The effect of hydrostatic pressure on antiferromagnetic ordering of Cr spins, magnetic compensation, and exotic spin switching in the single-crystal $\mathrm{GdCr}{\mathrm{O}}_3$ ferrimagnet is studied. The Néel temperature $T_{\mathrm{N}}=168\mathrm{K}$ increases under pressure with the rate of 0.42 K/kbar, and the compensation temperature $T_{\mathrm{comp}}=144\mathrm{K}$ at which the canted ferromagnetic moment of Cr spins and antiparallel polarized moment of Gd spins cancel each other also increases by 0.3 K/kbar. It was found that the spin switching temperature $T_{\mathrm{sw}}$, at which the ferromagnetic moment is reversed, noticeably increases under pressure, and the spin switching energy required for magnetization reversal reduces significantly because of a decrease in magnetic anisotropy. Due to this mechanism, the spin switching line, described in the $T$−$H$ diagram by a certain ratio of switching energy to canted ferromagnetic moment, is shifted under pressure towards higher temperatures. Thus, switching between two opposite spin configurations in the $\mathrm{GdCr}{\mathrm{O}}_3$ ferrimagnet can be controlled by both applied magnetic field and external pressure.

We study the energy and spin transport of the classical spin liquid hosted by the pyrochlore Heisenberg antiferromagnet in the large $S$ limit. Molecular dynamics calculation suggests that both the energy and spin diffusion constants approach finite limits as the temperature tends to zero. We explain our results in terms of an effective disorder model, where the energy/spin-carrying normal modes propagate in a quasistatic disordered spin background. The finite zero temperature limits of the diffusion constants are then naturally understood as a result of the finite mean free path of the normal modes due to the effective disorder.

We construct the effective field theory for the $A$ phase of superfluid ${}^3\mathrm{He}$ up to the next-to-leading order in the derivative expansion. To this end, we gauge the internal global symmetries of the theory on curved space by introducing the background gauge fields and spatial metric so as to expose a hidden local symmetry known as nonrelativistic diffeomorphism. Nonrelativistic diffeomorphism is particularly useful to yield an additional constraint on the effective field theory and reveal a universal expression for the Hall viscosity in the $A$ phase. We find it five orders of magnitude larger than that in the $B$ phase under a magnetic field so that its experimental observation is more feasible by measuring the induced elliptic polarization of sound waves.

We investigate the electronic band structure of graphene on a series of two-dimensional hexagonal nitride insulators $\mathrm{h}X\mathrm{N}, X=\text{B}$, Al, and Ga, with first-principles calculations. A symmetry-based model Hamiltonian is employed to extract orbital parameters and spin-orbit coupling (SOC) from the low-energy Dirac bands of the proximitized graphene. While commensurate hBN induces a staggered potential of about 10 meV into the Dirac band structure, less lattice-matched hAlN and hGaN disrupt the Dirac point much less, giving a staggered gap below 100 $\mu \mathrm{eV}$. Proximitized intrinsic SOC surprisingly does not increase much above the pristine graphene value of 12 $\mu \mathrm{eV}$; it stays in the window of 1–16 $\mu \mathrm{eV}$, depending strongly on stacking. However, Rashba SOC increases sharply when increasing the atomic number of the boron group, with calculated maximal values of 8, 15, and 65 $\mu \mathrm{eV}$ for B-, Al-, and Ga-based nitrides, respectively. The individual Rashba couplings also depend strongly on stacking, vanishing in symmetrically sandwiched structures, and can be tuned by a transverse electric field. The extracted spin-orbit parameters were used as input for spin transport simulations based on Chebyshev expansion of the time-evolution of the spin expectation values, yielding interesting predictions for the electron spin relaxation. Spin lifetime magnitudes and anisotropies depend strongly on the specific ($\mathrm{h}X\mathrm{N}$)/graphene/$\mathrm{h}X\mathrm{N}$ system, and they can be efficiently tuned by an applied external electric field as well as the carrier density in the graphene layer. A particularly interesting case for experiments is graphene/hGaN, in which the giant Rashba coupling is predicted to induce spin lifetimes of 1–10 ns, short enough to dominate over other mechanisms, and lead to the same spin relaxation anisotropy as that observed in conventional semiconductor heterostructures: 50%, meaning that out-of-plane spins relax twice as fast as in-plane spins.

We investigate the current flow of microcavity polariton condensates loaded into concentric ring-shaped potentials. The tunneling of the condensates between different potential rings results in different phase-locked states, depending on the separation of the potential rings. As a consequence, the condensate currents in different rings can flow either in the same or opposite direction, depending on the specific configuration of the ring-shaped potentials. In two concentric standard ring-shaped potentials, the condensates always circulate in the same direction (coflowing current), and the vortices formed in the two rings share the same topological charge because of the azimuthally uniform distribution of their phase difference. In this case, increasing the number of the potential rings enables the excitation of Bessel-like solutions. If the two ring-shaped potentials are engineered into an eye shape, with the inner ring being standard ring-shaped and the outer ring being elliptically ring-shaped, the phase differences of the condensates in the two rings along the major and minor axes of the ellipse can be opposite, which gives rise to counterflowing condensate currents.

Subwavelength plasmonic wires provide a very simple, ultracompact, and efficient platform for active plasmonics. We theoretically investigate magnetically switched transparency in subwavelength semiconductor wires. The main difficulty limiting the performance of magnetoplasmonic devices and subwavelength optical isolators is the need to use a strong magnetic field. We reveal that the required magnetic field intensity in subwavelength semiconductor wires is twice higher than that in planar semiconductor interfaces and is determined by the carrier mobility of the constituent semiconductor and does not directly depend on other material parameters such as the coating dielectric permittivity, the semiconductor carrier density, and the electron's effective mass. The azimuthal magnetic field can be excited by a direct electric current or by an optical pump through the semiconductor wires. Our finding points to the important limitations on the performance of active plasmonic devices based on magneto-optical transparency in the plasmonic wires.

We demonstrate by time-resolved resonance fluorescence measurements on a single self-assembled quantum dot an internal photoeffect that emits electrons from the dot by an intraband excitation. We find a linear dependence of the optically generated emission rate on the excitation intensity and use a rate equation model to deduce the involved rates. The emission rate is tunable over several orders of magnitude by adjusting the excitation intensity. Our findings show that a process that is well known in single atom spectroscopy (i.e., photoionization) can also be observed in the solid state. The results also quantify an important, but mostly neglected, mechanism that may fundamentally limit the coherence times in solid-state quantum optical devices.

We report a soft x-ray resonant magnetic scattering study of the spin configuration in multiferroic thin films of ${\mathrm{Co}}_{0.975}{\mathrm{Ge}}_{0.025}{\mathrm{Cr}}_2{\mathrm{O}}_4$ (Ge-CCO) and ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ (CCO) under low and high magnetic fields from 0.2 to 6.5 T. A characterization of Ge-CCO at a low magnetic field was performed, and the results were compared with those of pure CCO. The ferrimagnetic phase transition temperature $T_C\approx 95\mathrm{K}$ and the multiferroic transition temperature $T_S\approx 27\mathrm{K}$ in Ge-CCO are comparable with those observed in CCO. In Ge-CCO, the ordering wave vector ($qq0$) observed below $T_S$ is slightly larger compared with that of CCO, and unlike CCO, the diffraction intensity consists of two contributions that show a dissimilar x-ray polarization dependence. In Ge-CCO, the coercive field observed at low temperatures was larger than the one reported for CCO. In both compounds, an unexpected reversal of the spiral helicity, and therefore the electric polarization, was observed on simply magnetic field cooling. In addition, we find a change in the helicity as a function of momentum transfer in the magnetic diffraction peak of Ge-CCO, indicative of the presence of multiple magnetic spirals.

In a metal, electron interactions can cause a perfect spin polarization of the Fermi surface. For such a phase, only half of the noninteracting Fermi surface is available, and thus this state is commonly referred to as a “half metal.” Here we argue that in multiband electronic systems with nesting, further “fractionalization” of the Fermi surface is possible. Taking the AA bilayer graphene as a convenient test case, we demonstrate that under suitable conditions imposed on the electron interactions, doped AA bilayer graphene can host a “quarter-metal” phase. Its Fermi surface (Fermi contour) is simultaneously polarized relative to spin-related and valley-related operators. The resultant state is nematic and possesses peculiar transport properties: the electric current carries both valley-related and spin-related quanta. In addition, since the two polarizations can be controlled independently of each other, the quarter metal is a promising candidate for spintronics and valleytronics applications. Other types of Fermi-surface fractionalization are also discussed.

Graphene and bilayer graphene quantum dots are promising hosts for spin qubits with long coherence times. Although recent technological improvements make it possible to confine single electrons electrostatically in bilayer graphene quantum dots and their spin and valley texture of the single-particle spectrum has been studied in detail, their relaxation dynamics remains still unexplored. Here, we report on transport through a high-frequency gate-controlled single-electron bilayer graphene quantum dot. By transient current spectroscopy of single-electron spin states, we extract a lower bound of the spin relaxation time of 0.5 $\mu \mathrm{s}$. This result represents an important step towards the investigation of spin coherence times in graphene-based quantum dots and the implementation of spin qubits.

We study the effects of disorder on semi-infinite Weyl and Dirac semimetals where the presence of a boundary leads to the formation of either Fermi arcs/rays or Dirac surface states. Using a local version of the self-consistent Born approximation, we calculate the profile of the local density of states and the surface group velocity. This allows us to explore the full phase diagram as a function of boundary conditions and disorder strength. While in all cases we recover the sharp criticality in the bulk, we unveil a critical behavior at the surface of Dirac semimetals, which is smoothed out by Fermi arcs in Weyl semimetals.

Negative thermal expansion (NTE) materials have been at the center of attention for the past few decades as thermal expansion compensators in the fields of engineering, photonics, electronics, and medicine. Numerous crystalline materials exhibit NTE, wherein a combination of positive and negative linear thermal expansion coefficients results from their highly anisotropic elasticity. In this study, we selected SnSe, an anisotropic uniaxial NTE material as a model system where theoretical studies have linked its NTE along the $c$ direction to transverse phonons and to positive Grüneisen parameters along all crystallographic axes. However, the fundamental origin of NTE in SnSe have not been experimentally resolved. Here we performed temperature-dependent resonant ultrasound spectroscopy (between 295–773 K) on single-crystalline SnSe to experimentally measure all nine independent elastic constants ($C_{11}, C_{22}, C_{33}, C_{44}, C_{55}, C_{66}, C_{12}, C_{13}, C_{23}$). Our data revealed a high degree of anisotropy in the temperature-dependent elastic constants with shear anisotropic factors show a contrasting pattern with increasing temperature. From this data we also deduced its material compressibility and negative Poisson's ratios in the major crystallographic directions that could explain its colossal linear thermal expansion coefficient along the $c$ direction, reaching $\sim –12\times {10}^{-5}{\mathrm{K}}^{-1}$ at 773 K as reported in this study. Furthermore, we confirmed positive Grüneisen parameters along all the crystallographic axes and observe that SnSe behaves like a semicompressible parallelepiped with elastically coupled $a$ and $b$ axes, with the NTE being driven by the displacement of Sn atoms in the $c$ direction.

We study ultrafast x-ray diffraction on the charge density wave (CDW) of ${\mathrm{SmTe}}_3$ using an x-ray free-electron laser. The high momentum and time resolution afforded by the x-ray laser enabled capturing fine wave-vector and time-dependent features of the CDW that originate from fast (in time) and sharp (in real space) variations of the CDW lattice distortion, which we attribute to an inversion of the order parameter. These domain inversions occur near the surface and are caused by the short penetration depth of the near-infrared pump with the wavelength centered at 800 nm, resulting in CDW domain walls perpendicular to the sample surface. These domain walls break the CDW long-range order on the scale of the x-ray probe depth, controlled experimentally by the x-ray incidence angle and suppress the diffraction intensity of the CDW for times much longer than the $\sim 1\text{−}\mathrm{ps}$ recovery of the electronic gap observed in time and angle-resolved photoemission spectroscopy. We model the spatial and temporal dependences of the order parameter using a simple Ginzburg-Landau model with all the parameters obtained from the published literature. We find reasonable agreement between the calculated and the measured diffraction across the momentum, time, fluence, and incidence angle dependence without adjusting any parameters. We reconstruct the spatial and temporal dependences of the lattice order parameter and find that at long times, depending on the pump fluence, multiple domain walls remain at distances of a few nanometers from the surface.

Prethermalization, by introducing emergent quasiconserved observables, plays a crucial role in protecting periodically driven (Floquet) many-body phases over an exponentially long time, while the ultimate fate of such quasiconserved operators can signal thermalization to infinite temperature. To elucidate the properties of prethermal quasiconservation in many-body Floquet systems, here we systematically analyze infinite-temperature correlations between observables. We numerically show that the late-time behavior of the autocorrelations unambiguously distinguishes quasiconserved observables from nonconserved ones, allowing one to single out a set of linearly independent quasiconserved observables. By investigating two Floquet spin models, we identify two different mechanisms underlying the quasiconservation law. First, we numerically verify energy quasiconservation when the driving frequency is large, so that the system dynamics is approximately described by a static prethermal Hamiltonian. More interestingly, under moderate driving frequency, another quasiconserved observable can still persist if the Floquet driving contains a large global rotation. We show theoretically how to calculate this conserved observable and provide numerical verification. Having systematically identified all quasiconserved observables, we can finally investigate their behavior in the infinite-time limit and thermodynamic limit, using autocorrelations obtained from both numerical simulation and experiments in solid-state nuclear magnetic resonance systems.

The electronic structure of the enigmatic iron-based superconductor FeSe has puzzled researchers since spectroscopic probes failed to observe the expected electron pocket at the $Y$ point in the 1-Fe Brillouin zone. It has been speculated that this pocket, essential for an understanding of the superconducting state, is either absent or incoherent. Here, we perform a theoretical study of the preferred nematic order originating from nearest-neighbor Coulomb interactions in an electronic model relevant for FeSe. We find that at low temperatures the dominating nematic components are of interorbital $d_{xz}\text{−}{d}_{xy}$ and $d_{yz}\text{−}{d}_{xy}$ character, with spontaneously broken amplitudes for these two components. This interorbital nematic order naturally leads to distinct hybridization gaps at the $X$ and $Y$ points of the 1-Fe Brillouin zone, and may thereby produce highly anisotropic Fermi surfaces with only a single electron pocket at one of these momentum-space locations. The associated superconducting gap structure obtained with the generated low-energy electronic band structure from spin-fluctuation mediated pairing agrees well with that measured experimentally. Finally, from a comparison of the computed spin susceptibility to available neutron scattering data, we discuss the necessity of additional self-energy effects, and explore the role of orbital-dependent quasiparticle weights as a minimal means to include them.

The electronic structure in unconventional superconductors holds a key to understanding the momentum-dependent pairing interactions and the resulting superconducting gap function. In superconducting Fe-based chalcogenides, there have been controversial results regarding the importance of the $k_z$ dependence of the electronic dispersion, the gap structure, and the pairing mechanisms. Here, we use density functional theory to investigate the underlying structural properties in combination with a sophisticated real-space treatment of magnetic disorder for the prototype system FeSe. Our calculations demonstrate that interlayer and intralayer interactions need to be considered and that charge-driven van der Waals interactions between Se atoms instead of magnetic coupling effects drive the interlayer binding. The methodological advances and physical insights are important for upcoming investigations of the three-dimensional effects, including nontrivial topology, of $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{Te}}_x$ and $\mathrm{Fe}{\mathrm{Se}}_{1-x}{\mathrm{S}}_x$ systems.

Hyperbolic dispersion enables unprecedented abilities for wave-field engineering which so far chiefly has been realized by man-made metamaterials. Recent classical explorations of topological media and semimetals suggest that these exotic structures may enable a novel route toward hyperbolic sound control. Here, we demonstrate that a three-dimensional acoustic semimetal implementation exhibits nodal lines of hyperbolic shape that are topologically protected due to translational symmetry. The structure comprises a cubic arrangement of crossed hollow channels whose hopping strengths are determined by their cross sections, which facilitate analytically exact prediction of tunable spin-Hall textures. Interestingly, thanks to these nodal signatures of hyperbolic shape, we are able to acquire an array of unusual emission features, spanning from directional collimation to either horizontally or vertically split radiation. We foresee that our findings will provide remarkable opportunities for advanced wave control with hyperbolic and topological attributes.

We present evidence of an unprecedented room temperature positive exchange bias (EB) and hysteresis loop inversion in monolithic ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ films grown epitaxially on a (111)-oriented ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ crystal. The growth-induced interfacial ${\mathrm{Gd}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ layer couples antiparallel to the ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ layer and leads to positive EB and in-turn loop inversion. An exchange shift as large as $H_{EB}\sim 30H_C$ is realized at 300 K. We observe a critical field value of $H_{CF}\sim 600$ Oe, above which the hysteresis loop inversion takes place. Our findings may have strong implication for spintronics device applications.

A family of layered manganese-pnictide antiferromagnets ${\mathrm{BaMn}}_2{Pn}_2$ ($\mathit{Pn}$ stands for P, As, Sb, and Bi) has been recently shown to host anomalously large magnetoresistance (MR) with both positive and negative MR components. In search for the microscopic picture of MR in this family, we here report a local-probe ${}^{55}\mathrm{Mn}$ nuclear magnetic resonance (NMR) study. The zero-field NMR spectra and the temperature dependence of the spin-lattice relaxation rates are fully consistent with the proposed G-type antiferromagnetic order. However, a close inspection of the ${}^{55}\mathrm{Mn}$ NMR spectra reveals a fine structure, which is due to the weakly localized charge carriers. As these carriers localize and segregate in the presence of electron correlations, they also undergo collective spin fluctuations which freeze-out at low temperatures and thus contribute to the ${}^{55}\mathrm{Mn}$ spin-lattice relaxation. The characteristic temperatures of the appearance of these additional features in the magnetic response probed by ${}^{55}\mathrm{Mn}$ NMR correlate well with the anomalies in the resistivity and anomalously large MR, which hints that the two phenomena are connected.