High-precision angle-resolved dc magnetization and magnetic torque studies were performed on a single-crystalline sample of URhGe, an orthorhombic Ising ferromagnet with the $c$ axis being the magnetization easy axis, in order to investigate the phase diagram around the ferromagnetic (FM) reorientation transition in a magnetic field near the $b$ axis. We have clearly detected a first-order transition in both the magnetization and the magnetic torque at low temperatures, and determined detailed profiles of the wing structure of the three-dimensional $T-H_b-H_c$ phase diagram, where $H_c$ and $H_b$ denote the field components along the $c$ and the $b$ axes, respectively. The quantum wing critical points are located at ${\mu }_0{H}_c\sim \pm 1.1$ T and ${\mu }_0{H}_b\sim 13.5$ T. Two second-order transition lines at the boundaries of the wing planes rapidly tend to approach each other with increasing temperature up to $\sim 3$ K. Just at the zero conjugate field ($H_c=0$), however, a signature of the first-order transition can still be seen in the field derivative of the magnetization at $\sim 4$ K, indicating that the tricritical point exists in a rather high temperature region above 4 K. This feature of the wing plane structure is consistent with the theoretical expectation that three second-order transition lines merge tangentially at the tricritical point.

Willis fluids, or more generally Willis materials, are homogenized composites that exhibit coupling between momentum and strain. This coupling is intrinsic to inhomogeneous media and can play a significant role in the overall response in acoustic metamaterials. In this paper, we draw connections between bianisotropy in electromagnetism and Willis coupling in elastodynamics to provide a qualitative understanding. Building upon these analogies, we introduce a homogenization technique for acoustic metamaterials based on a source-driven, multiple scattering approach that highlights the physical origins of Willis coupling. Moreover, through numerical examples, we compare several macroscopic material descriptions of acoustic metamaterials with non-negligible Willis coupling. The descriptions neglecting Willis coupling may not satisfy restrictions stemming from reciprocity, passivity, and causality, which suggests that including Willis coupling in macroscopic descriptions is necessary to realize physically meaningful macroscopic parameters.

We study the anomalous Nernst and thermal Hall effects in a linearized low-energy model of a tilted Weyl semimetal, with two Weyl nodes separated in momentum space. For inversion symmetric tilt, we give analytic expressions in two opposite limits: For a small tilt, corresponding to a type-I Weyl semimetal, the Nernst conductivity is finite and independent of the Fermi level; for a large tilt, corresponding to a type-II Weyl semimetal, it acquires a contribution depending logarithmically on the Fermi energy. This result is in a sharp contrast to the nontilted case, where the Nernst response is known to be zero in the linear model. The thermal Hall conductivity similarly acquires Fermi surface contributions, which add to the Fermi level-independent, zero-tilt result, and is suppressed as one over the tilt parameter at half filling in the type-II phase. In the case of inversion-breaking tilt, with the tilting vector of equal modulus in the two Weyl cones, all Fermi surface contributions to both anomalous responses cancel out, resulting in zero Nernst conductivity. We discuss two possible experimental setups, representing open and closed thermoelectric circuits.

There have been a large number of papers on bismuth ferrite $\left(\mathrm{BiFe}{\mathrm{O}}_3\right)$ over the past few years, trying to exploit its room-temperature magnetoelectric multiferroic properties. Although these are attractive, $\mathrm{BiFe}{\mathrm{O}}_3$ is not the ideal multiferroic due to weak magnetization and the difficulty in limiting leakage currents. Thus there is an ongoing search for alternatives, including such materials as gallium ferrite $\left(\mathrm{GaFe}{\mathrm{O}}_3\right)$. In the present work we report a comprehensive study of the perovskite $\mathrm{PbT}{\mathrm{i}}_{1-x}\mathrm{P}{\mathrm{d}}_x{\mathrm{O}}_3$ with $0<x<0.3$. Our study includes dielectric, impedance, and magnetization measurements, conductivity analysis, and study of crystallographic phases present in the samples, with special attention paid to minor phases identified as PdO, $\mathrm{PbPd}{\mathrm{O}}_2$, and $\mathrm{P}{\mathrm{d}}_3\mathrm{Pb}$. The work is remarkable in two ways: Pd is difficult to substitute into $AB{O}_3$ perovskite oxides (where it might be useful for catalysis), and Pd is magnetic under only unusual conditions (under strain or internal electric fields).

We construct fixed-point wave functions and exactly solvable commuting-projector Hamiltonians for a large class of bosonic symmetry-enriched topological (SET) phases, based on the concept of equivalent classes of symmetric local unitary transformations. We argue that for onsite unitary symmetries, our construction realizes all SETs free of anomaly, as long as the underlying topological order itself can be realized with a commuting-projector Hamiltonian. We further extend the construction to antiunitary symmetries (e.g., time-reversal symmetry), mirror-reflection symmetries, and to anomalous SETs on the surface of three-dimensional symmetry-protected topological phases. Mathematically, our construction naturally leads to a generalization of group extensions of unitary fusion categories to antiunitary symmetries.

We generalize the recently introduced TRILEX approach (TRiply irreducible local EXpansion) to superconducting phases. The method treats simultaneously Mott and spin-fluctuation physics using an Eliashberg theory supplemented by local vertex corrections determined by a self-consistent quantum impurity model. We show that, in the two-dimensional Hubbard model, at strong coupling, TRILEX yields a $d$-wave superconducting dome as a function of doping. Contrary to the standard cluster dynamical mean field theory (DMFT) approaches, TRILEX can capture $d$-wave pairing using only a single-site effective impurity model. We also systematically explore the dependence of the superconducting temperature on the bare dispersion at weak coupling, which shows a clear link between strong antiferromagnetic (AF) correlations and the onset of superconductivity. We identify a combination of hopping amplitudes particularly favorable to superconductivity at intermediate doping. Finally, we study within $GW$+EDMFT the low-temperature $d$-wave superconducting phase at strong coupling in a region of parameter space with reduced AF fluctuations.

Iron-based chalcogenides are complex superconducting systems in which orbitally dependent electronic correlations play an important role. Here, using high-resolution angle-resolved photoemission spectroscopy, we investigate the effect of these electronic correlations outside the nematic phase in the tetragonal phase of superconducting ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ $\left(x=0,0.18,1\right)$. With increasing sulfur substitution, the Fermi velocities increase significantly and the band renormalizations are suppressed towards a factor of $1.5–2$ for FeS. Furthermore, the chemical pressure leads to an increase in the size of the quasi-two-dimensional Fermi surface, compared with that of FeSe, however, it remains smaller than the predicted one from first-principles calculations for FeS. Our results show that the isoelectronic substitution is an effective way to tune electronic correlations in ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$, being weakened for FeS with a lower superconducting transition temperature. This suggests indirectly that electronic correlations could help to promote higher-$T_c$ superconductivity in FeSe.

The ability to perform first-principles calculations of electronic and vibrational properties of two-dimensional heterostructures in a field-effect setup is crucial for the understanding and design of next-generation devices. We present here an implementation of density functional perturbation theories tailored for the case of two-dimensional heterostructures in field-effect configuration. Key ingredients are the inclusion of a truncated Coulomb interaction in the direction perpendicular to the slab and the possibility of simulating charging of the slab via field effects. With this implementation we can access total energies, force and stress tensors, the vibrational properties and the electron-phonon interaction. We demonstrate the relevance of the method by studying flexural acoustic phonons and their coupling to electrons in graphene doped by field effect. In particular, we show that while the electron-phonon coupling to those phonons can be significant in neutral graphene, it is strongly screened and negligible in doped graphene, in disagreement with other recent first-principles reports. Consequently, the gate-induced coupling with flexural acoustic modes would not be detectable in transport measurements on doped graphene.

The evolution of electrical transport properties, the electronic band structure, and lattice dynamics of $\mathrm{PdS}{\mathrm{e}}_2$ is studied under high pressure. The emergence of superconductivity is reported in the high-pressure pyrite-type phase of $\mathrm{PdS}{\mathrm{e}}_2$. In this transition-metal dichalcogenide, the critical temperature of superconductivity rapidly increases with pressure up to 13.1 K. Ab initio electronic band structure calculations indicate the presence of Dirac and nodal-line fermions in the vicinity of the Fermi energy protected by the pyrite structure symmetry, which can lead to interesting superconducting states. Raman spectroscopy shows a direct correlation between critical temperature and bonding strength of Se-Se dumbbells in $\mathrm{PdS}{\mathrm{e}}_2$, underlining the crucial role of bonding for tuning the superconductivity.

We report a systematic dependence of the spin-transfer torque (STT) switching threshold on junction size and its resistance-area product (RA, or $r_{\mathrm{Amtj}}$) for the CoFeB-MgO-CoFeB type of magnetic tunnel juctions. The RA dependence of the switching efficiency is seen to become stronger for junctions of larger size (in the range of 15 to 50 nms). Here, the STT switching efficiency is defined as the ratio of the nanojunction free-layer energy barrier for thermal activation reversal to its STT switching threshold current. That is, the efficiency $\kappa =E_b/I_{c0}$, and it is seen to follow the junction $r_{\mathrm{Amtj}}$ in the form of $1/\kappa =A_0+A_1/r_{\mathrm{Amtj}}$, with $A_1\propto a-a_0$, where $a$ is the device diameter and $a_0$ is of the order of 10 nm. The $1/r_{\mathrm{Amtj}}$ dependence is consistent with a tunnel-conductance-dependent spin-pumping-like action, although the spin-pumping mixing conductance originating from the $r_{\mathrm{Amtj}}$ of a magnetic tunnel interface is orders of magnitudes below that of the natural damping of the free layer. We postulate that the spin-torque-induced dynamics in these magnetic tunnel junctions must involve high-frequency spin dynamics near the tunnel interface beyond the average free-layer dynamics frequencies. Such interface high-energy process gives rise to a two-step process for spin-current transmission into the free layer and causes visible spin-pumping spin-current loss across a tunnel barrier.

The formation of coplanar spin spirals is a common motif in the magnetic ordering of many frustrated magnets. For classical antiferromagnets, geometric frustration can lead to a massively degenerate ground state manifold of spirals whose propagation vectors can be described, depending on the lattice geometry, by points (triangular), lines (fcc), surfaces (frustrated diamond), or completely flat bands (pyrochlore). Here we demonstrate an exact mathematical correspondence of these spiral manifolds of classical antiferromagnets with the Fermi surfaces of free-fermion band structures. We provide an explicit lattice construction relating the frustrated spin model to a corresponding free-fermion tight-binding model. Examples of this correspondence relate the ${120}^{\circ }$ order of the triangular lattice antiferromagnet to the Dirac nodal structure of the honeycomb tight-binding model or the spiral line manifold of the fcc antiferromagnet to the Dirac nodal line of the diamond tight-binding model. We discuss implications of topological band structures in the fermionic system for the corresponding classical spin system.

We study an odd-parity magnetic multipole order in ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Mn}}_2{\mathrm{As}}_2$ and related materials. Although ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ is a seemingly conventional Mott insulator with $\mathrm{G}$-type antiferromagnetic order, we identify the ground state as a magnetic hexadecapole ordered state accompanied by simultaneous time-reversal and space-inversion symmetry breaking. A symmetry argument and microscopic calculations reveal the ferroic ordering of leading magnetic hexadecapole moment and admixed magnetic quadrupole moment. Furthermore, we clarify electromagnetic responses characterizing the magnetic hexadecapole state of semiconducting ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ and doped metallic systems. A magnetoelectric effect and antiferromagnetic Edelstein effect are shown. Interestingly, a counterintuitive current-induced nematic order occurs in the metallic state. The electric current along the $z$ axis induces the $xy$-plane nematicity in sharp contrast to the spontaneous nematic order in superconducting Fe-based 122 compounds. Thus, the magnetic hexadecapole state of doped ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ is regarded as a magnetopiezoelectric metal. Other candidate materials for magnetic hexadecapole order are proposed.

When a fermionic quantum Hall system is projected into the lowest Landau level, there is an exact particle-hole symmetry between filling fractions $\nu$ and $1-\nu$. We investigate whether a similar symmetry can emerge in bosonic quantum Hall states, where it would connect states at filling fractions $\nu$ and $2-\nu$. We begin by showing that the particle-hole conjugate to a composite fermion “Jain state” is another Jain state, obtained by reverse flux attachment. We show how information such as the shift and the edge theory can be obtained for states which are particle-hole conjugates. Using the techniques of exact diagonalization and infinite density matrix renormalization group, we study a system of two-component (i.e., spinful) bosons, interacting via a $\delta$-function potential. We first obtain real-space entanglement spectra for the bosonic integer quantum Hall effect at $\nu =2$, which plays the role of a filled Landau level for the bosonic system. We then show that at $\nu =4/3$ the system is described by a Jain state which is the particle-hole conjugate of the Halperin (221) state at $\nu =2/3$. We show a similar relationship between nonsinglet states at $\nu =1/2$ and $3/2$. We also study the case of $\nu =1$, providing unambiguous evidence that the ground state is a composite Fermi liquid. Taken together our results demonstrate that there is indeed an emergent particle-hole symmetry in bosonic quantum Hall systems.

We use scanning tunneling microscopy to investigate the interactions between atomic chains, of two different types, formed by the adsorption of submonolayer Au onto a stepped Si surface. The first chain consists of a double row of Au atoms. The second is a single row of Si dangling bonds at the edges of the steps. The two chains are interspersed and hence each could, in principle, influence the other structurally as well as electronically. However, we find this interaction to be highly unidirectional: The Au chains modulate the Si chains, breaking their parity and lending them directionality, while the Si chains leave the Au chains unaffected.

The antiferromagnetic Ising chain in both transverse and longitudinal magnetic fields is one of the paradigmatic models of a quantum phase transition. The antiferromagnetic system exhibits a zero-temperature critical line separating an antiferromagnetic phase and a paramagnetic phase; the critical line connects an integrable quantum critical point at zero longitudinal field and a classical first-order transition point at zero transverse field. Using a strong-disorder renormalization group method formulated as a tree tensor network, we study the zero-temperature phase of the quantum Ising chain with bond randomness. We introduce a new matrix product operator representation of high-order moments, which provides an efficient and accurate tool for determining quantum phase transitions via the Binder cumulant of the order parameter. Our results demonstrate an infinite-randomness quantum critical point in zero longitudinal field accompanied by pronounced quantum Griffiths singularities, arising from rare ordered regions with anomalously slow fluctuations inside the paramagnetic phase. The strong Griffiths effects are signaled by a large dynamical exponent $z>1$, which characterizes a power-law density of low-energy states of the localized rare regions and becomes infinite at the quantum critical point. Upon application of a longitudinal field, the quantum phase transition between the paramagnetic phase and the antiferromagnetic phase is completely destroyed. Furthermore, quantum Griffiths effects are suppressed, showing $z<1$, when the dynamics of the rare regions is hampered by the longitudinal field.

A solid-state system combining a stable spin degree of freedom with an efficient optical interface is highly desirable as an element for integrated quantum-optical and quantum-information systems. We demonstrate a bright color center in diamond with excellent optical properties and controllable electronic spin states. Specifically, we carry out detailed optical spectroscopy of a germanium-vacancy ($\mathrm{GeV}$) color center demonstrating optical spectral stability. Using an external magnetic field to lift the electronic spin degeneracy, we explore the spin degree of freedom as a controllable qubit. Spin polarization is achieved using optical pumping, and a spin relaxation time in excess of $20\mu \mathrm{s}$ is demonstrated. We report resonant microwave control of spin transitions, and use this as a probe to measure the Autler-Townes effect in a microwave-optical double-resonance experiment. Superposition spin states were prepared using coherent population trapping, and a pure dephasing time of about $19\mathrm{ns}$ was observed at a temperature of 2.0 K.

Confinement is a process by which particles with fractional quantum numbers bind together to form quasiparticles with integer quantum numbers. The constituent particles are confined by an attractive interaction whose strength increases with increasing particle separation and, as a consequence, individual particles are not found in isolation. This phenomenon is well known in particle physics where quarks are confined in baryons and mesons. An analogous phenomenon occurs in certain spatially anisotropic magnetic insulators. These can be thought of in terms of weakly coupled chains of spins $S=1/2$, and a spin flip thus carries integer spin $S=1$. The collective excitations in these systems, called spinons, turn out to carry fractional spin quantum number $S=1/2$. Interestingly, at sufficiently low temperatures the weak coupling between chains can induce an attractive interaction between pairs of spinons that increases with their separation and thus leads to confinement. In this paper, we employ inelastic neutron scattering to investigate the spinon-confinement process in the quasi-one-dimensional, spin-1/2 antiferromagnet with Heisenberg-Ising (XXZ) anisotropy ${\mathrm{SrCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$. A wide temperature range both above and below the long-range ordering temperature $T_N=5.2$ K is explored. Spinon excitations are observed above $T_N$ in quantitative agreement with established theory. Below $T_N$ pairs of spinons are confined and two sequences of meson-like bound states with longitudinal and transverse polarizations are observed. Several theoretical approaches are used to explain the data. These are based on a description in terms of a one-dimensional, $S=1/2$ XXZ antiferromagnetic spin chain, where the interchain couplings are modeled by an effective staggered magnetic mean field. A wide range of exchange anisotropies are investigated and the parameters specific to ${\mathrm{SrCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ are identified. Recently developed theoretical technique based on tangent-space matrix product states gives a very complete description of the data and provides good agreement not only with the energies of the bound modes but also with their intensities. We also successfully explain the effect of temperature on the excitations including the experimentally observed thermally induced resonance between longitudinal modes below $T_N$ and the transitions between thermally excited spinon states above $T_N$. In summary, our work establishes ${\mathrm{SrCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ as a beautiful paradigm for spinon confinement in a quasi-one-dimensional quantum magnet and provides a comprehensive picture of this process.

In most semiconductors and insulators the presence of a small density of charged impurities cannot be avoided, but their effect can be reduced by compensation doping, i.e., by introducing defects of opposite charge. Screening in such a system leads to the formation of electron-hole puddles, which dominate bulk transport, as first recognized by Efros and Shklovskii. Metallic surface states of topological insulators (TIs) contribute extra screening channels, suppressing puddles. We investigate the typical length ${\ell }_P$, which determines the distance between puddles and the suppression of puddle formation close to metallic surfaces in the limit where the gap $\mathrm{\Delta }$ is much larger than the typical Coulomb energy $E_c$ of neighboring dopants $\mathrm{\Delta }\gg E_c$. In particular, this is relevant for three-dimensional Bi-based topological insulators, where $\mathrm{\Delta }/E_c\sim 100$. Scaling arguments predict ${\ell }_P\sim {(\mathrm{\Delta }/E_c)}^2$. In contrast, we find numerically that ${\ell }_P$ is much smaller and grows in an extended crossover regime approximately linearly with $\mathrm{\Delta }/E_c$ for numerically accessible values $\mathrm{\Delta }/E_c\lesssim 35$. We show how a quantitative scaling argument can be used to extrapolate to larger $\mathrm{\Delta }/E_c$, where ${\ell }_P\sim {(\mathrm{\Delta }/E_c)}^2/ln(\mathrm{\Delta }/E_c)$. Our results can be used to predict a characteristic thickness of TI thin films, below which the sample quality is strongly enhanced.

We investigate the effects of disorder in Floquet topological insulators (FTIs) occurring in semiconductor quantum wells. Such FTIs are induced by resonantly driving a transition between the valence and conduction bands. We show that when disorder is added, the topological nature of such FTIs persists as long as there is a mobility gap at the resonant quasienergy. For strong enough disorder, this gap closes and all the states become localized as the system undergoes a transition to a trivial insulator. Interestingly, the effects of disorder are not necessarily adverse: we show that in the same quantum well, disorder can also induce a transition from a trivial to a topological system, thereby establishing a Floquet topological Anderson insulator (FTAI). We identify the conditions on the driving field necessary for observing such a transition.

We consider a theory of a two-component Dirac fermion localized on a (2+1)-dimensional brane coupled to a (3+1)-dimensional bulk. Using the fermionic particle-vortex duality, we show that the theory has a strong-weak duality that maps the coupling $e$ to $\tilde{e}=\left(8\pi \right)/e$. We explore the theory at $e^2=8\pi$ where it is self-dual. The electrical conductivity of the theory is a constant independent of frequency. When the system is at finite density and magnetic field at filling factor $\nu =\frac{1}{2}$, the longitudinal and Hall conductivity satisfies a semicircle law, and the ratio of the longitudinal and Hall thermal electric coefficients is completely determined by the Hall angle. The thermal Hall conductivity is directly related to the thermal electric coefficients.