The anomalous ground-state phase shift in superconductor/antiferromagnet/superconductor Josephson junctions in the presence of the Rashba spin-orbit coupling is predicted and numerically investigated. It is found to be a consequence of the uncompensated magnetic moment at the superconductor/antiferromagnet interfaces. The anomalous phase shift exhibits a strong dependence on the value of the spin-orbit coupling and the sublattice magnetization with the simultaneous existence of stable and metastable branches. It depends strongly on the orientation of the Néel vector with respect to the superconductor/antiferromagnet interfaces via the dependence on the orientation of the interface uncompensated magnetic moment. This effect opens a way to control the Néel vector by supercurrent in Josephson systems.

We develop a formalism to study the resonant inelastic x-ray scattering (RIXS) response in metals based on the diagrammatic expansion for its cross section. The standard approach to the solution of the RIXS problem relies on two key approximations: short-range potentials and noninteracting conduction electrons. However, these approximations are inaccurate for charged particles in metals, where the long-range Coulomb interaction and dynamic screening effects are very important. In this work, we study how to extract important information about collective excitations in the Coulomb plasma, plasmons, and electron-hole pairs from RIXS data. We find that single- and multi-plasmon excitations can easily be distinguished by positions of the corresponding peaks, singularities, and their intensities. We also discuss the hybrid processes, where plasmon emission is accompanied by excitation of electron-hole pairs and study how they manifest themselves.

The efficient validation of quantum devices is critical for emerging technological applications. In a wide class of use cases the precise engineering of a Hamiltonian is required both for the implementation of gate-based quantum information processing as well as for reliable quantum memories. Inferring the experimentally realized Hamiltonian through a scalable number of measurements constitutes the challenging task of Hamiltonian learning. In particular, assessing the quality of the implementation of topological codes is essential for quantum error correction. Here, we introduce a neural-net-based approach to this challenge. We capitalize on a family of exactly solvable models to train our algorithm and generalize to a broad class of experimentally relevant sources of errors. We discuss how our algorithm scales with system size and analyze its resilience toward various noise sources.

We propose a method to probe and control the interactions between an ensemble of magnetic impurities in a superconductor via microwave radiation. Our method relies upon the presence of subgap Yu-Shiba-Rusinov (YSR) states associated with the impurities. Depending on the sign of the detuning, radiation generates either a ferro- or antiferromagnetic contribution to the exchange interaction. This contribution can bias the statistics of the random exchange constants stemming from the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. Moreover, by measuring the microwave response at the YSR resonance, one gains information about the magnetic order of the impurities. To this end, we estimate the absorption coefficient as well as the achievable strength of the microwave-induced YSR interactions using off-resonant radiation. The ability to utilize microwave fields to both probe and control impurity spins in a superconducting host may open new paths to studying metallic spin glasses.

We consider a system of weakly coupled Rashba nanowires in the strong spin-orbit interaction (SOI) regime. The nanowires are arranged into two tunnel-coupled layers proximitized by a top and bottom superconductor such that the superconducting phase difference between them is $\pi$. We show that in such a system strong electron-electron interactions can stabilize a helical topological superconducting phase hosting Kramers partners of ${\mathbb{Z}}_{2m}$ parafermion edge modes, where $m$ is an odd integer determined by the position of the chemical potential. Furthermore, upon turning on a weak in-plane magnetic field, the system is driven into a second-order topological superconducting phase hosting zero-energy ${\mathbb{Z}}_{2m}$ parafermion bound states localized at two opposite corners of a rectangular sample. As a special case, zero-energy Majorana corner states emerge in the noninteracting limit $m=1$, where the chemical potential is tuned to the SOI energy of the single nanowires.

We investigate supercurrent interference patterns measured as a function of magnetic field in ballistic graphene Josephson junctions. At high doping, the expected ${\mathrm{\Phi }}_0$-periodic “Fraunhofer” pattern is observed, indicating a uniform current distribution. Close to the Dirac point, we find anomalous interferences that are close to a $2{\mathrm{\Phi }}_0$ periodicity, similar to that predicted for topological Andreev bound states carrying a charge of $e$ instead of $2e$. This feature persists with increasing temperature, ruling out a nonsinusoidal current-phase relationship. It also persists in junctions in which sharp vacuum edges are eliminated. Our results indicate that the observed behavior may originate from an intrinsic property of ballistic graphene Josephson junctions, though the exact mechanism remains unclear.

The proper description of step structures in the exchange correlation potential, of charge localization, and a reasonable prediction of band gaps have been long-standing, serious challenges for semilocal density functionals. In practice, obtaining all of these properties from the functional derivative of an energy functional was possible only at the price of incorporating exact exchange. We here show that they can be achieved at significantly lower, semilocal computational expense by using kinetic-energy density-dependent functionals. The key to obtaining these features is a functional construction strategy that focuses on the derivative discontinuity and the density response.

Magnetization, magnetocaloric, calorimetric, neutron- and x-ray-diffraction, and inelastic-neutron-scattering measurements are performed on single crystals of $\mathrm{BaCdVO}{\left({\mathrm{PO}}_4\right)}_2$. The low-temperature crystal structure is found to be of a lower symmetry than previously assumed. The result is a more complicated model spin Hamiltonian, which we infer from measurements of the spin wave dispersion spectrum. The main finding is a spin state which emerges in high magnetic fields after antiferromagnetic order is terminated at $H_{c1}\simeq 4.0\mathrm{T}$. It is a distinct thermodynamic phase with a well-defined phase boundary at $H_{c2}\simeq 6.5\mathrm{T}$ and is clearly separate from the fully saturated phase, yet it shows no conventional (dipolar) magnetic long-range order. We argue that it is fully consistent with the expectations for a quantum bond-nematic state.

We present a far-infrared magneto-optical study of the gapped nodal-line semimetal ZrSiS in magnetic fields $B$ up to 7 T. The observed field-dependent features, which represent intra- (cyclotron resonance) and interband transitions, develop as $\sqrt{B}$ in increasing field and can be consistently explained within a simple two-dimensional Dirac band model with a gap of 26 meV and an averaged Fermi velocity of $3\times {10}^5$ m/s. This indicates a rather narrow distribution of these parameters along the in-plane portions of the nodal line in the Brillouin zone. A field-induced feature with an energy position that does not depend on $B$ is also detected in the spectra. Possible origins of this feature are discussed.

Band engineering in twisted bilayers of the five generic two-dimensional Bravais networks is demonstrated. We first derive constraints on the interlayer coupling that determines an effective potential for low-energy physics. The constraints are originated from symmetry and fixed without reference to microscopic details. Having the constraints on the potential helps us to predict and understand electron behavior under the influence of the moiré structure, like formation of flat bands. It is also pointed out that the generic constraints become most relevant when the typical length scale of the microscopic interlayer coupling is moderate. The concepts are numerically tested in simple tight-binding models, showing the band flattening compatible with the potential profile fixed by the generic constraints. On the basis of the generic theory, we propose a possibility of anisotropic band flattening, in which quasi-one-dimensional band dispersion is generated from relatively isotropic original band dispersion. In the strongly correlated regime, the anisotropic band flattening leads to a spin-orbital model where intertwined magnetic and orbital ordering can give rise to rich physics. Overall, our theory and demonstration give useful information for screening out interesting materials from the material database.

We study two-dimensional spinful insulating phases of matter that are protected by time-reversal and crystalline symmetries. To characterize these phases, we employ the concept of corner charge fractionalization: corners can carry charges that are fractions of even multiples of the electric charge. The charges are quantized and topologically stable as long as all symmetries are preserved. We classify the different corner charge configurations for all point groups, and match them with the corresponding bulk topology. For this we employ symmetry indicators and (nested) Wilson loop invariants. We provide formulas that allow for a convenient calculation of the corner charge from Bloch wave functions and illustrate our results using the example of arsenic and antimony monolayers. Depending on the degree of structural buckling, these materials can exhibit two distinct obstructed atomic limits. We present density functional theory calculations for open flakes to support our findings.

Twisted bilayer graphene (TBLG) has emerged as an important platform for studying correlated phenomena, including unconventional superconductivity, in two-dimensional systems. The complexity of the atomic-scale structures in TBLG has made even the study of single-particle physics at low energies around the Fermi level, quite challenging. Our goal here is to provide a convenient and physically motivated picture of single-particle physics in TBLG using reduced models with the smallest possible number of localized orbitals. The reduced models exactly reproduce the low-energy bands of ab initio tight-binding models, including the effects of atomic relaxations. Furthermore, we obtain for the first time the corresponding Wannier orbitals that incorporate all symmetries of TBLG, which are also calculated as a function of angle, a requisite first step towards incorporating electron interaction effects. We construct eight-band and five-band models for the low-energy states for twist angles between $1.3^{\circ }$ and $0.6^{\circ }$. The models are created using a multistep Wannier projection technique starting with appropriate ab initio $\mathit{k}·\mathit{p}$ continuum Hamiltonians. Our procedure can also readily capture the perturbative effects of substrates and external displacement fields while offering a significant reduction in complexity for studying electron-electron correlation phenomena in realistic situations.

We formulate a quantum theory of vorticity (hydro)dynamics on a general two-dimensional bosonic lattice. In the classical limit of a bosonic condensate, it reduces to conserved plasma-like vortex-antivortex dynamics. The nonlocal topological character of the vorticity flows is reflected in the bulk-edge correspondence dictated by the Stokes theorem. This is exploited to establish physical boundary conditions that realize, in the coarse-grained thermodynamic limit, an effective chemical-potential bias of vorticity. A Kubo formula is derived for the vorticity conductivity, which could be measured in a suggested practical device, in terms of quantum vorticity-flux correlators of the original lattice model. As an illustrative example, we discuss the superfluidity of vorticity, exploiting the particle-vortex duality at a bosonic superfluid-insulator transition.

We study the single-particle spectrum of three-dimensional Weyl semimetals taking into account electron-phonon interactions that are the result of straining the material. We find that a well-defined fermionic excitation appears in addition to the standard peak corresponding to quasiparticle states as suggested by Landau-Fermi liquid theory. Contrary to the case of Dirac systems interacting via the Coulomb interaction, these satellite peaks appear even at lowest order in perturbation theory. The new excitations are anisotropic, as opposed to the single-particle spectrum, and their behavior is dictated by the Debye frequency, which naturally regulates the electron-phonon coupling.

It is well known that an interface created by two topologically distinct structures could host nontrivial edge states that are immune to defects. In this paper, we introduce a one-dimensional space-time phononic crystal and study the associated anomalous topological edge states when the phononic crystal's density is modulated both in space and time. While preserving the key topological feature of the system, the time modulation also duplicates the edge state mode across the spectrum, both inside and outside the band gap. It is shown that, in contrast to conventional topological edge states which are excited by frequencies in the Bragg regime, the time-modulation-induced frequency conversion can be leveraged to access topological edge states at a deep subwavelength scale where the entire phononic crystal size can be as small as 1/5.1 of the wavelength. This feature could open another route for designing miniature devices that are based on topological physics.

Generic quantum many-body systems typically show a linear growth of the entanglement entropy after a quench from a product state. While entanglement is a property of the wave function, it is generated by the unitary time-evolution operator and is therefore reflected in its increasing complexity as quantified by the operator entanglement entropy. Using numerical simulations of a static and a periodically driven quantum spin chain, we show that there is a robust correspondence between the entanglement entropy growth of typical product states and the operator entanglement entropy of the unitary evolution operator, while special product states, e.g., ${\sigma }_z$ basis states, can exhibit faster entanglement production. In the presence of a disordered magnetic field in our spin chains, we show that both the wave function and operator entanglement entropies exhibit a power-law growth with the same disorder-dependent exponent and clarify the apparent discrepancy in previous results. These systems, in the absence of conserved densities, provide further evidence for slow information spreading on the ergodic side of the many-body localization transition.

We consider a Floquet triple-layer setup composed of a two-dimensional electron gas with spin-orbit interactions, proximity coupled to an $s$-wave superconductor and to a ferromagnet driven at resonance. The ferromagnetic layer generates a time-oscillating Zeeman field which competes with the induced superconducting gap and leads to a topological phase transition. The resulting Floquet states support a second-order topological superconducting phase with a pair of localized zero-energy Floquet Majorana corner states. Moreover, the phase diagram comprises a Floquet helical topological superconductor, hosting a Kramers pair of Majorana edge modes protected by an effective time-reversal symmetry, as well as a gapless Floquet Weyl phase. The topological phases are stable against disorder as well as against parameter variations and are within experimental reach.

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 ${\mathbb{Z}}_2$ 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 ${\mathbb{Z}}_2$ 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 ${\mathbb{Z}}_2$ 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.

We consider an autonomous implementation of Maxwell's demon in a quantum dot architecture acting on a system without changing its number of particles or its energy. As in the original thought experiment, only the second law of thermodynamics is seemingly violated when disregarding the demon. The autonomous architecture allows us to compare descriptions in terms of information to a more traditional, thermoelectric characterization. Our detailed investigation of information-to-work conversion is based on fluctuation relations and second-law-like inequalities in addition to the average heat and charge currents. By introducing a time reversal on the level of individual electrons, we find a fluctuation relation that is not connected to any symmetry of the moment-generating function of heat and particle flows. Furthermore, we show how an effective Markovian master equation with broken detailed balance for the system alone can emerge from a full description, allowing for an investigation of the entropic cost associated with breaking detailed balance. Interestingly, while the entropic cost of performing a perfect measurement diverges, the entropic cost of breaking detailed balance does not. Our results connect various approaches and idealized scenarios found in the literature and can be tested experimentally with present-day technology.

We report on the magnetocaloric effect and ultrasound studies of the frustrated quasi-one-dimensional spin-1/2 compound ${\mathrm{LiCuVO}}_4$, evidencing a spin-nematic state. The magnetic Grüneisen parameter diverges at the transition to the spin-nematic phase, ${\mu }_0{H}_{\mathrm{c}3}\approx 40$ T, showing quantum criticality accompanied by entropy accumulation. The observed high-field anomalies in the acoustic properties clearly evidence a strong involvement of the lattice in the spin dynamics. The theoretical approach, based on exchange-striction coupling with dipolar and quadrupolar contributions, suggests that the spin-dipole-strain and quadrupole-strain interactions govern the spin-nematicity in ${\mathrm{LiCuVO}}_4$.