We report numerical simulations of a trapped elastic vortex driven by a strong ac magnetic field $H\left(t\right)=Hsin\omega t$ parallel to the surface of a superconducting film. The surface resistance and the power dissipated by an oscillating vortex perpendicular to the film surface were calculated as functions of $H$ and $\omega$ for different spatial distributions, densities, and strengths of pinning centers, including bulk pinning, surface pinning, and cluster pinning. Our simulations were performed for both the Bardeen-Stephen viscous vortex drag and the Larkin-Ovchinnikov (LO) drag coefficient $\eta \left(v\right)$ decreasing with the vortex velocity $v$. The local residual surface resistance $R_i\left(H\right)$ calculated for different statistical realizations of the pinning potential exhibits strong mesoscopic fluctuations caused by local depinning jumps of a vortex segment as $H$ increases, but the global surface resistance ${\overline{R}}_i\left(H\right)$ obtained by averaging $R_i\left(H\right)$ over different pin configurations increases smoothly with the field amplitude at small $H$ and levels off at higher fields. For strong pinning, the LO decrease of $\eta \left(v\right)$ with $v$ can result in a nonmonotonic field dependence of $R_i\left(H\right)$ which decreases with $H$ at higher fields, but cause a runaway instability of the vortex in a thick film for weak pinning. It is shown that overheating of a single moving vortex can produce the LO-like velocity dependence of $\eta \left(v\right)$, but can mask the decrease of the surface resistance with $H$ at a higher density of trapped vortices.

A nonzero non-Hermitian winding number indicates that a gapped system is in a nontrivial topological class due to the non-Hermiticity of its Hamiltonian. While for Hermitian systems nontrivial topological quantum numbers are reflected by the existence of edge states, a nonzero non-Hermitian winding number impacts a system's bulk response. To establish this relation, we introduce the bulk Green function, which describes the response of a gapped system to an external perturbation on timescales where the induced excitations have not propagated to the boundary yet, and show that it will grow in space if the non-Hermitian winding number is nonzero. Such spatial growth explains why the response of non-Hermitian systems on longer timescales, where excitations have been reflected at the boundary repeatedly, may be highly sensitive to boundary conditions. This exponential sensitivity to boundary conditions explains the breakdown of the bulk-boundary correspondence in non-Hermitian systems: topological invariants computed for periodic boundary conditions no longer predict the presence or absence of boundary states for open boundary conditions.

Focusing specifically on the recently retracted work by Zhang et al. [H. Zhang et al., Nature (London) 556, 74 (2018); Retraction, Nature (London) 591, E30 (2021)] and the related recently available correctly analyzed data from this Delft experiment (H. Zhang et al., arXiv:2101.11456), we discuss the general problem of confirmation bias in experiments verifying various theoretical topological quantization predictions. We show that the Delft Majorana experiment is most likely dominated by disorder, which produces trivial (but quite sharp and large) zero-bias Andreev tunneling peaks with large conductance $\sim 2e^2/h$ in the theory, closely mimicking the data. Thus, although the corrected Delft data are by far the best tunnel spectroscopy results available in the literature, manifesting large and sharp zero-bias peaks rising above the background with an impressive hard superconducting gap, our theory shows that the most natural explanation for these zero-bias peaks is that they are disorder induced and not topological Majorana modes. It is possible to misinterpret such disorder-induced zero-bias trivial peaks as the apparent Majorana quantization, as was originally done arising from confirmation bias. One characteristic of the disorder-induced trivial peaks is that they manifest little stability as a function of Zeeman field, chemical potential, and tunnel barrier, distinguishing their trivial behavior from the expected topological robustness of non-Abelian Majorana zero modes. We also analyze a more recent nanowire experiment [P. Yu et al., Nat. Phys. 17, 482 (2021)] which is known to have a huge amount of disorder, showing that such highly disordered nanowires may produce very small above-background trivial peaks with conductance values $\sim 2e^2/h$ in a dirty system manifesting very soft superconducting gap with substantial in-gap conduction, as were already reported by several groups almost 10 years ago. Removing disorder and producing cleaner samples through materials quality improvement and better fabrication is the only way for future progress in this field.

We introduce an algorithm, the orthogonal operator polynomial expansion (OOPEX), to approximately compute expectation values in energy eigenstates at finite energy density of nonintegrable quantum many-body systems with polynomial effort, whereas exact diagonalization (ED) of the Hamiltonian $H$ is exponentially hard. The OOPEX relies on the eigenstate thermalization hypothesis, which conjectures that eigenstate expectation values of physical observables in such systems vary smoothly with the eigenstate energy (and other macroscopic conserved quantities, if any), and it computes them through a series generated by repeated multiplications, rather than diagonalization, of $H$ and whose successive terms oscillate faster with the energy. The hypothesis guarantees that only the first few terms of this series contribute appreciably. We further show that the OOPEX, in a sense, is the most optimum algorithm based on series expansions of $H$ as it avoids computing the many-body density of states, which plagues other similar algorithms. Then, we argue nonrigorously that working in the Fock space of operators, rather than that of states as is usually done, yields convergent results with computational resources that scale polynomially with $N$. We demonstrate the polynomial scaling by applying the OOPEX to the nonintegrable Ising chain and comparing with ED and high-temperature expansion (HTX) results. The OOPEX provides access to much larger $N$ than ED and HTX do, which facilitates overcoming finite-size effects that plague the other methods to extract correlation lengths in chaotic eigenstates. In addition, access to large systems allows us to test a recent conjecture that the Renyi entropy of chaotic eigenstates has positive curvature if the Renyi index is greater than 1, and we find encouraging supporting evidence.

Recently, SnSe has been found to possess a remarkable thermoelectric performance. As an isostructural compound to SnSe, SnS contains a more environmentally compatible and earth abundant element, which enables SnS a promising thermoelectric material for commercial application. In this work, we have performed systematic studies of the crystal structures and the lattice vibrations by means of neutron total scattering, Raman scattering measurements, and first-principle calculations. A structural transition has been observed by both temperature dependent neutron diffraction and pressure dependent Raman scattering. The lattice anharmonicity has been revealed by the local distortion induced by the Sn $5s^2$ lone pair and is also evidenced from the temperature dependent atomic displacement parameter of the Sn atom. By separating the intrinsic anharmonicity and lattice thermal expansion effects, we found that the former plays the primary role in the softening of the Raman active modes. Such anharmonicity has also been observed experimentally by the linewidth broadening of the high-energy optical modes and confirmed by frozen phonon calculations. Furthermore, our frozen phonon calculation reveals the presence of the quartic anharmonicity potential of those high-energy optical modes vibrating along the $b$ axis. Our data will contribute to a better understanding of the thermal conduction in SnS, which will be beneficial for the enhancement of the thermoelectric performance of this material.

We study theoretically the spin-orbit interaction of low-energy electrons in semiconducting nanowires with a zinc-blende lattice. The effective Dresselhaus term is derived for various growth directions, including $\langle 11\overline{2}\rangle$-oriented nanowires. While a specific configuration exists where the Dresselhaus spin-orbit coupling is suppressed even at confinement potentials of low symmetry, many configurations allow for a strong Dresselhaus coupling. In particular, we discuss qualitative and quantitative results for nanowire cross-sections modeled after sectors of rings or circles. The parameter dependence is analyzed in detail, enabling predictions for a large variety of setups. For example, we gain insight into the spin-orbit coupling in recently fabricated GaAs-InAs nanomembrane-nanowire structures. By combining the effective Dresselhaus and Rashba terms, we find that such structures are promising platforms for applications where an electrically controllable spin-orbit interaction is needed. If the nanowire cross-section is scaled down and InAs replaced by InSb, remarkably high Dresselhaus-based spin-orbit energies of the order of millielectronvolt are expected. A Rashba term that is similar to the effective Dresselhaus term can be induced via electric gates, providing means to switch the spin-orbit interaction on and off. By varying the central angle of the circular sector, we find, among other things, that particularly strong Dresselhaus couplings are possible, for example, when nanowire cross-sections resemble half-disks.

We study the formation of magnon-polaron excitations and the consequences of different timescales between the magnon and lattice dynamics. The spin-spin interactions along the one-dimensional lattice are ruled by a Heisenberg Hamiltonian in the anisotropic form XXZ, in which each spin exhibits a vibrational degree of freedom around its equilibrium position. By considering a magnetoelastic coupling as a linear function of the relative displacement between nearest-neighbor spins, results provide an original framework for achieving a hybridized magnon-polaron state. Such a state is characterized by high cooperation between the underlying excitations, where nondispersive profiles of the magnon wave function and the lattice deformations exhibit a bound dynamics. Traveling or stationary formation of a magnon-polaron depends on the effective magnetoelastic coupling, and the critical amount $\left({\chi }_c\right)$ separating both regimes is shown. Different characteristic timescales of the magnon and the vibrational dynamics unveiled the threshold between such behaviors, as well as a limit value of critical magnetoelastic interaction, above which the magnon velocity no longer interferes at the critical magnetoelastic coupling capable of inducing the stationary regime.

We theoretically consider Fermi surface anomalies manifesting in the temperature-dependent quasiparticle properties of two-dimensional (2D) interacting electron systems, comparing and contrasting with the corresponding three-dimensional (3D) Fermi liquid situation. In particular, employing microscopic many-body perturbative techniques, we obtain analytically the leading-order and the next-to-leading-order interaction corrections to the renormalized effective mass for three distinct physical interaction models: electron-phonon, electron-paramagnon, and electron-electron Coulomb coupling. We find that the 2D renormalized effective mass does not develop any Fermi surface anomaly due to electron-phonon interaction, manifesting $\mathcal{O}\left(T^2\right)$ temperature correction and thus remaining consistent with the Sommerfeld expansion of the noninteracting Fermi function, in contrast to the corresponding 3D situation where the temperature correction to the renormalized effective mass has the anomalous $T^{2}logT$ behavior. In contrast, both electron-paramagnon and electron-electron interactions lead to the anomalous $\mathcal{O}\left(T\right)$ corrections to the 2D effective mass renormalization in contrast to $T^{2}logT$ behavior in the corresponding 3D interacting systems. We provide detailed analytical results, and comment on the conditions under which a $T^{2}logT$ term could possibly arise in the 2D quasiparticle effective mass from electron-phonon interactions. We also compare results for the temperature-dependent specific heat in the interacting 2D and 3D Fermi systems, using the close connection between the effective mass and specific heat.

The understanding of complex quantum many-body systems has been vastly boosted by tensor network (TN) methods. Among others, excitation spectrum and long-range interacting systems can be studied using TNs, where one however confronts the intricate summation over an extensive number of tensor diagrams. Here, we introduce a set of generating functions, which encode the diagrammatic summations as leading-order series expansion coefficients. Combined with automatic differentiation, the generating function allows us to solve the problem of TN diagrammatic summation. We illustrate this scheme by computing variational excited states and the dynamical structure factor of a quantum spin chain, and further investigating entanglement properties of excited states. Extensions to infinite-size systems and higher dimension are outlined.

Spatiotemporal spin noise spectroscopy is combined with dynamic light scattering in order to reach spatial resolutions down to $\sim \lambda /10$. Applied to a system of localized electron spins, an insulating n-doped CdTe layer, this allows us to reveal long spin jump distances $\ell \sim 2.7\mu \mathrm{m}$. Spin noise spectra at large wave vectors $q$ ($q\ell \gg 1$) provide a snapshot of the spin dynamics before jump (therefore not affected by spin motion), while at smaller $q$, spin motion sets in. This allows us to unravel the contributions of spin-orbit and hyperfine fields in the electron spin relaxation and to determine self-consistently all parameters relevant to the spin dynamics. We propose a phenomenological equation inspired by studies of atomic jump diffusion by neutron scattering, which includes the relevant spin relaxation mechanisms and the effect of time of flight of the spin fluctuation across the laser spot. This modeling reproduces all experimental results.

We investigate the dynamics of a charge qubit consisting of two Coulomb-blockaded islands hosting Majorana zero modes. The frequency of the qubit is determined by coherent single-electron tunneling between two islands originating from the hybridization of two Majorana zero modes localized at the junction. We calculate the sequential tunneling current $I$ through the double-island device coupled to normal metal leads. We demonstrate that the $I\text{−}V$ characteristics in the large-bias regime can be used as a measurement of the coherent Majorana coupling $E_M$. We propose dc and ac transport experiments for measuring $E_M$, and discuss their limitations due to intrinsic dephasing mechanisms such as excited quasiparticles.

We perform Floquet spectroscopy in a GaAs double quantum dot system coupled to a high-impedance superconducting resonator. By applying microwave-induced consecutive passages under a double-resonance condition, we observe Landau-Zener-Stückelberg-Majorana interference patterns that stem from a cavity-assisted interference pattern modified by the depletion of the ground state. Our experimental results reveal the stationary state behavior of a strongly driven two-level system and are consistent with the simulations based on our theoretical model. This study provides an excellent platform for investigating the dynamics of Floquet states in the presence of strong driving.

We propose and analyze a physical system capable of performing topological quantum computation with Majorana zero modes (MZMs) in a one-dimensional topological superconductor (1DTS). One of the leading methods to realize quantum gates in a 1DTS is to use T junctions, which allows one to maneuver MZMs in such a manner as to achieve braiding. In this paper, we propose a scheme for implementing quantum logical gates in a purely one-dimensional geometry without T junctions, instead replacing it with an auxiliary qubit. This has the additional benefit of introducing a non-Clifford gate, corresponding to one- and two-logical-qubit $Z$ rotations. We first design a topologically protected logical $Z$ gate based entirely on local interactions within the 1DTS. Using an auxiliary qubit coupled to the topological superconductors, we extend the $Z$ gate to non-Clifford single- and multiqubit arbitrary rotations with partial topological protection. Finally, to perform universal quantum computing, we introduce a scheme for performing arbitrary unitary rotations, albeit without topological protection. We develop a formalism based on unitary braids which creates transitions between different topological phases of the 1DTS system. The unitary formalism can be simply converted to an equivalent adiabatic scheme, which we numerically simulate, and we show that high-fidelity operations should be possible with reasonable parameters.

The list of semiconductor materials with spectroscopically fingerprinted self-interstitials is very short. The M center in 4H-SiC, a bistable defect responsible for a family of electron traps, has been deprived of a model which could unveil its real importance for almost two decades. Using advanced first-principles calculations and junction spectroscopy, we demonstrate that the properties of M, including bistability, annealing, reconfiguration kinetics, and electronic levels match those of the carbon self-interstitial.

We demonstrate mechanisms of reciprocity breaking in nonlinear optics driven by the toroidal dipole moment which characterizes nontrivial spatial distributions of spins in solids. Using high-resolution femtosecond spectroscopy at electronic resonances in the magnetoelectric antiferromagnet $\mathrm{Cu}{\mathrm{B}}_2{\mathrm{O}}_4$, we show that nonreciprocity reaches 100% for opposite magnetic fields due to the interference of nonlinear coherent sources of second harmonic generation originating from the toroidal dipole moment, applied magnetic field, and noncentrosymmetric crystal structure. The experimental results are corroborated by theoretical analysis based on the crystal and magnetic symmetry of $\mathrm{Cu}{\mathrm{B}}_2{\mathrm{O}}_4$. Our findings open degrees of freedom in nonlinear optics and pave the way for future nonreciprocal spin-optronic devices operating on the femtosecond timescale.

The stacking of different two-dimensional (2D) materials provides a promising approach to realize new states of quantum matter. In this combined scanning tunneling microscopy (STM) and density functional theory (DFT) study we show that the structure in weakly bound, purely van der Waals (vdW) interacting systems is strongly influenced by screening and relaxation. We studied in detail the physisorption of lead phthalocyanine (PbPc) molecules on epitaxial monolayer graphene on SiC(0001) as well as on highly ordered pyrolytic graphite (HOPG), resembling truly 2D and anisotropic, semi-infinite three-dimensional (3D) supports. Our analysis demonstrates that the different deformation ability of the vdW coupled systems, i.e., their actual thickness and buckling, triggers the molecular morphology and exhibits a proximity coupled band structure. It thus provides important implications for future 2D design concepts.

Giant magnetostriction in insulating magnetic materials is highly required for applications but is rarely observed. Here we show that giant magnetostriction $\left(>1500\mathrm{ppm}\right)$ can be achieved in an insulating transition metal oxide $\mathrm{CaBa}{\mathrm{Co}}_4{\mathrm{O}}_7$ where the ferrimagnetic ordering at $T_{\mathrm{C}}\sim 62\mathrm{K}$ is associated with a huge change in the lattice. Moreover, because this material is pyroelectric with a nonswitchable electric polarization ($P$), the giant magnetostriction results in a pronounced magnetoelectric effect—a huge change of electric polarization $(\mathrm{\Delta }P\sim 1.6\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2)$ in response to the applied magnetic field up to 60 T. Geometric frustration as well as the orbital instability of ${\mathrm{Co}}^{2+}/{\mathrm{Co}}^{3+}$ ions is believed to play a crucial role in the giant magnetostriction. Our study provides insights on how to achieve both giant magnetostriction and pronounced magnetoelectric effect in insulating transition metal oxides.

Structural parameters in quasi-one-dimensional organic conductors ${\left(\mathrm{TMTTF}\right)}_{2}X$ ($X=\mathrm{Nb}{\mathrm{F}}_6, \mathrm{As}{\mathrm{F}}_6, \mathrm{P}{\mathrm{F}}_6$, and Br) are investigated by synchrotron x-ray diffraction. The temperature dependences of the crystal structures differ significantly between octahedral and monatomic anion systems, corresponding to the presence or absence of a charge-ordering transition. Changes in temperature and chemical pressure control the dimensionality of the lattice and the degree of dimerization. Furthermore, the antiferromagnetic and spin-Peierls ground states in this system depend on the spin frustration due to transfer integral paths between the one-dimensional chain molecules.

In this work, we present bilayer flat-band Hamiltonians, in which all bulk states are localized and specified by extensive local integrals of motion (LIOMs). The present systems are bilayer extension of the Creutz ladder, which is studied previously. In order to construct models, we employ building blocks, cube operators, which are linear combinations of fermions defined in each cube of the bilayer lattice. There are eight cubic operators, and the Hamiltonians are composed of the number operators of them, the LIOMs. A suitable arrangement of locations of the cube operators is needed to have exact projective Hamiltonians. The projective Hamiltonians belong to a topological classification class, the BDI class. With the open boundary condition, the constructed Hamiltonians have gapless edge modes, which commute with each other as well as the Hamiltonian. This result comes from a symmetry analogous to the one-dimensional chiral symmetry of the BDI class. These results indicate that the projective Hamiltonians describe a kind of symmetry-protected topological phase matter. Careful investigation of topological indexes, such as the Berry phase and string operator, is given. We also show that, by using the gapless edge modes, a generalized Sachdev-Ye-Kitaev model is constructed.