| 1. |
- Holmvall, Patric, 1988, et al.
(författare)
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SuperConga: An open-source framework for mesoscopic superconductivity
- 2023
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Ingår i: Applied Physics Reviews. - : AIP Publishing. - 1931-9401. ; 10:1
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Forskningsöversikt (refereegranskat)abstract
- We present SuperConga, an open-source framework for simulating equilibrium properties of unconventional and ballistic singlet superconductors, confined to two-dimensional (2D) mesoscopic grains in a perpendicular external magnetic field, at arbitrary low temperatures. It aims at being both fast and easy to use, enabling research without access to a computer cluster, and visualization in real-time with OpenGL. The core is written in C++ and CUDA, exploiting the embarrassingly parallel nature of the quasiclassical theory of superconductivity by utilizing the parallel computational power of modern graphics processing units. The framework self-consistently computes both the superconducting order-parameter and the induced vector potential and finds the current density, free energy, induced flux density, local density of states (LDOS), and the magnetic moment. A user-friendly Python frontend is provided, enabling simulation parameters to be defined via intuitive configuration files, or via the command-line interface, without requiring a deep understanding of implementation details. For example, complicated geometries can be created with relative ease. The framework ships with simple tools for analyzing and visualizing the results, including an interactive plotter for spectroscopy. An overview of the theory is presented, as well as examples showcasing the framework's capabilities and ease of use. The framework is free to download from https://gitlab.com/superconga/superconga, which also links to the extensive user manual, containing even more examples, tutorials, and guides. To demonstrate and benchmark SuperConga, we study the magnetostatics, thermodynamics, and spectroscopy of various phenomena. In particular, we study flux quantization in solenoids, vortex physics, surface Andreev bound-states, and a "phase crystal."We compare our numeric results with analytics and present experimental observables, e.g., the magnetic moment and LDOS, measurable with, for example, scanning probes, STM, and magnetometry.
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| 2. |
- Hahn, Oliver, 1993, et al.
(författare)
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Deterministic Gaussian conversion protocols for non-Gaussian single-mode resources
- 2022
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Ingår i: Physical Review A. - : American Physical Society. - 2469-9934 .- 2469-9926. ; 105:6
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Tidskriftsartikel (refereegranskat)abstract
- In the context of quantum technologies over continuous variables, Gaussian states and operations are typically regarded as freely available, as they are relatively easily accessible experimentally. In contrast, the generation of non-Gaussian states, as well as the implementation of non-Gaussian operations, pose significant challenges. This divide has motivated the introduction of resource theories of non-Gaussianity. As for any resource theory, it is of practical relevance to identify free conversion protocols between resources, namely, Gaussian conversion protocols between non-Gaussian states. Via systematic numerical investigations, we address the approximate conversion between experimentally relevant single-mode non-Gaussian states via arbitrary deterministic one-to-one mode Gaussian maps. First we show that cat and binomial states are approximately equivalent for finite energy, while this equivalence was previously known only in the infinite-energy limit. Then we consider the generation of cat states from photon-added and photon-subtracted squeezed states, improving over known schemes by introducing additional squeezing operations. The numerical tools that we develop also allow one to devise conversions of trisqueezed into cubic-phase states beyond previously reported performances. Finally, we identify various other conversions which instead are not viable.
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| 3. |
- Holmvall, Patric, 1988, et al.
(författare)
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Broken translational symmetry at edges of high-temperature superconductors
- 2018
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- Flat bands of zero-energy states at the edges of quantum materials have a topological origin. However, their presence is energetically unfavorable. If there is a mechanism to shift the band to finite energies, a phase transition can occur. Here we study high-temperature superconductors hosting flat bands of midgap Andreev surface states. In a second-order phase transition at roughly a fifth of the superconducting transition temperature, time-reversal symmetry and continuous translational symmetry along the edge are spontaneously broken. In an external magnetic field, only translational symmetry is broken. We identify the order parameter as the superfluid momentum p_s, that forms a planar vector field with defects, including edge sources and sinks. The critical points of the vector field satisfy a generalized Poincaré-Hopf theorem, relating the sum of Poincaré indices to the Euler characteristic of the system.
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| 4. |
- Holmvall, Patric, 1988, et al.
(författare)
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Collinear cluster tri-partition: Kinematics constraints and stability of collinearity
- 2017
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Ingår i: Physical Review C. - 2469-9985 .- 2469-9993. ; 95:014602
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Tidskriftsartikel (refereegranskat)abstract
- A new mode of nuclear fission has been proposed by the FOBOS collaboration, called Collinear Cluster Tri-partition (CCT), suggesting that three heavy fission fragments can be emitted perfectly collinearly in low-energy fission. It is surprising that CCT escaped observation for so long given the relatively high reported yield, of roughly 0.5% relative to binary fission. These claims call for an independent verification with a different experimental technique. Verification experiments based on direct observation of CCT fragments with fission fragment spectrometers require guidance with respect to the allowed kinetic energy range, which we present in this paper. We discuss corresponding model calculations which, if CCT is found in such verification experiments, could indicate how the breakups proceed. We also study the intrinsic stability of collinearity. Three different decay models are used, which span together the timescales of three-body fission. These models are used to calculate the possible kinetic energy ranges of CCT fragments in 235U(n,f) and 252Cf(sf). We use semi-classical trajectory calculations with a Monte-Carlo method to study the intrinsic stability of collinearity. CCT has a high net Q-value, but in a sequential decay, the intermediate steps are energetically and geometrically unfavorable or even forbidden. Moreover, perfect collinearity is extremely unstable, and broken by the slightest perturbation. According to our results, the central fragment would be very difficult to detect due to its low kinetic energy, raising the question of why previous experiments could not detect a missing-mass signature corresponding to CCT. We find that a realization of CCT would require an unphysical fine-tuning of the initial conditions. Our results enable independent experimental verification and encourage further critical theoretical studies of CCT.
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| 5. |
- Holmvall, Patric, 1988
(författare)
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Crystallization of the superconducting phase in unconventional superconductors
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Superconductivity is a macroscopic quantum phenomenon, in the sense that a macroscopic number of electrons form a pair condensate, that occupies a single ground state. The electrons in this state are phase-coherent, breaking global U (1)-symmetry, and spatial variations of the phase imply superflows that usually cost kinetic energy, resulting in a uniform and rigid phase. It would therefore be surprising if a more ordered state with a non-uniform phase existed. This thesis proposes that such a ground state can occur in the absence of external perturbations, deep inside the superconducting state, where a periodic pattern is spontaneously imprinted on the superconducting phase, breaking continuous translational invariance. The resulting phase gradients break time-reversal symmetry, manifested through finite superflows and equilibrium charge currents with peculiar patterns. In analogy to crystallization in solids, the new order parameter is defined as a finite Fourier amplitude at the wavevector corresponding to the phase-periodicity. This ground state is hence referred to as a phase crystal. The thesis employs the quasiclassical theory of superconductivity, combined with a non-local Ginzburg-Landau theory, to derive the inhomogeneous superfluid density tensor and the conditions under which phase crystallization can occur. It is shown how the phase can be realized at certain interfaces of unconventional superconductors, and in conventional superconductor-ferromagnet structures. The instability phase diagram is obtained, and the transition classified as second-order, surviving moderately strong external fields. The phase is tied to critical points in the superflow field, satisfying a generalized Poincaré-Hopf theorem. Geometric perturbations and disorder are studied, and characteristic signatures identified, in an attempt to aid experimental efforts in potential realization of the phase. In conclusion, the model based on the non-local superfluid tensor provides a unified approach to studying surface phenomena, e.g. topological states and inhomogeneous superconductivity, and is used to both verify and explain several previous numerical observations. The model directly highlights the role of non-local correlations and phase variations as drivers in phase transitions, motivating a search for new non-local phenomena in various condensed matter systems.
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| 6. |
- Holmvall, Patric, 1988
(författare)
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Modeling mesoscopic unconventional superconductors
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High-temperature superconducting materials are often experimentally realized as thin films that can be patterned into devices operating in the mesoscopic regime. On this length scale, various finite-size and surface effects heavily influence the nature of the superconducting state, and can induce new ground states with spontaneously broken symmetries. Motivated by the wide technological application of such mesoscopic devices and the many open questions regarding the new emergent ground states, this thesis sets out to study mesoscopic grains. In particular, a recently discovered phase which spontaneously breaks translational and time-reversal symmetries will be studied, referred here to as the "loop-current phase". The aim is to study how this phase responds to magnetic and geometric perturbations. The quasiclassical theory of superconductivity is used to simulate mesoscopic thin-film grains in equilibrium, with a strong emphasis on d-wave superconductors, e.g. the cuprates. The properties of the loop-current phase are cataloged, with an explanation of how and why it occurs. Various phase diagrams are produced, and the magnetic-field dependent thermodynamics is studied.In conclusion, the loop-current phase occurs at pairbreaking interfaces that host quasiparticle midgap states. The phase is associated with a spontaneous superfluid momentum which drives circulating current loops that break continuous translational symmetry, providing an energetically favorable Doppler shift of the midgap states. The phase is found to be robust against external fields in the whole Meissner state, but not against very high fields in the mixed state. The phase is lost when there is a competing effect which significantly broadens the spectrum, e.g. a strong external vector potential. The phase transition is associated with a large jump in the heat capacity, serving as a hallmark for the phase to be observed experimentally. It is predicted that the phase leads to a broadening of the spectrum which is consistent with experimental findings.
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| 7. |
- Holmvall, Patric, 1988, et al.
(författare)
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Phase crystals
- 2020
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Ingår i: Physical Review Research. - 2643-1564. ; 2:1
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Tidskriftsartikel (refereegranskat)abstract
- Superconductivity owes its properties to the phase of the electron pair condensate that breaks the U(1) symmetry. In the most traditional ground state, the phase is uniform and rigid. The normal state can be unstable towards special inhomogeneous superconducting states: the Abrikosov vortex state and the Fulde-Ferrell-Larkin-Ovchinnikov state. Here we show that the phase-uniform superconducting state can go into a fundamentally different and more ordered nonuniform ground state, which we refer to as a phase crystal. This state breaks translational invariance through formation of a spatially periodic modulation of the phase, manifested by unusual superflow patterns and circulating currents, that also break time-reversal symmetry. We list the general conditions needed for realization of phase crystals. Using microscopic theory, we then derive an analytic expression for the superfluid density tensor for the case of a nonuniform environment in a semi-infinite superconductor. We demonstrate how the surface quasiparticle states enter the superfluid density and identify phase crystallization as the main player in several previous numerical observations in unconventional superconductors, and predict the existence of a similar phenomenon in superconductor-ferromagnetic structures. This analytic approach provides a unifying aspect for the exploration of boundary-induced quasiparticles and collective excitations in superconductors. More generally, we trace the origin of phase crystallization to nonlocal properties of the gradient energy, which implies the existence of similar pattern-forming instabilities in many other contexts.
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| 8. |
- Holmvall, Patric, 1988, et al.
(författare)
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Robust and tunable coreless vortices and fractional vortices in chiral d-wave superconductors
- 2023
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 108:9
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Tidskriftsartikel (refereegranskat)abstract
- Chiral Formula Presented-wave superconductivity has recently been proposed in a wide range of materials based on both experiment and theoretical studies. Chiral superconductors host a finite Chern number set by the winding of the superconducting order parameter and associated topologically protected chiral edge modes. However, the chiral edge currents and orbital angular momentum (OAM) generated by the edge modes are not topologically protected and another, more robust, experimental probe is therefore needed to facilitate experimental verification of chiral Formula Presented-wave superconductors. We have recently shown the appearance of quadruply quantized coreless vortices (CVs) in chiral Formula Presented-wave superconductors, consisting of a closed domain wall decorated with eight fractional vortices, and generating a smoking-gun signature of the Chern number, chirality, and the superconducting pairing symmetry [ P. Holmvall and A. M. Black-Schaffer , Phys. Rev. B 108, L100506 (2023)10.1103/PhysRevB.108.L100506 ]. Specifically, the CV spontaneously breaks axial symmetry for parallel chirality and vorticity, with a signature appearing directly in the local density of states (LDOS) measurable with scanning tunneling spectroscopy (STS). In this paper, we first demonstrate a strong tunability of the CV size and shape directly reflected in the LDOS and then show that the LDOS signature is robust in the presence of regular Abrikosov vortices, strong confinement, system and normal-state anisotropy, different Fermi surfaces (FSs), nondegenerate order parameters, and even nonmagnetic impurities. In conclusion, our paper establishes CVs as a tunable and robust signature of chiral Formula Presented-wave superconductivity.
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| 9. |
- Holmvall, Patric, 1988, et al.
(författare)
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Spontaneous generation of fractional vortex-Antivortex pairs at single edges of high-Tc superconductors
- 2018
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 969:1
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Konferensbidrag (refereegranskat)abstract
- Unconventional d-wave superconductors with pair-breaking edges are predicted to have ground states with spontaneously broken time-reversal and translational symmetries. We use the quasiclassical theory of superconductivity to demonstrate that such phases can exist at any single pair-breaking facet. This implies that a greater variety of systems, not necessarily mesoscopic in size, should be unstable to such symmetry breaking. The density of states averaged over the facet displays a broad peak centered at zero energy, which is consistent with experimental findings of a broad zero-bias conductance peak with a temperature-independent width at low temperatures.
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| 10. |
- Holmvall, Patric, 1988, et al.
(författare)
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Spontaneous symmetry breaking at surfaces of d-wave superconductors: Influence of geometry and surface ruggedness
- 2019
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 99:18
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Tidskriftsartikel (refereegranskat)abstract
- Surfaces of d -wave superconductors may host a substantial density of zero-energy Andreev states. The zero-energy flat band appears due to a topological constraint, but comes with a cost in free energy. We have recently found that an adjustment of the surface states can drive a phase transition into a phase with finite superflow that breaks time-reversal symmetry and translational symmetry along the surface. The associated Doppler shifts of Andreev states to finite energies lower the free energy. Direct experimental verification of such a phase is still technically difficult and controversial, however. To aid further experimental efforts, we use the quasiclassical theory of superconductivity to investigate how the realization and the observability of such a phase are influenced by sample geometry and surface ruggedness. Phase diagrams are produced for relevant geometric parameters. In particular, critical sizes and shapes are identified, providing quantitative guidelines for sample fabrication in the experimental hunt for symmetry-breaking phases.
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| 11. |
- Wennerdal, Niclas, 1986, et al.
(författare)
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Breaking time-reversal and translational symmetry at edges of d -wave superconductors: Microscopic theory and comparison with quasiclassical theory
- 2020
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Ingår i: Physical Review Research. - 2643-1564. ; 2:4
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Tidskriftsartikel (refereegranskat)abstract
- We report results of a microscopic calculation of a second-order phase transition into a state-breaking time-reversal and translational invariance along pair-breaking edges of d-wave superconductors. By solving a tight-binding model through exact diagonalization with the Bogoliubov–de Gennes method, we find that such a state with current loops having a diameter of a few coherence lengths is energetically favorable below T∗ between 10%–20% of Tc of bulk superconductivity, depending on model parameters. This extends our previous studies of such a phase crystal within the quasiclassical theory of superconductivity, and shows that the instability is not qualitatively different when including a more realistic band structure and the fast oscillations on the scale of the Fermi wavelength. Effects of size quantization and Friedel oscillations are not detrimental. We also report on a comparison with quasiclassical theory with the Fermi surfaces extracted from the tight-binding models used in the microscopic calculation. There are quantitative differences in for instance the value of T∗ between the different models, but we can explain the predicted transition temperature within each model as due to the different spectral weights of zero-energy Andreev bound states and the resulting gain in free energy by breaking time-reversal and translational invariance below T∗.
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| 12. |
- Zheng, Yu, 1992, et al.
(författare)
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Gaussian Conversion Protocols for Cubic Phase State Generation
- 2021
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Ingår i: PRX Quantum. - 2691-3399. ; 2:1
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Tidskriftsartikel (refereegranskat)abstract
- Universal quantum computing with continuous variables requires non-Gaussian resources, in addition to a Gaussian set of operations. A known resource enabling universal quantum computation is the cubic phase state, a non-Gaussian state whose experimental implementation has so far remained elusive. In this paper, we introduce two Gaussian conversion protocols that allow for the conversion of a non-Gaussian state that has been achieved experimentally, namely the trisqueezed state [Chang et al., Phys. Rev. X 10, 011011 (2020)], to a cubic phase state. The first protocol is deterministic and it involves active (inline) squeezing, achieving large fidelities that saturate the bound for deterministic Gaussian protocols. The second protocol is probabilistic and it involves an auxiliary squeezed state, thus removing the necessity of inline squeezing but still maintaining significant success probabilities and fidelities even larger than for the deterministic case. The success of these protocols provides strong evidence for using trisqueezed states as resources for universal quantum computation.
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