
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Towards holographic flat bands
- 2021
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Ingår i: Journal of High Energy Physics (JHEP). - : Springer Nature. - 1126-6708 .- 1029-8479. ; :5
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Tidskriftsartikel (refereegranskat)abstract
- Motivated by the phenomenology in the condensed-matter flat-band Dirac systems, we here construct a holographic model that imprints the symmetry breaking pattern of a rather simple Dirac fermion model at zero chemical potential. In the bulk we explicitly include the backreaction to the corresponding Lifshitz geometry and compute the dynamical critical exponent. Most importantly, we find that such a geometry is unstable towards a nematic phase, exhibiting an anomalous Hall effect and featuring a Drude-like shift of its spectral weight. Our findings should motivate further studies of the quantum phases emerging from such holographic models.
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| 5. |
- Khaetskii, Alexander, et al.
(författare)
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Thermal magnetic fluctuations of a ferroelectric quantum critical point
- 2021
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 33:4
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Tidskriftsartikel (refereegranskat)abstract
- Entanglement of two different quantum orders is of an interest of the modern condensed matter physics. One of the examples is the dynamical multiferroicity, where fluctuations of electric dipoles lead to magnetization. We investigate this effect at finite temperature and demonstrate an elevated magnetic response of a ferroelectric near the ferroelectric quantum critical point (FE QCP). We calculate the magnetic susceptibility of a bulk sample on the paraelectric side of the FE QCP at finite temperature and find enhanced magnetic susceptibility near the FE QCP. We propose quantum paraelectric strontium titanate as a candidate material to search for dynamic multiferroicity. We estimate the magnitude of the magnetic susceptibility for this material and find that it is detectable experimentally.
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| 6. |
- Nag, Tanay, et al.
(författare)
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Hierarchy of higher-order Floquet topological phases in three dimensions
- 2021
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 103:11
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Tidskriftsartikel (refereegranskat)abstract
- Following a general protocol of periodically driving static first-order topological phases (supporting surface states) with suitable discrete symmetry breaking Wilson-Dirac masses, here we construct a hierarchy of higher-order Floquet topological phases in three dimensions. In particular, we demonstrate realizations of both second-order and third-order Floquet topological states, respectively supporting dynamic hinge and corner modes at zero quasienergy, by periodically driving their static first-order parent states with one and two discrete symmetry breaking Wilson-Dirac mass(es). While the static surface states are characterized by codimension d(c) = 1, the resulting dynamic hinge (corner) modes, protected by antiunitary spectral or particle-hole symmetries, live on the boundaries with d(c) = 2 (3). We exemplify these outcomes for three-dimensional topological insulators and Dirac semimetals, with the latter ones following an arbitrary spin-j representation.
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| 7. |
- Roy, Bitan, et al.
(författare)
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Dislocation as a bulk probe of higher-order topological insulators
- 2021
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Ingår i: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 3:3
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Tidskriftsartikel (refereegranskat)abstract
- Topological materials occupy the central stage in the modern condensed matter physics because of their robust metallic edge or surface states protected by the topological invariant, characterizing the electronic band structure in the bulk. Higher-order topological (HOT) states extend this usual bulk-boundary correspondence, so they host the modes localized at lower-dimensional boundaries, such as corners and hinges. Here we theoretically demonstrate that dislocations, ubiquitous defects in crystalline materials, can probe higher-order topology, recently realized in various platforms. We uncover that HOT insulators respond to dislocations through symmetry protected finite-energy in-gap electronic modes, localized at the defect core, which originate from an interplay between the orientation of the HOT mass domain wall and the Burgers vector of the dislocation. As such, these modes become gapless only when the Burgers vector points toward lower-dimensional gapless boundaries. Our findings are consequential for the systematic probing of the extended bulk-boundary correspondence in a broad range of HOT crystals and photonic and phononic or mechanical metamaterials through the bulk topological lattice defects.
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| 8. |
- Roy, Bitan, et al.
(författare)
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Mixed-parity octupolar pairing and corner Majorana modes in three dimensions
- 2021
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 104:18
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Tidskriftsartikel (refereegranskat)abstract
- We identify time-reversal symmetry breaking mixed-parity superconducting states that feature eight Majorana corner modes in properly cleaved three-dimensional cubic crystals. Namely, when an odd-parity isotropic p-wave pairing coexists with cubic symmetry preserving even-parity octupolar d(x2-y2) + id(3z2-r2) pairing, the gapless surface Majorana modes of the former get localized at the eight corners, thus yielding an intrinsic third-order topological superconductor (TOTSC). A cousin d(xy) + id(3z2-r2) pairing also accommodating eight corner Majorana modes, by virtue of breaking the cubic symmetry, in contrast, yields an extrinsic TOTSC. We identify a doped octupolar (topological or trivial) Dirac insulator as a suitable platform to sustain such unconventional superconductors, realized from an intraunit cell pairing. Finally, we argue that the proposed TOTSC can be experimentally realizable in NaCl and other structurally similar compounds under high pressure.
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