The origin of incommensurate structural modulation in Ni-Mn based Heusler-type magnetic shape-memory alloys (MSMAs) is still an unresolved issue in spite of intense focus on it due to its role in the magnetic field induced ultrahigh strains. In the archetypal MSMA $\mathrm{N}{\mathrm{i}}_2\mathrm{MnGa}$, the observation of “nonuniform displacement” of atoms from their mean positions in the modulated martensite phase, premartensite phase, and charge density wave as well as the presence of phason broadening of satellite peaks has been taken in support of the electronic instability model linked with a soft acoustic phonon. We present here results of a combined high-resolution synchrotron x-ray powder diffraction (SXRPD) and neutron powder diffraction (NPD) study on $\mathrm{N}{\mathrm{i}}_2\mathrm{M}{\mathrm{n}}_{1.4}\mathrm{I}{\mathrm{n}}_{0.6}$ using a (3+1)D superspace group approach, which reveals not only uniform atomic displacements in the modulated structure of the martensite phase with physically acceptable ordered magnetic moments in the antiferromagnetic phase at low temperatures, but also the absence of any premartensite phase and phason broadening of the satellite peaks. Our HRTEM studies and first-principles calculations of the ground state also support uniform atomic displacements predicted by powder diffraction studies. All these observations suggest that the structural modulation in the martensite phase of $\mathrm{N}{\mathrm{i}}_2\mathrm{M}{\mathrm{n}}_{1.4}\mathrm{I}{\mathrm{n}}_{0.6}$ MSMA can be explained in terms of the adaptive phase model. The present study underlines the importance of superspace group analysis using complementary SXRPD and NPD in understanding the physics of the origin of modulation as well as the magnetic and the modulated ground states of the Heusler-type MSMAs. Our work also highlights the fact that the mechanism responsible for the origin of modulated structure in different Ni-Mn based MSMAs may not be universal and it must be investigated thoroughly in different alloy compositions.

We consider the ability of local quantum dynamics to solve the “energy-matching” problem: given an instance of a classical optimization problem and a low-energy state, find another macroscopically distinct low-energy state. Energy matching is difficult in rugged optimization landscapes, as the given state provides little information about the distant topography. Here, we show that the introduction of quantum dynamics can provide a speedup over classical algorithms in a large class of hard optimization problems. Tunneling allows the system to explore the optimization landscape while approximately conserving the classical energy, even in the presence of large barriers. Specifically, we study energy matching in the random $p$-spin model of spin-glass theory. Using perturbation theory and exact diagonalization, we show that introducing a transverse field leads to three sharp dynamical phases, only one of which solves the matching problem: (1) a small-field “trapped” phase, in which tunneling is too weak for the system to escape the vicinity of the initial state; (2) a large-field “excited” phase, in which the field excites the system into high-energy states, effectively forgetting the initial energy; and (3) the intermediate “tunneling” phase, in which the system succeeds at energy matching. The rate at which distant states are found in the tunneling phase, although exponentially slow in system size, is exponentially faster than classical search algorithms.

We classify the topology of the quench dynamics by homotopy groups. A relation between the topological invariant of a postquench order parameter and the topological invariant of a static Hamiltonian is shown in $d+1$ dimensions ($d=1,2,3$). The midgap states in the entanglement spectrum of the postquench states reveal their topological nature. When a trivial quantum state is under a sudden quench to a Chern insulator, the midgap states in the entanglement spectrum form rings. These rings are analogous to the boundary Fermi rings in the Hopf insulators. Finally, we show a postquench order parameter in 3+1 dimensions can be characterized by the second Chern number. The number of Dirac cones in the entanglement spectrum is equal to the second Chern number.

The recently introduced concept of “surface Berry plasmons” is studied in the concrete instance of a ferromagnetic conductor in which the Berry curvature, generated by spin-orbit (SO) interaction, has opposite signs for carrier with spins parallel or antiparallel to the magnetization. By using collisionless hydrodynamic equations with appropriate boundary conditions, we study both the surface plasmons of a three-dimensional ferromagnetic conductor and the edge plasmons of a two-dimensional one. The anomalous velocity and the broken inversion symmetry at the surface or the edge of the conductor create a “handedness” whereby the plasmon frequency depends not only on the angle between the wave vector and the magnetization, but also on the direction of propagation along a given line. In particular, we find that the frequency of the edge plasmon depends on the direction of propagation along the edge. These Berry curvature effects are compared and contrasted with similar effects on plasmon dispersions induced by an external magnetic field in the absence of Berry curvature. We argue that Berry curvature effects may be used to control the direction of propagation of the surface plasmons via coupling with the magnetization of ferromagnetic conductors, and thus create a link between plasmonics and spintronics.

The pyrochlore magnet ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ has been proposed as a quantum spin ice candidate, a spin liquid state expected to display emergent quantum electrodynamics with gauge photons among its elementary excitations. However, ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$'s ground state is known to be very sensitive to its precise stoichiometry. Powder samples, produced by solid-state synthesis at relatively low temperatures, tend to be stoichiometric, while single crystals grown from the melt tend to display weak “stuffing” wherein $\sim 2\%$ of the ${\mathrm{Yb}}^{3+}$, normally at the $A$ site of the $A_2{B}_2{\text{O}}_7$ pyrochlore structure, reside as well at the $B$ site. In such samples ${\mathrm{Yb}}^{3+}$ ions should exist in defective environments at low levels and be subjected to crystalline electric fields very different from those at the stoichiometric $A$ sites. Neutron scattering measurements of ${\mathrm{Yb}}^{3+}$ in four compositions of ${\mathrm{Yb}}_{2+x}{\mathrm{Ti}}_{2-x}{\mathrm{O}}_{7-y}$ show the spectroscopic signatures for these defective ${\mathrm{Yb}}^{3+}$ ions and explicitly demonstrate that the spin anisotropy of the ${\mathrm{Yb}}^{3+}$ moment changes from $XY$-like for stoichiometric ${\mathrm{Yb}}^{3+}$ to Ising-like for “stuffed” $B$ site ${\mathrm{Yb}}^{3+}$ or for $A$ site ${\mathrm{Yb}}^{3+}$ in the presence of oxygen vacancies.

Signatures of nodal line/point superconductivity [Kim et al., Sci. Adv. 4, eaao4513 (2018); Brydon et al., Phys. Rev. Lett. 116, 177001 (2016)] have been observed in half-Heusler compounds, such as LnPtBi (Ln = Y, Lu). Topologically nontrivial band structures, as well as topological surface states, have also been confirmed by angular-resolved photoemission spectroscopy in these compounds [Liu et al., Nat. Commun. 7, 12924 (2016)]. In this paper, we present a systematical classification of possible gap functions of bulk states and surface states in half-Heusler compounds and the corresponding topological properties based on the representations of crystalline symmetry group. Different from all the previous studies based on the four band Luttinger model, our study starts with the six-band Kane model, which involves both four p-orbital type of ${\mathrm{\Gamma }}_8$ bands and two s-orbital type of ${\mathrm{\Gamma }}_6$ bands. Although the ${\mathrm{\Gamma }}_6$ bands are away from the Fermi energy, our results reveal the importance of topological surface states, which originate from the band inversion between ${\mathrm{\Gamma }}_6$ and ${\mathrm{\Gamma }}_8$ bands, in determining surface properties of these compounds in the superconducting regime by combining topological bulk state picture and nontrivial surface state picture.

The transport properties of relativisticlike fermions have been extensively studied in solid-state systems with isotropic energy dispersions. Recently, several two-dimensional and three-dimensional Dirac-Weyl (DW) materials exhibiting tilted energy dispersions around their DW cones have been explored. Here, we demonstrate that such a tilt character could induce drastically different transport phenomena, compared to the isotropic-dispersion cases. Indeed, the Klein tunneling of DW fermions of opposite chiralities is predicted to appear along two separated oblique directions. In addition, valley filtering and beam splitting effects are easily tailored by dopant engineering techniques whereas the refraction of electron waves at a $\left(p\text{−}n\right)$-doped interface is dramatically modified by the tilt, thus paving the way for emerging applications in electron optics and valleytronics.

We show how a recently proposed solid-state Majorana platform comprising a planar Josephson junction proximitized to a 2D electron gas (2DEG) with Rashba spin-orbit coupling and Zeeman field can be viewed as an effectively one-dimensional (1D) Kitaev chain with long-range pairing and hopping terms. We highlight how the couplings of the 1D system may be tuned by changing experimentally realistic parameters. We also show that the mapping is robust to disorder by computing the Clifford pseudospectrum index in real space for the long-range Kitaev chain across several topological phases. This mapping opens up the possibility of using current experimental setups to explore 1D topological superconductors with nonstandard and tunable couplings.

We investigate Kondo correlations in a quantum dot with normal and superconducting electrodes, where a spin bias voltage is applied across the device and the local interaction $U$ is either attractive or repulsive. When the spin current is blockaded in the large-gap regime, this nonequilibrium strongly correlated problem maps into an equilibrium model solvable by the numerical renormalization group method. The Kondo spectra with characteristic splitting due to the nonequilibrium spin accumulation are thus obtained at high precision. It is shown that while the bias-induced decoherence of the spin Kondo effect is partially compensated by the superconductivity, the charge Kondo effect is enhanced out of equilibrium and undergoes an additional splitting by the superconducting proximity effect, yielding four Kondo peaks in the local spectral density. In the charge Kondo regime, we find a universal scaling of charge conductance in this hybrid device under different spin biases. The universal conductance as a function of the coupling to the superconducting lead is peaked at and hence directly measures the Kondo temperature. Our results are of direct relevance to recent experiments realizing a negative-$U$ charge Kondo effect in hybrid oxide quantum dots [Nat. Commun. 8, 395 (2017)].

We used single-crystal x-ray and neutron diffraction to investigate the crystal and magnetic structures of trigonal lattice iridate ${\mathrm{Ca}}_2{\mathrm{Sr}}_2{\mathrm{IrO}}_6$. The crystal structure is determined to be $R\overline{3}$ with two distinct Ir sites. The system exhibits long-range antiferromagnetic order below $T_N=13.1$ K. The magnetic wave vector is identified as (0,0.5,1) with ferromagnetic coupling along the $a$ axis and antiferromagnetic correlation along the $b$ axis. Spins align dominantly within the basal plane along the [1,2,0] direction and tilt ${34}^{\circ }$ toward the $c$ axis. The ordered moment is 0.66(3) ${\mu }_B$/Ir, larger than other iridates where iridium ions form corner- or edge-sharing ${\mathrm{IrO}}_6$ octahedral networks. The tilting angle is reduced to $\approx {19}^{\circ }$ when a magnetic field of 4.9 T is applied along the $c$ axis. Density functional theory calculations confirm that the experimentally determined magnetic configuration is the most probable ground state with an insulating gap $\sim 0.5$ eV.

We report results of isothermal magnetotransport and susceptibility measurements at elevated magnetic fields $B$ down to very low temperatures $T$ on single crystals of the frustrated Kondo-lattice system CePdAl. They reveal a $B^{*}\left(T\right)$ line within the paramagnetic part of the phase diagram. This line denotes a thermally broadened “small”–to-“large” Fermi-surface crossover which substantially narrows upon cooling. At ${B_0}^{*}=B^{*}\left(T=0\right)=\left(4.6\pm 0.1\right)\mathrm{T}$, this $B^{*}\left(T\right)$ line merges with two other crossover lines, viz. $T_{\mathrm{p}}\left(B\right)$ below and $T_{\mathrm{FL}}\left(B\right)$ above ${B_0}^{*}$. $T_{\mathrm{p}}$ characterizes a frustration-dominated spin-liquid state, while $T_{\mathrm{FL}}$ is the Fermi-liquid temperature associated with the lattice Kondo effect. Non-Fermi-liquid phenomena which are commonly observed near a “Kondo-destruction” quantum-critical point cannot be resolved in CePdAl. Our observations reveal a rare case where Kondo coupling, frustration, and quantum criticality are closely intertwined.

The pyrochlore ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ has recently attracted significant attention as a candidate Weyl semimetal. The previous reports on this compound unanimously show a thermally induced metal-to-insulator (MI) transition, concomitant with antiferromagnetic long-range ordering of the Ir moments below $T_{\mathit{N}}\sim 120$ K. However, there are contradictory reports concerning the slope $d\rho /dT$ of the resistivity plots ($\rho$) in the “metallic” state above the MI transition, and the value of $\rho$ in the insulating state, both of which show significant sample dependence. Here, we explore this issue by investigating six different ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ samples with slightly varying Eu:Ir ratio. High-resolution synchrotron powder diffraction is done to probe minor variations in the cell parameters of the various ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ samples investigated here. Specific heat ($C_p$) and magnetic susceptibility of all the samples showed long-range antiferromagnetic ordering upon cooling below $T_{\mathit{N}}\sim 120$ K. The transitions are, however, found to be smeared out for the off-stoichiometric samples. We show that the sign of $d\rho /dT$ above the MI transition is highly sensitive to the unit-cell length, which, in turn, depends on the level of Eu stuffing at the Ir site. Samples with composition close to the ideal stoichiometry (Eu:$\mathrm{Ir}=1$) showed a change of sign of $d\rho /dT$ from negative to positive upon cooling below a certain temperature $T^{*}>T_{\text{MI}}$. With increasing Eu stuffing, $T^{*}$ decreased until a negative $d\rho /dT$ persisted without any sign change down to $T_{\text{MI}}$.

Motivated by RIXS experiments on a wide range of complex heavy oxides, including rhenates, osmates, and iridates, we discuss the theory of RIXS for site-localized $t_{2g}$ orbital systems with strong spin-orbit coupling. For such systems, we present exact diagonalization results for the spectrum at different electron fillings, showing that it accesses “single-particle” and “multiparticle” excitations. This leads to a simple picture for the energies and intensities of the RIXS spectra in Mott insulators such as double perovskites which feature highly localized electrons, and yields estimates of the spin-orbit coupling and Hund's coupling in correlated $5d$ oxides. We present new higher resolution RIXS data at the Re $L_3$ edge in ${\mathrm{Ba}}_2{\mathrm{YReO}}_6$ which finds a previously unresolved peak splitting, providing further confirmation of our theoretical predictions. Using ab initio electronic structure calculations on ${\mathrm{Ba}}_2\mathcal{M}{\mathrm{ReO}}_6$ (with $\mathcal{M}=\mathrm{Re}$, Os, Ir) we show that while the atomic limit yields a reasonable effective Hamiltonian description of the experimental observations, effects such as $t_{2g}-e_g$ interactions and hybridization with oxygen are important. Our ab initio estimate for the strength of the intersite exchange coupling shows that, compared to the $d^3$ systems, the exchange is one or two orders of magnitude weaker in the $d^2$ and $d^4$ materials, which may partly explain the suppression of long-range magnetic order in the latter compounds. As a way to interpolate between the site-localized picture and our electronic structure band calculations, we discuss the spin-orbital levels of the $\mathcal{M}{\mathrm{O}}_6$ cluster. This suggests a possible role for intracluster excitons in ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$ which may lead to a weak breakdown of the atomic $J_{\mathrm{eff}}=0$ picture and to small magnetic moments.

A deterministic and scalable array of single photon nonlinearities in the solid state holds great potential for both fundamental physics and technological applications, but its realization has proved extremely challenging. Despite significant advances, leading candidates such as quantum dots and group III-V quantum wells have yet to overcome their respective bottlenecks in random positioning and weak nonlinearity. Here we consider a hybrid light-matter platform, marrying an atomically thin two-dimensional material to a photonic crystal cavity, and analyze its second-order coherence function. We identify several mechanisms for photon antibunching under different system parameters, including one characterized by large dissipation and weak nonlinearity. Finally, we show that by patterning the two-dimensional material into different sizes, we can drive our system dynamics from a coherent state into a regime of strong antibunching with second-order coherence function $g^{\left(2\right)}\left(0\right)\sim {10}^{-3}$, opening a possible route to scalable, on-chip quantum simulations with correlated photons.

The understanding of the fundamentals of spin and charge densities and currents interconversion by spin-orbit coupling can enable efficient applications beyond the possibilities offered by conventional electronics. For this purpose we consider various forms of the frequency-dependent inverse spin galvanic effect in semiconductor quantum wells and epilayers taking into account the cubic in the electron momentum spin-orbit coupling in the Rashba and Dresselhaus forms, concentrating on the current-induced spin polarization (CISP). We find that including the cubic terms qualitatively explains recent findings of the CISP in InGaAs epilayers being the strongest if the internal spin-orbit coupling field is the smallest and vice versa [Norman et al., Phys. Rev. Lett. 112, 056601 (2014); Luengo-Kovac et al., Phys. Rev. B 96, 195206 (2017)], in contrast to the common understanding. Our results provide a promising framework for the control of spin transport in future spintronics devices.

Plasmons and excitons, bound states of electrons and holes, are collective charge excitations in solids. In this study, we numerically show that in most metallic carbon nanotubes, which are called narrow-gap carbon nanotubes, excitons cross over to plasmons as the wave vector increases. This indicates that resonance with the excitons changes to that with the plasmons by changing the nanotube length, which can explain the origin of observed peaks in the terahertz or far-infrared region in the optical absorption spectra of metallic carbon nanotubes. In the crossovers from excitons to plasmons, a depolarization effect on the many-body wave functions of the plasmons and excitons is clarified.

Unique intensity features arising from dynamical diffraction arise in coherent x-ray nanobeam diffraction patterns of crystals having thicknesses larger than the x-ray extinction depth or exhibiting combinations of nanoscale and mesoscale features. We demonstrate that dynamical scattering effects can be accurately predicted using an optical model combined with the Darwin theory of dynamical x-ray diffraction. The model includes the highly divergent coherent x-ray nanobeams produced by Fresnel zone plate focusing optics and accounts for primary extinction, multiple scattering, and absorption. The simulation accurately reproduces the dynamical scattering features of experimental diffraction patterns acquired from a GaAs/AlGaAs epitaxial heterostructure on a GaAs (001) substrate.

We calculate the chemical potential dependence of the renormalized Fermi velocity and static dielectric function for Dirac quasiparticles in graphene nonperturbatively at finite temperature. By reinterpreting the chemical potential as a flow parameter in the spirit of the functional renormalization group (fRG) we obtain a set of flow equations, which describe the change of these functions upon varying the chemical potential. In contrast to the fRG the initial condition of the flow is nontrivial and has to be calculated separately. Our results are consistent with a charge carrier-independent Fermi velocity $v\left(k\right)$ for small densities $n\lesssim k^2/\pi$, supporting the comparison of the zero-density fRG calculation of Bauer et al. [Phys. Rev. B 92, 121409 (2015)], with the experiment of Elias et al. [Nat. Phys. 7, 701 (2011)].

Very recently, the half-metallic compound ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ was proposed to be a magnetic Weyl semimetal (WSM) with Weyl points only 60 meV above the Fermi level $E_F$. Owing to the low charge carrier density and large Berry curvature induced, ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ possesses both a large anomalous Hall conductivity and a large anomalous Hall angle, which provide strong evidence for the existence of Weyl points in ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$. In this work, we theoretically study the surface topological feature of ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ and its counterpart ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{Se}}_2$. By cleaving the sample at the weak Sn-S/Se bonds, one can achieve two different surfaces terminated with Sn and S/Se atoms, respectively. The resulting Fermi-arc-related states can range from the energy of the Weyl points to $E_F-0.1$ eV in the Sn-terminated surface. Therefore, it should be possible to observe the Fermi arcs in angle-resolved photoemission spectroscopy (ARPES) measurements. Furthermore, in order to simulate quasiparticle interference in scanning tunneling microscopy (STM) measurements, we also calculate the joint density of states for both terminals. This work should be helpful for a comprehensive understanding of the topological properties of these two magnetic WSMs and further ARPES and STM measurements.

We present a microscopic description of plasmon-enhanced optical absorption in graphene, which is based on perturbation theory. We consider the interaction of graphene with a lattice of plasmonic nanoparticles, as was previously realized experimentally. By using tight-binding wave functions for the electronic states of graphene and the dipole approximation for the plasmon, we obtain analytic expressions for the coupling matrix element and enhanced optical absorption. The plasmonic nanostructure induces nonvertical optical transitions in the band structure of graphene with selection rules for the momentum transfer that depend on the periodicity of the plasmonic lattice. The plasmon-mediated optical absorption leads to an anisotropic carrier population around the $K$ point in phase space, which depends on the polarization pattern of the plasmonic near field in the graphene plane. Using Fourier optics, we draw a connection to a macroscopic approach, which is independent from graphene-specific parameters. Each Fourier component of the plasmonic near field corresponds to the momentum transfer of an optical transition. Both approaches lead to the same expression for the integrated optical absorption enhancement, which is relevant for the photocurrent enhancement in graphene-based optoelectronic devices.