Boundary obstructed topological insulators are an unusual class of higher-order topological insulators with topological characteristics determined by the so-called Wannier bands. Boundary obstructed phases can harbor hinge/corner modes, but these modes can often be destabilized by a phase transition on the boundary instead of the bulk. While there has been much work on the stability of topological insulators in the presence of disorder, the topology of a disordered Wannier band, and disorder-induced Wannier transitions have not been extensively studied. In this Rapid Communication, we focus on the simplest example of a Wannier topological insulator: a mirror-symmetric $\pi$-flux ladder in one dimension. We find that the Wannier topology is robust to disorder, and derive a real-space renormalization group procedure to understand a different type of strong disorder-induced transition between nontrivial and trivial Wannier topological phases. We also establish a connection between the Wannier topology of the ladder and the energy band topology of a related system with a physical boundary cut, something which has generally been conjectured for clean models, but still needs to be studied in the presence of disorder.

We show that spontaneous strain and magnetization can arise in the doped topological insulators with a two-component superconducting vector order parameter. The details of the effects depend on the symmetry of the order parameter, whether it is nematic or chiral. The transition from the nematic state to the chiral one can be performed by application of a magnetic field, while the transition from the chiral state to the nematic is tuned by the external strain. These transitions are associated with a jump of the magnetic susceptibility and mechanical stiffness. Possible experimental observations of the predicted effects are discussed.

We show that a supercurrent carried by spinless singlet Cooper pairs can induce a spin accumulation in the normal metal interlayer of a Josephson junction. This phenomenon occurs when a nonequilibrium spin-energy mode is excited in the normal metal, for instance, by an applied temperature gradient between ferromagnetic electrodes. Without supercurrent, the spin accumulation vanishes in the Josephson junction. With supercurrent, a spatially antisymmetric spin accumulation is generated that can be measured by tunneling to a polarized detector electrode or by a nano-superconducting quantum interference device. We explain the physical origin of the induced spin accumulation by the combined effect of a Doppler shift induced by a flow of singlet Cooper pairs, and the spin-energy mode excited in the normal metal. This effect shows that spin control is possible even with singlet Cooper pairs in conventional superconductors, a finding which could open interesting perspectives in superconducting spintronics.

It has been commonly accepted that a magnetic field suppresses superconductivity by inducing the ordered motion of Cooper pairs. We demonstrate that a magnetic field can instead provide a generation of superconducting correlations by inducing the motion of a superconducting condensate. This effect arises in superconductor/ferromagnet heterostructures in the presence of Rashba spin-orbital coupling. We predict the odd-frequency spin-triplet superconducting correlations called the Berezinskii order to be switched on at large distances from the superconductor/ferromagnet interface by the application of a magnetic field. This is shown to result in the unusual behavior of Josephson effect and local density of states in superconductor/ferromagnet structures.

The oxygen octahedral rotations in ${\mathrm{Ca}}_2\mathrm{Mn}{\mathrm{O}}_4$, the first member of the $\mathrm{Ca}\mathrm{O}{\left({\mathrm{CaMnO}}_3\right)}_n$ Ruddlesden-Popper family, is probed through a set of complementary techniques, including temperature-dependent neutron and x-ray diffraction, combined with local probe studies and ab initio calculations. Here we demonstrate the enhancement of the uniaxial negative thermal expansion coefficient from $-1.26\pm 0.25$ to $-21\pm 1.8$ ppm/K at the second order $I{4}_1/acd$ to $I4/mmm$ structural phase transition, providing direct evidence for the corkscrew atomic mechanism. We establish, also, that the predicted $I4/mmm$ high symmetry is attained around 1050 K. At lower temperatures, within the 10–1000 K temperature range, our first-principles calculations and detailed analysis of the Ca local environment reveals that the reported $Aba2$ structural phase, coexisting with the $I{4}_1/acd$ one, cannot describe correctly this compound. On the other hand, our data allow for the coexistence of the locally identical $I{4}_1/acd$ and $Acam$ structural phases.

Recent studies have demonstrated that the polarization states of high harmonics from solids can differ from those of the driving pulses. To gain insights on the microscopic origin of this behavior, we perform one-particle intraband-only calculations and reproduce some of the most striking observations. For instance, our calculations yield circularly polarized harmonics from elliptically polarized pulses that sensitively depend on the driving conditions. Furthermore, we perform experiments on ZnS and find characteristics partly similar to those reported from silicon. Comparison to our intraband-only calculations shows reasonable qualitative agreement for a below-band-gap harmonic. We show that intraband dynamics predict depolarization effects that gain significance with higher field strengths and we observe such effects in the experimental data. For harmonics above the band gap, interband dynamics become important and the high-harmonic response to elliptical excitation looks systematically different. Our work proposes a method to distinguish between different high-harmonic generation mechanisms and it could pave the way to compact solid-state high-harmonic sources with controllable polarization states.

Unlike a real magnetic field, which separates the energy levels of a particle with opposite spin polarization, a complex field can lead to a special kind of spectral degeneracy, known as exceptional point (EP), at which two spin eigenmodes coalesce. It allows an EP impurity to be an invisible scattering center for a fermion with the resonant spin polarization, but an amplifying emitter for opposite polarization. We show that a pair of conjugate EP modes supports resonant mutual stimulation, acting as a resonant amplifier based on the underlying mechanism of a positive-feedback loop. Together with other Hermitian eigenmodes, a fermion with EP polarization exhibits some exclusive dynamics, referred to as EP dynamics. We construct several typical superlattices, which are built up by embedding EP-impurity arrays into a Hermitian two-dimensional square lattice. Numerical simulations are performed to demonstrate resonant amplification and invisibility of Bragg scattering.

We report magnetization, heat capacity, thermal expansion, and magnetostriction measurements down to millikelvin temperatures on the triangular antiferromagnet $\mathrm{Yb}\mathrm{Mg}\mathrm{Ga}{\mathrm{O}}_4$. Our data exclude the formation of the distinct $\frac{1}{3}$ plateau phase observed in other triangular antiferromagnets, but reveal plateaulike features in second derivatives of the free energy, magnetic susceptibility, and specific heat, at ${\mu }_{0}H=1.0–2.5$ T for $H\parallel c$ and $2–5$ T for $H\perp c$. Using Monte Carlo simulations of a realistic spin Hamiltonian, we ascribe these features to nonmonotonic changes in the magnetization and the $\frac{1}{2}$ plateau that is smeared out by the random distribution of exchange couplings in $\mathrm{Yb}\mathrm{Mg}\mathrm{Ga}{\mathrm{O}}_4$.

Underdoped PrFeAs(O,F), one of the lesser known members of the 1111 family of iron-based superconductors, was investigated in detail by means of transport, magnetometry, nuclear magnetic resonance (NMR) measurements, and point-contact Andreev-reflection spectroscopy (PCARS). PCARS measurements on single crystals evidence the multigap nature of PrFeAs(O,F) superconductivity, shown to host at least two isotropic gaps, clearly discernible in the spectra, irrespective of the direction of current injection (i.e., along the $ab$ planes or along the $c$ axis). Additional features at higher energy can be interpreted as signatures of a strong electron-boson coupling, as demonstrated by a model which combines Andreev reflection with the Eliashberg theory. Magnetic resonance measurements in the normal phase indicate the lack of a magnetic order in underdoped PrFeAs(O,F), while ${}^{75}\mathrm{As}$ NMR spin-lattice relaxation results suggest the presence of significant electronic spin fluctuations, peaking above $T_c$ and expected to mediate the superconducting pairing.

We investigate out-of-equilibrium dynamics in an excitonic insulator (EI) with a finite-momentum pairing perturbed by a laser-pulse excitation and a sudden coupling to fermionic baths. The transient dynamics of the excitonic order parameter is resolved using the full nonequilibrium Green's function approach and the generalized Kadanoff-Baym ansatz (GKBA) within the second-order Born approximation. The comparison between the two approaches after a laser-pulse excitation shows a good agreement in the weak and the intermediate photodoping regime. In contrast, the laser-pulse dynamics resolved by the GKBA does not show a complete melting of the excitonic order after a strong excitation. Instead we observe persistent oscillations of the excitonic order parameter with a predominant frequency given by the renormalized equilibrium band gap. This anomalous behavior can be overcome within the GKBA formalism by coupling to an external bath, which leads to a transition of the EI system toward the normal state. We analyze the long-time evolution of the system and distinguish decay timescales related to dephasing and thermalization.

We report on the observation of a nonsaturating, extremely large magnetoresistance (XMR) and the Fermi surface topology of a high quality $\mathrm{W}{\mathrm{Si}}_2$ single crystal grown by the Czochralski method. The magnetoresistance at $T=2\mathrm{K}$ reaches a value $\approx {10}^5\%$ in 14 T magnetic field with no sign of saturation. The Hall resistivity data of $\mathrm{W}{\mathrm{Si}}_2$ was found to be highly anisotropic. The analysis of magnetoconductivity data of $\mathrm{W}{\mathrm{Si}}_2$ revealed a near compensation of charge carrier with relatively low carrier density as compared to that of a normal metal. The observed anisotropic Hall resistivity in $\mathrm{W}{\mathrm{Si}}_2$ is due to the presence of multiple bands and Fermi pockets responsible for the transport phenomena in it. The extremely large carrier mobility and near compensation of charge carriers are responsible for the nonsaturating XMR behavior in $\mathrm{W}{\mathrm{Si}}_2$ crystal. The band structure calculation and de Haas-van Alphen effect measurement depict a cylindrical Fermi surface from which the associated quantum parameters have been obtained. The magnetotransport data of $\mathrm{W}{\mathrm{Si}}_2$ along both the crystallographic directions follows the universal temperature-field triangular phase diagram as observed in other materials exhibiting XMR behavior.

We present a theory of charge density wave (CDW) states in Weyl semimetals and their interplay with the chiral anomaly. In particular, we demonstrate a special nature of the shortest-period CDW state, which is obtained when the separation between the Weyl nodes equals exactly half a primitive reciprocal lattice vector. Its topological properties are shown to be distinct from all other Weyl CDW states. We make a connection between this observation and the three-dimensional fractional quantum Hall state, which was recently proposed to exist in magnetic Weyl semimetals.

Mott insulators with strong spin-orbit coupling display a strongly anisotropic response to applied magnetic fields. This applies in particular to Kitaev materials, with $\alpha -{\mathrm{RuCl}}_3$ and ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ representing two important examples. Both show a magnetically ordered zigzag state at low temperatures, and considerable effort has been devoted to properly modeling these systems in order to identify routes towards realizing a quantum spin liquid. Here, we investigate the relevant Heisenberg-Kitaev-$\mathrm{\Gamma }$ model primarily at elevated temperatures, focusing on the characteristic anisotropy between the in-plane and out-of-plane uniform susceptibility. For $\alpha -{\mathrm{RuCl}}_3$, we find that the experimentally observed anisotropy, including its temperature dependence, can be reproduced by combining a large off-diagonal ${\mathrm{\Gamma }}_1$ coupling with a moderate $g$-factor anisotropy. Moreover, we study in detail the effect of magnetic dilution and provide predictions for the doping evolution of the temperature-dependent susceptibilities.

A self-consistent theory for the classical description of the interaction of light and matter at the nanoscale is presented, which takes into account spatial dispersion. Up to now, the Maxwell equations in nanostructured materials with spatial dispersion have been solved by the introduction of the so-called additional boundary conditions which, however, lack generality and uniqueness. In this paper, we derive an approach where nonlocal effects are studied in a precise and uniquely defined way, thus allowing the treatment of all solid-solid interfaces (among metals, semiconductors or insulators), as well as solid-vacuum interfaces in the same framework. The theory is based on the derivation of a potential energy for an ensemble of electrons in a given potential, where the deformation of the ensemble is treated as in a solid, including both shear and compressional deformations, instead of a fluid described only by a bulk compressibility like in the hydrodynamical approach. The derived classical equation of motion for the ensemble describes the deformation vector and the corresponding polarization vector as an elastodynamic field, including viscous forces, from which a generalized nonlocal constitutive equation for the dielectric constant is derived. The required boundary conditions are identical to that of elastodynamics and they emerge in a natural way, without any physical hypothesis outside the current description, as is commonly required in other nonlocal approaches. Interestingly, this description does not require the discontinuity of any component of the electric, magnetic, or polarization fields and, consequently, no bounded currents or charges are present at the interface, which is a more suitable description from the microscopic point of view. It is shown that the method converges to the local boundary conditions in the low spatial dispersion limit for insulators and conductors, quantified by means of a parameter defined as the characteristic length. A brief discussion about the inclusion of the spill out of electrons across surfaces is discussed. Finally, the planar interface is studied and numerical examples of the behavior of the different fields at the interfaces are presented, showing the limiting situations in which the local limit is recovered, reinforcing the self-consistency of this description.

In most studies on all-optical diodes, spatial asymmetry has been by necessity applied to break Lorentz reciprocity. Here we suggest a paradigm for optically induced nonreciprocity in semiconductor wires which are spatially asymmetry-free and provide a very simple and efficient platform for plasmonic devices. An azimuthal magnetic field induced by a plasmonic pump in the semiconductor wire alters the material parameters and thus results in a cross-nonlinear modulation of the plasmonic signal. Peculiarly, the nonlinear wave-number shift has opposite signs for forward and backward signals whereas Kerr or Kerr-like nonlinearity does not break Lorentz reciprocity in spatially symmetric structures. This principle may open an avenue toward highly integrated all-optical nonreciprocal devices.

A Datta-Das spin field-effect transistor is built of a heterostructure with a Rashba spin-orbit interaction (SOI) at the interface (or quantum well) separating two possibly magnetized reservoirs. The particle and spin currents between the two reservoirs are driven by chemical potentials that are (possibly) different for each spin direction. These currents are also tuned by varying the strength of the SOI, which changes the amount of the rotation of the spins of electrons crossing the heterostructure. Here we investigate the dependence of these currents on additional Zeeman fields on the heterostructure and on variations of the reservoir magnetizations. In contrast to the particle current, the spin currents are not necessarily conserved; an additional spin polarization is injected into the reservoirs. If a reservoir has a finite (equilibrium) magnetization, then we surprisingly find that the spin current into that reservoir can only have spins which are parallel to the reservoir magnetization, independent of all the other fields. This spin current can be enhanced by increasing the magnetization of the other reservoir, and can also be tuned by the SOI and the various magnetic fields. When only one reservoir is magnetized then the spin current into the other reservoir has arbitrary tunable size and direction. In particular, this spin current changes as the magnetization of the other reservoir is rotated. The optimal conditions for accumulating spin polarization on an unpolarized reservoir are to either apply a Zeeman field in addition to the SOI, or to polarize the other reservoir.

We use a fully self-consistent laterally resolved Hartree-Fock approximation for numerically addressing the electron configurations at higher Landau levels in the quantum Hall regime for near-macroscopic sample sizes. At low disorder we find spatially resolved, stripe- and bubblelike charge-density modulations and show how these emerge depending on the filling factor. The microscopic details of these boundary regions determine the geometrical boundary conditions for aligning the charge-density modulation either as stripes or bubbles. Transport is modeled using a nonequilibrium network model giving a pronounced anisotropy in the direction of the injected current in the stripe regime close to half filling. We obtain a stripe period of 2.9 cyclotron radii. Our results provide an intuitive understanding of its consequences in strong magnetic fields and indicate the dominance of many particle physics in the integer quantum Hall regime.

Chirality manifests in various forms in nature. However, there is no evidence of the chirality in one-dimensional charge density wave (CDW) systems. Here, we have explored the chirality among quasi-one-dimensional CDW ground states with the aid of scanning tunneling microscopy, symmetry analysis, and density functional theory calculations. We discovered three distinct chiralities emerging in the form of two-dimensional chiral stacking orders composed of degenerate CDW ground states: right-, left-, and nonchiral stacking orders. Such chiral stacking orders correspond to newly introduced chiral winding numbers. Furthermore, we observed that these chiral stacking orders are intertwined with chiral vortices and chiral domain walls, which play a crucial role in engineering the chiral stacking orders. Our findings suggest that the unexpected chiral stacking orders can open a way to investigate the chirality in CDW systems, which can lead to diverse phenomena such as circular dichroism depending on chirality.

We introduce variational wave functions to evaluate the ground-state properties of spin-phonon coupled systems described by the Su-Schrieffer-Heeger model. Quantum spins and phonons are treated on equal footing within a Monte Carlo sampling, and different regimes are investigated. We show that the proposed variational Ansatz yields good agreement with previous density-matrix renormalization group results in one dimension and is able to accurately describe the spin-Peierls transition. This variational approach is constrained neither by the magnetoelastic-coupling strength nor by the dimensionality of the systems considered, thus allowing future investigations in more general cases, which are relevant to spin-liquid and topological phases in two spatial dimensions.

There is growing interest in warm dense matter (WDM), an exotic state on the border between condensed matter and plasmas. Due to the simultaneous importance of quantum and correlation effects, WDM is complicated to treat theoretically. A key role has been played by ab initio path integral Monte Carlo (PIMC) simulations, and recently extensive results for thermodynamic quantities have been obtained. The first extension of PIMC simulations to the dynamic structure factor of the uniform electron gas was reported by Dornheim et al. [Phys. Rev. Lett. 121, 255001 (2018)]. This was based on an accurate reconstruction of the dynamic local field correction. Here we extend this concept to other dynamical quantities of the warm dense electron gas including the dynamic susceptibility, the dielectric function, and the conductivity.