${\mathrm{YbCo}}_2{\mathrm{Ge}}_4$ is a clean paramagnetic Kondo lattice which displays non-Fermi-liquid behavior. We report a detailed investigation of the specific heat, magnetic Grüneisen parameter (${\mathrm{\Gamma }}_{\mathrm{mag}}$), and temperature derivative of the magnetization ($M$) on a high-quality single crystal at temperatures down to $0.1$ K and magnetic fields up to 7 T. ${\mathrm{\Gamma }}_{\mathrm{mag}}$ and $dM/dT$ display a divergence upon cooling and obey $T/B$ scaling. Similar behavior has previously been found in several other Yb-based Kondo lattices and related to a zero-field quantum critical point without fine tuning of pressure or composition. However, in the approach of $B\to 0$ the electronic heat capacity coefficient of ${\mathrm{YbCo}}_2{\mathrm{Ge}}_4$ saturates at low $T$, excluding ferromagnetic quantum criticality. This indicates that $T/B$ scaling is insufficient to prove a zero-field quantum critical point.

We present, theoretically and experimentally, the observation of the Rashba effect in photonic lattices, where the effect is brought about by an artificial gauge field, induced by the geometry of the system. In doing that, we demonstrate a particular form of coupling between pseudospin and momentum, resulting in spin-dependent shifts in the spectrum. Our system consists of two coupled, oppositely tilted waveguide arrays, where the evolution of an optical beam allows for probing the dynamics of the evolving wave packets, and the formation of spectral splitting. We show that the Rashba effect can be amplified or decreased through optical nonlinear effects, which correspond to mean-field interactions in various systems such as cold-atom lattices and exciton-polariton condensates.

We report the relationship between epitaxial strain and the crystallographic orientation of the in-phase rotation axis and $A$-site displacements in Pbnm-type perovskite films. Synchrotron diffraction measurements of ${\mathrm{EuFeO}}_3$ films under strain states ranging from 2% compressive to 0.9% tensile on cubic or rhombohedral substrates exhibit a combination of $a^{-}{a}^{+}{c}^{-}$ and $a^{+}{a}^{-}{c}^{-}$ rotational patterns. We compare the ${\mathrm{EuFeO}}_3$ behavior with previously reported experimental and theoretical work on strained Pbnm-type films on nonorthorhombic substrates, as well as additional measurements from ${\mathrm{LaGaO}}_3,{\mathrm{LaFeO}}_3$, and ${\mathrm{Eu}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ films on ${\mathrm{SrTiO}}_3$. Compiling the results from various material systems reveals a general strain dependence in which compressive strain strongly favors $a^{-}{a}^{+}{c}^{-}$ and $a^{+}{a}^{-}{c}^{-}$ rotation patterns and tensile strain weakly favors $a^{-}{a}^{-}{c}^{+}$ structures. In contrast, ${\mathrm{EuFeO}}_3$ films grown on Pbnm-type ${\mathrm{GdScO}}_3$ under 2.3% tensile strain take on a uniform $a^{-}{a}^{+}{c}^{-}$ rotation pattern imprinted from the substrate, despite strain considerations that favor the $a^{-}{a}^{-}{c}^{+}$ pattern. These results point to the use of substrate imprinting as a more robust route than strain for tuning the crystallographic orientations of the octahedral rotations and $A$-site displacements needed to realize rotation-induced hybrid improper ferroelectricity in oxide heterostructures.

Conventional wisdom expects that making semiconductors ferromagnetic requires doping with magnetic ions and that superconductivity cannot coexist with magnetism. However, recent concerted efforts exploring new classes of materials have established that intrinsic ferromagnetic semiconductors exist and that certain types of strongly correlated metals can be ferromagnetic and superconducting at the same time. Here we show that the trifecta of semiconducting behavior, ferromagnetism, and superconductivity can be achieved in a single material. Samarium nitride (SmN) is a well-characterized intrinsic ferromagnetic semiconductor, hosting strongly spin-ordered $4f$ electrons below a Curie temperature of 27 K. We have now observed that it also hosts a superconducting phase below 4 K when doped to electron concentrations above ${10}^{21}{\mathrm{cm}}^{-3}$. The large exchange splitting of the conduction band in SmN favors equal-spin triplet pairing with $p$-wave symmetry. Significantly, superconductivity is enhanced in superlattices of gadolinium nitride (GdN) and SmN. An analysis of the robustness of such a superconducting phase against disorder leads to the conclusion that the $4f$ bands are crucial for superconductivity, making SmN a heavy-fermion-type superconductor.

The Schwinger boson theory of the frustrated square lattice antiferromagnet yields a stable, gapped ${\mathbb{Z}}_2$ spin liquid ground state with time-reversal symmetry, incommensurate spin correlations, and long-range Ising-nematic order. We obtain an equivalent description of this state using fermionic spinons (the fermionic spinons can be considered to be bound states of the bosonic spinons and the visons). Upon doping, the ${\mathbb{Z}}_2$ spin liquid can lead to a fractionalized Fermi liquid (FL*) with small Fermi pockets of electronlike quasiparticles, while preserving the ${\mathbb{Z}}_2$ topological and Ising-nematic orders. We describe a Higgs transition out of this deconfined metallic state into a confining superconducting state which is almost always of the Fulde-Ferrell-Larkin-Ovchinnikov type, with spatial modulation of the superconducting order.

We study both experimentally and using micromagnetic simulations how the spin excitation spectra of the vortex-state circular dots made of soft magnetic material change with the dot thickness $t$ in the range $t=20–80\mathrm{nm}$. It is found that in addition to higher-order gyrotropic modes which are nonuniform along the dot thickness and were observed earlier, azimuthal spin-wave modes having curled structure at the dot top and bottom faces appear in the spectrum when increasing the dot thickness. For the dot thickness $t>50\mathrm{nm}$ these “curled” modes become the lowest ones in the spin-wave excitation spectrum. It is also shown that all spin-wave modes with azimuthal index $m=\pm 1$ are hybridized with the vortex gyrotropic modes. However, while “common” azimuthal $(0,\pm 1)$ modes are hybridized with the main gyrotropic ${\mathrm{G}}_0$ mode and reveal large frequency splitting of their doublet, the curled modes can be hybridized with higher-order gyrotropic modes and the doublet frequency splitting vanishes with the dot thickness increase.

We have time resolved the spin-transfer-torque-induced switching in perpendicularly magnetized tunnel junctions of diameters from 50 to 250 nm in the subthreshold thermally activated regime. When the field and the spin torque concur to both favor the P to AP transition, the reversal yields monotonic resistance ramps that can be interpreted as a domain wall propagation through the device at velocities of the order of 17 to 30 nm/ns; smaller cells switch faster, and proportionally to their diameter. At the largest sizes, transient domain wall pinning can occasionally occur. When the field hinders the P to AP transition triggered by the spin torque, the P to AP switching is preceded by repetitive switching attempts, during which the resistance transiently increases until successful reversal occurs. At 50 nm, the P to AP switching proceeds reproducibly in 3 ns, with a monotonic featureless increase of the device resistance. In the reverse transition (AP to P), the variability of thermally activated reversal is not restricted to stochastic variations of incubation delays before the onset of reversal: several reversal paths are possible even in the smallest perpendicularly magnetized junctions. Besides, the nonuniform nature of the magnetic response seems still present at the nanoscale, with sometimes electrical signatures of strong disorder during the AP to P reversal. The AP to P transition is preceded by a strong instability of the AP states in devices larger than 100 nm. The resistance becomes extremely agitated before switching to P in a path yielding a slow (20 to 50 ns) and irregular increase of the conductance with substantial event-to-event variability. Unreversed bubbles of typical diameter 60 nm can persist a few additional microseconds in the largest junctions. The complexity of the AP to P switching is reduced but not suppressed when the junctions are downsized below 60 nm. The instability of the initial AP state is no longer detected but the other features are maintained. In the smallest junctions (50 nm) we occasionally observe much faster (sub-1 ns) AP to P switching events that could result from a macrospin process. We discuss the origin of the switching asymmetry and the size dependence, with an emphasis on the role of the nonuniformities of the stray field emanating from the reference layers of the tunnel junction, which affects the zones in which nucleation is favored.

In this study, we reanalyze the magnetic interactions in the Kitaev spin-liquid candidate materials ${\mathrm{Na}}_2{\mathrm{IrO}}_3,\alpha \text{−}{\mathrm{RuCl}}_3$, and $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ using nonperturbative exact diagonalization methods. These methods are more appropriate given the relatively itinerant nature of the systems suggested in previous works. We treat all interactions up to third neighbors on equal footing. The computed terms reveal significant long-range coupling, bond anisotropy, and/or off-diagonal couplings which we argue naturally explain the observed ordered phases in these systems. Given these observations, the potential for realizing the spin-liquid state in real materials is analyzed, and synthetic challenges are defined and explained.

We report neutron scattering experiments which reveal a large spin gap in the magnetic excitation spectrum of weakly-monoclinic double perovskite ${\mathrm{Sr}}_2{\mathrm{ScOsO}}_6$. The spin gap is demonstrative of appreciable spin-orbit-induced anisotropy, despite nominally orbitally-quenched $5d^3{\mathrm{Os}}^{5+}$ ions. The system is successfully modeled including nearest neighbor interactions in a Heisenberg Hamiltonian with exchange anisotropy. We find that the presence of the spin-orbit-induced anisotropy is essential for the realization of the type I antiferromagnetic ground state. This demonstrates that physics beyond the LS or JJ coupling limits plays an active role in determining the collective properties of $4d^3$ and $5d^3$ systems and that theoretical treatments must include spin-orbit coupling.

Motivated by the issue of particle-hole symmetry for the composite fermion Fermi sea at the half-filled Landau level, Son has made an intriguing proposal [Phys. Rev. X 5, 031027 (2015)] that composite fermions are Dirac particles. We ask what features of the Dirac-composite fermion theory and its various consequences may be reconciled with the well-established microscopic theory of the fractional quantum Hall effect and the $\frac{1}{2}$ state, which is based on nonrelativistic composite fermions. Starting from the microscopic theory, we derive the assertion of Son that the particle-hole transformation of electrons at filling factor $\nu =\frac{1}{2}$ corresponds to an effective time-reversal transformation (i.e., $\left\{{\mathit{k}}_j\right\}\to \{-{\mathit{k}}_j\}$) for composite fermions, and discuss how this connects to the absence of $2k_{\mathrm{F}}$ backscattering in the presence of a particle-hole symmetric disorder. By considering bare holes in various composite-fermion $\mathrm{\Lambda }$ levels (analogs of electronic Landau levels), we determine the $\mathrm{\Lambda }$ level spacing and find it to be very nearly independent of the $\mathrm{\Lambda }$ level index, consistent with a parabolic dispersion for the underlying composite fermions. Finally, we address the compatibility of the Chern-Simons theory with the lowest Landau level constraint, and find that the wave functions of the mean-field Chern-Simons theory, as well as a class of topologically similar wave functions, are surprisingly accurate when projected into the lowest Landau level. These considerations lead us to introduce a “normal form” for the unprojected wave functions of the $n/(2pn-1)$ states that correctly capture the topological properties even without lowest Landau level projection.

Recent work suggests that a sharp definition of “phase of matter” can be given for some quantum systems out of equilibrium, first for many-body localized systems with time-independent Hamiltonians and more recently for periodically driven or Floquet localized systems. In this work, we propose a classification of the finite Abelian symmetry-protected phases of interacting Floquet localized systems in one dimension. We find that the different Floquet phases correspond to elements of ${\text{Cl}}_G\times {\mathcal{A}}_G$, where ${\text{Cl}}_G$ is the undriven interacting classification, and ${\mathcal{A}}_G$ is a set of (twisted) one-dimensional representations corresponding to symmetry group $G$. We will address symmetry-broken phases in a subsequent paper C. W. von Keyserlingk and S. L. Sondhi, following paper, Phys. Rev. B 93, 245146 (2016).

Particle-vortex duality is a powerful theoretical tool that has been used to study bosonic systems. Here, we propose an analogous duality for Dirac fermions in 2+1 dimensions. The physics of a single Dirac cone is proposed to be described by a dual theory, ${\mathrm{QED}}_3$, with again a single Dirac fermion but coupled to a gauge field. This duality is established by considering two alternate descriptions of the three-dimensional topological insulator (TI) surface. The first description is the usual Dirac fermion surface state. The dual description is accessed via an electric-magnetic duality of the bulk TI coupled to a gauge field, which maps it to a gauged chiral topological insulator. This alternate description ultimately leads to a new surface theory, ${\mathrm{QED}}_3$, which provides a simple description of otherwise intractable interacting electronic states. For example, an explicit derivation of the T-Pfaffian state, a proposed surface topological order of the TI, is obtained by simply pair condensing the dual fermions. The roles of time-reversal and particle-hole symmetries are exchanged by the duality, which connects some of our results to a recent conjecture by Son on particle-hole symmetric quantum Hall states.

We study low-energy excitations in the quantum breathing pyrochlore antiferromagnet ${\mathrm{Ba}}_3{\mathrm{Yb}}_2{\mathrm{Zn}}_5{\mathrm{O}}_{11}$ by a combination of inelastic neutron scattering (INS) and thermodynamical property measurements. The INS spectra are quantitatively explained by spin-$\frac{1}{2}$ single-tetrahedron model having $XXZ$ anisotropy and Dzyaloshinskii-Moriya interactions. This model has a twofold degeneracy of the lowest-energy state per tetrahedron and well reproduces the magnetization curve at 0.5 K and heat capacity above 1.5 K. At lower temperatures, however, we observe a broad maximum in the heat capacity around 63 mK, demonstrating that a unique quantum ground state is selected due to extra perturbations with an energy scale smaller than the instrumental resolution of INS.

In three-dimensional (3D) topological materials, tuning of the bulk chemical potential is of crucial importance for observing their topological properties; for example, Weyl semimetals require chemical-potential tuning to the bulk Weyl nodes, while 3D topological insulators require tuning into the bulk band gap. Such tuning is often realized by compensation, i.e., by balancing the density of acceptors and donors. Here we show that in such a compensated 3D topological material, the possibility of local chemical-potential tuning is limited by the formation of self-organized charge puddles. The puddles arise from large fluctuations of the Coulomb potential of donors and acceptors. Their emergence is akin to the case of graphene, where charge puddles are already established as a key paradigm. However, there is an important difference: Puddles in graphene are simply dictated by the static distribution of defects in the substrate, whereas we find that puddles in 3D systems self-organize in a nontrivial way and show a strong temperature dependence. Such a self-organization is revealed by measurements of the optical conductivity of the bulk-insulating 3D topological insulator ${\mathrm{BiSbTeSe}}_2$, which pinpoints the presence of puddles at low temperatures as well as their surprising “evaporation” on a temperature scale of 30–40 K. The experimental observation is described semiquantitatively by Monte Carlo simulations. These show that the temperature scale is set by the Coulomb interaction between neighboring dopants and that puddles are destroyed by thermally activated carriers in a highly nonlinear screening process. This result indicates that understanding charge puddles is crucial for the control of the chemical potential in compensated 3D topological materials.

We report the magnetization, ac susceptibility, and specific heat of optically float-zoned single crystals of ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Si}$, $0.20\le x\le 0.50$. We determine the magnetic phase diagrams for all major crystallographic directions and cooling histories. After zero-field cooling, the phase diagrams resemble that of the archetypal stoichiometric cubic chiral magnet MnSi. Besides the helical and conical state, we observe a pocket of skyrmion lattice phase just below the helimagnetic ordering temperature. At the phase boundaries between these states evidence for slow dynamics is observed. When the sample is cooled in small magnetic fields, the phase pocket of skyrmion lattice may persist metastably down to the lowest temperatures. Taken together with the large variation in the transition temperatures, transition fields, and helix wavelength as a function of the composition, this hysteresis identifies ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Si}$ as an ideal material for future experiments exploring, for instance, the topological unwinding of the skyrmion lattice.

Quantum dots (QDs) are semiconductor nanostructures in which a three-dimensional potential trap produces an electronic quantum confinement, thus mimicking the behavior of single atomic dipole-like transitions. However, unlike atoms, QDs can be incorporated into solid-state photonic devices such as cavities or waveguides that enhance the light-matter interaction. A near unit efficiency light-matter interaction is essential for deterministic, scalable quantum-information (QI) devices. In this limit, a single photon input into the device will undergo a large rotation of the polarization of the light field due to the strong interaction with the QD. In this paper we measure a macroscopic ($\sim 6^{\circ }$) phase shift of light as a result of the interaction with a negatively charged QD coupled to a low-quality-factor ($Q\sim 290$) pillar microcavity. This unexpectedly large rotation angle demonstrates that this simple low-$Q$-factor design would enable near-deterministic light-matter interactions.

We have performed inelastic neutron scattering (INS) experiments to investigate the magnetic excitations in the weakly distorted face-centered-cubic (fcc) iridate double perovskites ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$ and ${\mathrm{La}}_2{\mathrm{MgIrO}}_6$, which are characterized by A-type antiferromagnetic ground states. The powder inelastic neutron scattering data on these geometrically frustrated $j_{\mathrm{eff}}=\frac{1}{2}$ Mott insulators provide clear evidence for gapped spin-wave excitations with very weak dispersion. The INS results and thermodynamic data on these materials can be reproduced by conventional Heisenberg-Ising models with significant uniaxial Ising anisotropy and sizeable second-neighbor ferromagnetic interactions. Such a uniaxial Ising exchange interaction is symmetry forbidden on the ideal fcc lattice, so that it can only arise from the weak crystal distortions away from the ideal fcc limit. This may suggest that even weak distortions in $j_{\mathrm{eff}}=\frac{1}{2}$ Mott insulators might lead to strong exchange anisotropies. More tantalizingly, however, we find an alternative viable explanation of the INS results in terms of spin models with a dominant Kitaev interaction. In contrast to the uniaxial Ising exchange, the highly directional Kitaev interaction is a type of exchange anisotropy which is symmetry allowed even on the ideal fcc lattice. The Kitaev model has a magnon gap induced by quantum order by disorder, while weak anisotropies of the Kitaev couplings generated by the symmetry lowering due to lattice distortions can pin the order and enhance the magnon gap. Our findings highlight how even conventional magnetic orders in heavy transition metal oxides may be driven by highly directional exchange interactions rooted in strong spin-orbit coupling.

We use ab initio coupled-cluster theory to compute the spectral function of the uniform electron gas at a Wigner-Seitz radius of $r_{\mathrm{s}}=4$. The coupled-cluster approximations we employ go significantly beyond the diagrammatic content of state-of-the-art $GW$ theory. We compare our calculations extensively to $GW$ and $GW$-plus-cumulant theory, illustrating the strengths and weaknesses of these methods in capturing the quasiparticle and satellite features of the electron gas. Our accurate calculations further allow us to address the long-standing debate over the occupied bandwidth of metallic sodium. Our findings indicate that the future application of coupled-cluster theory to condensed phase material spectra is highly promising.

We identify the leading processes of electron transport across finite-length segments of proximitized nanowires and build a quantitative theory of their two-terminal conductance. In the presence of spin-orbit interaction, a nanowire can be tuned across the topological transition point by an applied magnetic field. Due to a finite segment length, electron transport is controlled by the Coulomb blockade. Upon increasing of the field, the shape and magnitude of the Coulomb blockade peaks in the linear conductance are defined, respectively, by Andreev reflection, single-electron tunneling, and resonant tunneling through the Majorana modes emerging after the topological transition. Our theory provides the framework for the analysis of experiments with proximitized nanowires [such as reported in S. M. Albrecht et al., Nature (London) 531, 206 (2016)] and identifies the signatures of the topological transition in the two-terminal conductance.

By making use of a low-energy effective model of Weyl semimetals, we show that the Fermi arc transport is dissipative. The origin of the dissipation is the scattering of the surface Fermi arc states into the bulk of the semimetal. It is noticeable that the corresponding scattering rate is nonzero and can be estimated even in a perturbative theory, although in general the reliable calculations of transport properties necessitate a nonperturbative approach. Nondecoupling of the surface and bulk sectors in the low-energy theory of Weyl semimetals invalidates the usual argument of a nondissipative transport due to one-dimensional arc states. This property of Weyl semimetals is in drastic contrast to that of topological insulators, where the decoupling is protected by a gap in the bulk. Within the framework of the linear response theory, we obtain an approximate result for the conductivity due to the Fermi arc states and analyze its dependence on chemical potential, temperature, and other parameters of the model.