The magnetic properties of iron-based superconductors $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($A=\mathrm{K}$, Cs, and Rb), which are characterized by the V-shaped dependence of the critical temperature ($T_{\mathrm{c}}$) on pressure ($P$), were studied by means of the muon spin rotation/relaxation technique. In all three systems studied the magnetism was found to appear for pressures slightly below the critical one ($P_{\mathrm{c}}$), i.e., at pressure where $T_{\mathrm{c}}\left(P\right)$ changes the slope. Rather than competing, magnetism and superconductivity in $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ are coexisting at the $P\gtrsim P_{\mathrm{c}}$ pressure region. Our results support the scenario of a transition from one pairing state to another, with different symmetries on either side of $P_{\mathrm{c}}$.

Knowledge of lattice dynamics of acoustic flexural phonons is critically important in understanding the mechanical, electrical, and thermal properties of nanomaterials. However, the general nature of acoustic flexural phonons for the span of possible nanomaterials and the dependence of these on microscopic material properties remain unclear. Therefore, we develop general lattice chain theories to understand acoustic flexural vibrations in covalent and van der Waals lattices. The analytic theories suggest that the flexure branch has quadratic dispersion for low dimensional lattices in the long-wavelength limit, quite different from linear branches in bulk van der Waals bonded lattices like graphite. Surprisingly, interlayer van der Waals interactions in multilayer lattices do not determine the behavior of the flexural dispersion. The bonds of covalent interactions in multilayer and buckled monolayer lattices play different roles in determining the flexural dispersion with those in flat monolayer lattices, e.g., graphene. Further, based on the correspondence between microscopic and continuum dynamics, we provide a universal and effective approach to characterize the intrinsic bending rigidity of low dimensional nanomaterials using dispersion data.

We consider the spin-$\frac{1}{2}$ isotropic XY chain in an external magnetic field directed along the $z$ axis with periodically varying $g$ factors. To reveal the effects of regularly alternating $g$ factors, we calculate various static and dynamic equilibrium quantities in the ground state and at finite temperatures. We demonstrate that because of the regularly alternating $g$ factors the saturation field may disappear and the field dependence of the susceptibility in the ground state has additional logarithmic singularity at zero field. Moreover, the zero-field susceptibility has a logarithmic singularity as $T\to 0$. Furthermore, the dynamic structure factors exhibit much more structure in the “wave vector–frequency” plane that can be traced out to modifications of the two-fermion excitation continua which exclusively determine $S_{zz}(\kappa ,\omega )$ and dominate the properties of $S_{xx}(\kappa ,\omega )$. We discuss what changes can be observed in dynamic experiments on the corresponding substances.

We investigated the magnetostriction behaviors and magnetoelectric effect of spinel structured $\mathrm{Mn}{\mathrm{V}}_2{\mathrm{O}}_4$ using the magnetization, thermal expansion, magnetostriction, permittivity, and pyroelectricity measurements. Two successive transitions were observed at $T_S$ and $T^{*}$ in all measurements. The weak electric polarization begins to appear below $T_S$, and can be suppressed noticeably by applying a magnetic field below $T^{*}$, indicative of the obvious magnetoelectric effect in $\mathrm{Mn}{\mathrm{V}}_2{\mathrm{O}}_4$. Furthermore, $\mathrm{Mn}{\mathrm{V}}_2{\mathrm{O}}_4$ exhibits large low-field magnetostriction up to 6000 ppm at 7 kOe between $T^{*}$ and $T_S$. Except for the strain at the structural transition $T_S, \mathrm{Mn}{\mathrm{V}}_2{\mathrm{O}}_4$ also shows another large strain at $T^{*}$ under the magnetic field, originating from the rearrangement of tetragonal domains. Decreasing temperature allows the easy magnetization axis of $\mathrm{Mn}{\mathrm{V}}_2{\mathrm{O}}_4$ to rotate from parallel to the $a{b}_{\mathrm{T}}$ plane to parallel to the $c_T$ axis because of the disappearance of weak orbital fluctuations at $T^{*}$, which leads to the rearrangement of tetragonal domains at $T^{*}$ under the magnetic field.

We have evidenced unconventional anomalous Hall effects (AHEs) in $3d/5d$ (${\mathrm{Co}0.2\mathrm{nm}/\mathrm{Ni}0.6\mathrm{nm})}_N$ multilayers grown on a thin Pt layer or thin Au:W alloys with perpendicular magnetic anisotropy (PMA) properties. The inversion of AHEs observed with one Pt series is explained by considering the opposite sign of the effective spin-orbit coupling of Pt compared to Co/Ni combined with peculiar specular electronic reflections. Using advanced simulations methods for the description of the spin-current profiles based on the spin-dependent Boltzmann formalism, we extracted the spin-Hall angle (SHA) of Pt and Co/Ni of opposite sign. The extracted SHA for Pt, $+20\%$, is opposite to the one of Co/Ni, giving rise to an effective AHE inversion for thin Co/Ni multilayers (with the number of repetition layers $N<17$). The spin-Hall angle in Pt is found to be larger than the one previously measured by complementary spin-pumping inverse spin-Hall effect experiments in a geometry of current perpendicular to the plane. Whereas magnetic proximity effects cannot explain the effect, spin-current leakage and spin-orbit assisted electron scattering at Pt/(Co,Ni) interfaces fit the experiments. We also extract the main relevant electronic transport parameters governing the overall effects in current-in-plane (CIP) currents and demonstrate, in particular, that the specularity/nonspecularity in the electronic diffusion processes play an essential role to explain the observed results.

It is known that in some higher-order topological insulators (HOTIs), topological phases are distinguished not by gap closings of bulk states but by those of edge states, which are called boundary-obstructed topological phases (BOTPs). In this paper, we construct an effective theory of the BOTP transition of two-dimensional (2D) Su-Schrieffer-Heeger (SSH) model in a uniform magnetic field. At $\pi$ flux per plaquette, this model corresponds to the typical model of HOTIs proposed by Benalcazar, Bernevig, and Hughes (BBH). The BBH model can be approximated by Dirac fermions with two kinds of mass terms, which will be referred to as BBH Dirac insulator. To clarify the BOTP transition of the 2D SSH model around $\pi$ flux, we study such BBH Dirac insulator in the presence of a magnetic field. On the other hand, generically in continuum Dirac models, boundary conditions associated with the Hermiticity of Hamiltonians are known to play a crucial role in determining the edge states. We first demonstrate that for the conventional Dirac fermion with a single mass term, such boundary conditions indeed determine the edge states even in the presence of a magnetic field. Next, imposing boundary conditions consistent to the lattice terminations and symmetries of the BBH Hamiltonian as well as to the Hermiticity of the BBH Dirac insulator, we obtain the edge states of the BBH Dirac insulator in a magnetic field and reproduce its BOTP transition. In particular, we show that the unpaired Landau levels, which cause the spectral asymmetry, yield the edge states responsible for the BOTP transition.

We consider a system of one-dimensional fermions moving in one direction, such as electrons at the edge of a quantum Hall system. At sufficiently long timescales the system is brought to equilibrium by weak interactions between the particles, which conserve their total number, energy, and momentum. The time evolution of the system near equilibrium is described by hydrodynamics based on the three conservation laws. We find that the system supports three sound modes. In the low-temperature limit one mode is a pure oscillation of particle density, analogous to the ordinary sound. The other two modes involve oscillations of both particle and entropy densities. In the presence of disorder, the first sound mode is strongly damped at frequencies below the momentum relaxation rate, whereas the other two modes remain weakly damped.

An applied field can modulate optical signals by resonance shifting via the Stark effect. The optical Stark effect uses ultrafast light in the transparency region of a material to shift resonances with speeds limited by the pulse duration or system coherence. In this Rapid Communication, we investigate the optical Stark effect in resonant optical harmonic generation using the $A$ exciton transition of ${\mathrm{WS}}_2$. Multidimensional pump-harmonic-probe measurements, in which the probe is second- or third-harmonic emission, reveal not only large Stark shifts that are commensurate with the large optical susceptibilities common to ${\mathrm{WS}}_2$ excitons, but also behaviors more complex than simple optical Stark effect treatments predict. We show how another manifestation of the Stark effect, brought forth by coherent photon exchange between the pump and harmonic generation fundamental fields, can strongly enhance or suppress harmonic generation.

The Hall viscosity has been proposed as a topological property of incompressible fractional quantum Hall states and can be evaluated as Berry curvature. This paper reports on the Hall viscosities of composite-fermion Fermi seas at $\nu =1/m$, where $m$ is even for fermions and odd for bosons. A well-defined value for the Hall viscosity is not obtained by viewing the $1/m$ composite-fermion Fermi seas as the $n\to \infty$ limit of the Jain $\nu =n/(nm\pm 1)$ states, whose Hall viscosities $(\pm n+m)\hslash \rho /4$ ($\rho$ is the two-dimensional density) approach $\pm \infty$ in the limit $n\to \infty$. A direct calculation shows that the Hall viscosities of the composite-fermion Fermi sea states are finite and also relatively stable with system size variation, although they are not topologically quantized in the entire $\tau$ space. I find that the $\nu =1/2$ composite-fermion Fermi sea wave function for a square torus yields a Hall viscosity that is expected from particle-hole symmetry and is also consistent with the orbital spin of $1/2$ for Dirac composite fermions. I compare my numerical results with some theoretical conjectures.

We investigate using the Boltzmann transport equation the electrical resistivity of atomically smooth single-crystal $\mathrm{Cu}\left(111\right)$ and $\mathrm{Cu}\left(001\right)$ films with thickness 1–45 nm. Our transport calculations show that, in the absence of grain-boundary scattering and surface perturbations, the resistivity of Cu films of thickness comparable to the electron mean free path (40 nm for bulk copper) is close to the bulk resistivity. The resistivity increases trivially for sub-40-nm-thick films. These single-crystal Cu films show an intrinsic limit of resistivity. A steep increase in resistivity observed for ultrathin films is due to enhanced finite-size effects. We explain the cause of increase in resistivity by investigating the electronic structure at the Fermi level.

We observe a broadband and subwavelength sensitivity of the recently reported enhanced transmission through symmetric diffusive slabs [Chéron et al., Phys. Rev. Lett. 122, 125501 (2019)]. From a perfectly symmetric distribution of scatterers on both sides of an opaque barrier within a waveguide, a shift as small as $\lambda /200$ significantly alters the multiple interferences giving rise to the transmission enhancement, and this subwavelength sensitivity appears on a very broad frequency range and without averaging. Analyses, from either full wave numerics or random matrix theory, show how the minimum shifting distance to suppress the transmission enhancement scales with the wavelength and the strength of the barrier.

The demonstration of the non-Abelian properties of Majorana bound states (MBSs) is a crucial step toward topological quantum computing. We theoretically investigate how the fusion of MBSs manifests itself in the current-voltage characteristics of a topological Josephson junction. The junction is assumed to be built on U-shaped quantum spin Hall edges and is supposed to host a Majorana qubit formed by four MBSs. Inter- and intraedge couplings among adjacent MBSs provide two orthogonal components of the rotation axes of the Majorana qubit. We show that the interplay of the dynamics of the superconductor phase difference and the Majorana qubit governs the Josephson effect. Strikingly, we identify sequential jumps of the voltage across the junction with increasing DC current bias without external AC driving. Its role in the formation of ordinary Shapiro steps is replaced by the intrinsic Rabi oscillations of the Majorana qubit. We coin this phenomenon DC Shapiro steps.

A comprehensive study of electron and magnetic properties and spin transport in the epitaxial manganite/iridate heterostructure was carried out by using of the x-ray, dc resistance measurements, ferromagnetic resonance at microwaves, polarized neutron reflectivity, and spin-current transport. Epitaxial growth of the heterostructure proceeded according to the “cube-to-cube” mechanism with a small lattice turn. The dc current measurement indicates the presence of a conduction channel at the iridate/manganite interface due to the charge leak from iridate making it hole doped, while the manganite side can be doped by electron. This is confirmed by the first-principles calculations based on density-functional theory [S. Bhowal and S. Satpathy, AIP Conf. Proc. 2005, 020007 (2018)], which showed a charge transfer at the interface from the half-filled spin-orbit entangled $J_{\mathrm{eff}}=1/2$ state of the iridate to the empty $e^{\uparrow g}$ states of manganite. The neutron-scattering data show the turn of magnetization vector of the heterostructure by ${26}^{\circ }$ towards the external field with temperature reduction down to 10 K. An additional ferromagnetic state that appears at $T<100\mathrm{K}$ indicates an emergence of ferromagnetism at the interface of the paramagnetic $\mathrm{SrIr}{\mathrm{O}}_3$ film and ferromagnetic manganite. We measured the dc voltage that occurs on the $\mathrm{SrIr}{\mathrm{O}}_3$ film due to spin pumping and anisotropic magnetoresistance in the heterostructure.

In the antiferromagnetic phase of $\mathrm{Eu}\mathrm{Ti}{\mathrm{O}}_3$ ceramics, a seemingly anisotropic magnetodielectric effect (up to 2.5%) was observed via the low-frequency dielectric permittivity measured in external magnetic field below 1.9 T. We explain the effect theoretically by taking into account the demagnetizing field which effectively reduces the internal magnetic flux density by 0.6 T when the external magnetic field is applied perpendicular to the plane of the disk-shaped sample. This finding is also confirmed experimentally by magnetization measurements. The refractive index between 0.2 and 0.5 THz exhibits a large anisotropy in an external magnetic field of up to 7 T. This anisotropy is due to a shift of the ferromagnetic resonance from the microwave to the sub-THz region with external magnetic field applied perpendicular to the magnetic component of the THz radiation. This magnon observation is performed in the strong external magnetic field (above 2 T) and in both paramagnetic and antiferromagnetic phases.

In P-doped pnictide, ${\mathrm{EuFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ superconductivity coexists with a ferromagnetism of $\mathrm{Eu}\text{−}4f$ spins. Above $x\simeq 0.26$, superconductivity vanishes whereas ferromagnetism remains. Here, we focus on the crossover region and study two ${\mathrm{EuFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ single crystals with $x=0.21$ and $x=0.25$; the superconducting critical temperature of the first one is slightly higher and of the second one is slightly lower than the Curie temperature. Despite similar compositions, characteristic temperatures and bulk magnetic properties of the two systems, their local magnetic structures and superconducting vortex-antivortex phases are found drastically different. We demonstrate that the interplay between superconductivity and magnetism is strongly dominated by the superconducting order in the first system, whereas it is mainly governed by ferromagnetism in the second one. Our discovery raises several fundamental questions on the vortex nucleation and dynamics in magnetic superconductors.

As a potential window on transitions out of the ergodic, many-body-delocalized phase, we study the dephasing of weakly disordered, quasi-one-dimensional fermion systems due to a diffusive, non-Markovian noise bath. Such a bath is self-generated by the fermions, via inelastic scattering mediated by short-ranged interactions. The ergodic phase can be defined by the nonzero dephasing rate, which makes transport incoherent and classical on long length scales. We calculate the dephasing of weak localization perturbatively through second order in the bath coupling, obtaining a short-time expansion. However, no well-defined dephasing rate can be identified, and the expansion breaks down at long times. This perturbative expansion is not stabilized by including a mean-field cooperon “mass” (decay rate), signaling a failure of the self-consistent Born approximation. We also consider a many-channel quantum wire where short-ranged, spin-exchange interactions coexist with screened Coulomb interactions. We calculate the dephasing rate, treating the short-ranged interactions perturbatively and the Coulomb interaction exactly. The latter provides a physical infrared regularization that stabilizes perturbation theory at long times, giving the first controlled calculation of quasi-1D dephasing due to diffusive noise. At first order in the diffusive bath coupling, we find an enhancement of the dephasing rate, but at second order, we find a rephasing contribution. Our results differ qualitatively from those obtained via self-consistent calculations commonly employed in higher dimensions. Our results are relevant in two different contexts: first, in the search for precursors to many-body localization in the ergodic phase of an isolated many-fermion system. Second, our results provide a mechanism for the enhancement of dephasing at low temperatures in spin SU(2)-symmetric quantum wires, beyond the Altshuler-Aronov-Khmelnitsky result. The enhancement is possible due to the amplification of the triplet-channel interaction strength and provides an additional physical mechanism that could contribute to the experimentally observed low-temperature saturation of the dephasing time.

Electromagnetic dressing by a high-frequency field drastically modifies the electronic transport properties on Dirac systems. Here its effects on the energy spectrum of graphene with two possible phases of Kekulé distortion (namely, Kek-Y and Kek-O textures) are studied. Using Floquet theory it is shown how circularly polarized light modifies the gapless spectrum of the Kek-Y texture, producing dynamical band gaps at the Dirac point that depend on the amplitude and the frequency of the electric field, and breaks the valley degeneracy of the gapped spectrum of the Kek-O texture. To further explore the electronic properties under circularly polarized radiation, the dc conductivity is studied by using the Boltzmann approach and considering both intervalley and intravalley contributions. When linearly polarized light is considered, the band structure of both textures is always modified in a perpendicular direction to the electric field. While the band structure for the Kek-Y texture remains gapless, the gap for the Kek-O texture is reduced considerably. For this linear polarization it is also shown that nondispersive bands can appear by a precise tuning of the light field parameters, thus inducing dynamical localization. The present results suggest that optical measurements will allow one to distinguish between different Kekulé bond textures.

Perovskite and spinel structures are widely found in ferroelectric and magnetic oxides, respectively, making their combination important for multiferroic composites. In this study, the (111) interface between perovskite-type $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ and spinel-type $\mathrm{Ni}{\mathrm{Fe}}_2{\mathrm{O}}_4$, has been systematically investigated at the atomic scale combining aberration-corrected scanning transmission electron microscopy and first-principles calculations. The atomic terminations at the interface were determined to be the $\mathrm{Bi}{\mathrm{O}}_3$ layer on the $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ side and the tetrahedral Fe layer on the $\mathrm{Ni}{\mathrm{Fe}}_2{\mathrm{O}}_4$ side. The lattice mismatch between $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ and $\mathrm{Ni}{\mathrm{Fe}}_2{\mathrm{O}}_4$ is primarily accommodated by the first and second $\mathrm{Bi}{\mathrm{O}}_3$ layers inside $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$, indicating a stand-off of misfit dislocations in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$. A metallic interface is formed between the two insulating phases, with the $\mathrm{Bi}{\mathrm{O}}_3$ and tetrahedral Fe layer coupled antiferromagnetically across the interface. The magnetic moment in $\mathrm{Ni}{\mathrm{Fe}}_2{\mathrm{O}}_4$ and the ferroelectric polarization in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ drop slightly at the interface and return to the bulk values within two atomic layers from the interface.