Quantum-well (QW) devices have been extensively investigated in semiconductor structures. More recently, spin-polarized QWs were integrated into magnetic tunnel junctions (MTJs). In this Letter, we demonstrate the spin-based control of the quantized states in iron $3d$-band QWs, as observed in experiments and theoretical calculations. We find that the magnetization rotation in the Fe QWs significantly shifts the QW quantization levels, which modulate the resonant-tunneling current in MTJs, resulting in a tunneling anisotropic magnetoresistance (TAMR) effect of QWs. This QW-TAMR effect is sizable compared with other types of TAMR effect, and it is present above room temperature. In a QW MTJ of $\mathrm{Cr}/\mathrm{Fe}/{\mathrm{MgAl}}_2{\mathrm{O}}_4/\mathrm{top}$ electrode, where the QW is formed by a mismatch between Cr and Fe in the $d$ band with ${\mathrm{\Delta }}_1$ symmetry, a QW-TAMR ratio of up to 5.4% was observed at 5 K, which persisted to 1.2% even at 380 K. The magnetic control of QW transport can open new applications for spin-coupled optoelectronic devices, ultrathin sensors, and memory.

We report the ultrafast optical pump-probe spectroscopy measurements on the recently discovered quantum critical ferromagnet ${\mathrm{CeRh}}_6{\mathrm{Ge}}_4$. Our experimental results reveal the two-stage development of the hybridization between localized $f$ moments and conduction electrons with lowering temperature, as evidenced by (1) the presence of hybridization fluctuation for temperatures from $\sim 85\mathrm{K}$ ($T^{*}$) to $\sim 140\mathrm{K}$ ($T^{†}$), and (2) the emergence of collective hybridization below the coherence temperature, $T^{*}$, marked by the opening of an indirect gap of $2\mathrm{\Delta }\approx 12\mathrm{meV}$. We also observe three coherent phonon modes being softened anomalously below $T^{*}$, reflecting directly their coupling with the emergent coherent heavy electrons. Our findings establish the universal nature of fluctuation and coherence during the hybridization process in heavy fermion systems.

We determine the phase diagram of ${\mathrm{LaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{AsO}$ single crystals by using nuclear magnetic resonance (NMR). Up to a nominal doping of $x=0.03$, it follows the phase diagram for F-doped polycrystals. Above $x=0.03$, the F-doped samples become superconducting, whereas for Co doping the structural and magnetic transitions can be observed up to $x=0.042$, and superconductivity occurs only for higher doping levels and with reduced transition temperatures. For dopings up to $x=0.056$, we find evidence for short-range magnetic order. By means of relaxation-rate measurements, we map out the magnetic fluctuations that reveal the interplay of nematicity and magnetism. Above the nematic ordering, the spin fluctuations in ${\mathrm{LaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{AsO}$ are identical to those in $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$, suggesting that nematicity in LaFeAsO is a result of the fluctuating spin density wave as well.

We consider exciton-polaritons in a honeycomb lattice of micropillars subjected to circularly polarized $({\sigma }_{\pm }$) incoherent pumps, which are arranged to form two domains in the lattice. We predict that the nonlinear interaction between the polaritons and the reservoir excitons gives rise to the topological valley Hall effect where in each valley two counterpropagating helical edge modes appear. Under a resonant pump, ${\sigma }_{\pm }$ polaritons propagate in different directions without being reflected around bends. The polaritons propagating along the interface have extremely high effective lifetimes and show fair robustness against disorder. This paves the way for robust exciton-polariton spin separating and transporting channels in which polaritons attain and maintain high degrees of spin polarization, even in the presence of spin relaxation.

We report the observation of the Barnett field, rotational Doppler effect, and Berry phase using the rotating nuclear quadrupole resonance (NQR) method. We have developed coil-spinning techniques that enable us to systematically study the effects of rotation in setups involving rotation of the signal detector, rotation of the sample, and simultaneous rotation of both the signal detector and sample. Applying these setups to NQR measurements, we observe NQR line splittings in which the spectral structures are clearly distinct among the setups. By analyzing these structures, we clarify the origin of the NQR line splittings and discuss the relationship between the rotational Doppler effect, Barnett field effect, and Berry phase in terms of the rotational degrees of freedom, such as the relative rotation and the sample rotation itself, and the observation frame of reference. We also provide clear evidence of the difference between the rotational Doppler effect and the Barnett field and the equivalence of the Barnett field and the Berry phase.

We report the robustness of the intrinsic spin Hall effect in Pt against a change in the lattice constant. We measure spin-torque ferromagnetic resonance for $\mathrm{Pt}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ bilayers, where the lattice constant and carrier scattering time of the Pt layer are varied by incorporating the nitrogen atoms as interstitial impurities. We find that the spin Hall angle is enhanced by a factor of 3 as the resistivity of the Pt layer is increased by the nitrogen incorporation. The variation of the spin Hall conductivity is consistent with the prediction of the crossover of the intrinsic spin Hall effect from the moderately dirty regime to the dirty-metal regime. This result demonstrates the variation of the intrinsic spin Hall conductivity in Pt is dominated by the change in the scattering time, rather than the change in the band structure, under a few percent change in the lattice constant.

We study the nonequilibrium dynamics of a homogeneous quantum Ising ring after a quench, in which the transverse field $g$ suddenly changes from zero to a nonzero value. The long-timescale reduced dynamics of a single spin and of two nearest-neighbor spins, which involves the evaluation of expectation values of odd operators that break the fermion parity, is exactly obtained for finite-size but large rings through the use of a recently developed Pfaffian method [N. Wu, Phys. Rev. E 101, 042108 (2020)]. Time dependence of the transverse and longitudinal magnetizations (${\langle {\sigma }_j^z\rangle }_t$ and ${\langle {\sigma }_j^{x,y}\rangle }_t$), single-spin purity, expectation value of the string operator $X_j={\prod }_{l=1}^{j-1}{\sigma }_l^z{\sigma }_j^x$ (${\langle X_j\rangle }_t$), several equal-time two-site correlators (${\langle {\sigma }_j^{x,z}{\sigma }_{j+1}^{x,z}\rangle }_t$, ${\langle {\sigma }_j^x{\sigma }_{j+1}^y\rangle }_t$, and ${\langle {\sigma }_j^x{\sigma }_{j+1}^z\rangle }_t$), and pairwise concurrence after quenches to different phases are numerically studied. Our main findings are that (i) The expectation value of a generic odd operator approaches zero in the long-time limit; (ii) ${\langle X_j\rangle }_t$ exhibits $j$-independent exponential decay for a quench to $g=1$ and the time at which ${\langle X_j\rangle }_t$ reaches its first maximum scales linearly with $j$; (iii) The single-spin purity dynamics is mainly controlled by ${\langle {\sigma }_j^x\rangle }_t$ (${\langle {\sigma }_j^z\rangle }_t$) for a quench to $g<1$ ($g\ge 1$). For quenches to the disordered phase with $g\gg 1$, the single-spin tends to be in the maximally mixed state and the transverse and longitudinal correlators ${\langle {\sigma }_j^z{\sigma }_{j+1}^z\rangle }_t$ and ${\langle {\sigma }_j^x{\sigma }_{j+1}^x\rangle }_t$, respectively, approaches $-0.25$ and $0.5$ in the thermodynamic limit; (iv) The nearest-neighbor entanglement acquires a finite plateau value that increases with increasing $g$, and approaches a saturated value $\sim 0.125$ for $g\gg 1$.

We study laser-induced torques in the antiferromagnet ${\mathrm{Mn}}_2\mathrm{Au}$. We find that even linearly polarized light may induce laser-induced torques in ${\mathrm{Mn}}_2\mathrm{Au}$; that is, the light does not have to be circularly polarized. The laser-induced torques in ${\mathrm{Mn}}_2\mathrm{Au}$ are comparable in magnitude to those in the ferromagnets Fe, Co, and FePt at optical frequencies. We also compute the laser-induced torques at terahertz (THz) frequencies and compare them to the spin-orbit torques (SOTs) excited by THz laser pulses. We find the SOTs are dominant at THz frequencies for the laser-field strengths used in experiments. Additionally, we show that the matrix elements of the spin-orbit interaction (SOI) can be used to add SOI only during the Wannier interpolation, which we call Wannier interpolation of SOI. This technique allows us to perform the Wannier interpolation conveniently for many magnetization directions from a single set of Wannier functions.

The impressive absence of the skyrmion Hall effect of antiferromagnetic (AFM) skyrmions in clean systems has raised high expectations of AFM skyrmions as information carriers and the potential applications in data storage technology. Here, we report an undesired behavior of current-driven AFM skyrmion dynamics in granular thin films. Understanding the motion of AFM skyrmions in disordered systems is crucial because disorders are inevitable in all materials. Micromagnetic simulations show that AFM skyrmions deviate from their trajectory in the clean limit. Interestingly, the transverse (to the current direction) motion is diffusive, i.e., the mean square of transverse displacement is proportional to the traveling time. The longitudinal (along with the current direction) motion is always hindered by disorders. An effective theory based on the stochastic Thiele equation can account for the observed phenomena. At a very strong disorder above a critical value that depends on the current density, AFM skyrmions are pinned. Our findings should be important for future spintronic devices based on AFM skyrmions.

The application of pressure has been speculated to boost the search for high-temperature superconductors, especially in superhydrides. However, the applied pressure as high as hundreds of GPa needed to create superconductivity in those materials limits their technological application. Finding a route to achieve the high-temperature superconductivity at near-ambient conditions is attractive. By choosing a phase-change alloy ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$, we study the phase evolution of this material with pressure from the trigonal phase through the amorphous to the body-centered cubic one by the measurements of x-ray diffraction, Raman scattering, resistivity, and Hall coefficient. Superconductivity is observed to take place in the last two phases and can maintain at nearly ambient pressure in the decompression run. Pressure-induced disorder is found to be the key for holding superconductivity in the compressed phase-change alloy.

The high-pressure behavior and polymorphism of gold have attracted strong recent interest from theory as well as from both static and dynamic experiments. In this study, gold was dynamically ramp-compressed to 690 GPa via laser ablation using the Omega Laser Facility (University of Rochester) in order to investigate the stability regions of this noble metal's polymorphs. Stresses were determined from measurements of surface or interface velocities by laser interferometry. In situ x-ray diffraction was carried out to constrain the structure and density. The ambient face-centered-cubic (fcc) phase was observed at 162 GPa, followed by a mixed-phase region consisting of the fcc and body-centered-cubic (bcc) phases. The presence of only the bcc phase was observed between 377 and 690 GPa. Our results are consistent with recent shock-compression experiments and demonstrate that gold transforms to the bcc phase under conditions of both ramp and shock loading.

We study a unitary time evolution of a symmetry-broken state in a form of a charge density wave in a finite system of interacting hard-core bosons, which can be mapped onto the XXZ Heisenberg chain. Moreover, we introduce a spatially homogeneous and time-dependent vector potential that mimics a short laser pulse. We establish the range of amplitudes of the vector potential for which the onset of charge density wave order can be controlled. We propose a protocol that reveals nonthermal long-lived states, which are characterized by a nonzero charge density wave order translated by one lattice site with respect to its initial formation. The lifetimes of these states are large in comparison to all typical times given by the parameters of the system. They increase with the number of lattice sites, but are significantly suppressed by the integrability-breaking perturbations. In view of these findings, we speculate that the long-lived nonthermal states exist in the thermodynamic limit.

Quantum fluctuations in the effective spin-$\frac{1}{2}$ layered triangular-lattice quantum Heisenberg antiferromagnet ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ lift the classical degeneracy of the antiferromagnetic ground state in magnetic field, producing a series of novel spin structures for magnetic fields applied within the crystallographic $ab$ plane, including a celebrated collinear “up-up-down” spin ordering with magnetization equal to 1/3 of the saturation magnetization over an extended field range. Theoretically unresolved, however, are the effects of interlayer antiferromagnetic coupling and transverse magnetic fields on the ground states of this system. Additional magnetic field induced phase transitions are theoretically expected and in some cases have been experimentally observed, but details regarding their number, location, and physical character appear inconsistent with the predictions of existing models. Conversely, an absence of experimental measurements as a function of magnetic-field orientation has left other key predictions of these models untested. To address these issues, we have used specific heat, neutron diffraction, thermal conductivity, and magnetic torque measurements to map out the phase diagram as a function of magnetic field intensity and orientation relative to the crystallographic $ab$ plane. For $H\left|\right|ab$, we have discovered an additional magnetic field induced phase transition at low temperature and an unexpected tetracritical point in the high-field phase diagram, which coupled with the apparent second-order nature of the phase transitions eliminates several theoretically proposed spin structures for the high-field phases. Our calorimetric measurements as a function of magnetic field orientation are in general agreement with theory for field-orientation angles close to plane parallel ($H\left|\right|a$) but diverge at angles near plane perpendicular; a predicted convergence of two phase boundaries at finite angle and a corresponding change in the order of the field induced phase transition are not observed experimentally. Our results emphasize the role of interlayer coupling in selecting and stabilizing field induced phases, provide guidance on the nature of the magnetic order in each phase, and reveal the need for new physics to account for the nature of magnetic ordering in this archetypal two-dimensional spin-$\frac{1}{2}$ triangular-lattice quantum Heisenberg antiferromagnet.

Reactive molecular beam epitaxy has been employed to prepare ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ cuprate films via a Co-deposition method. Precise control of flux ratios among the solid sources leads us to establish a very narrow growth window for the cation stoichiometry. The hole doping by divalent ${\mathrm{Sr}}^{2+}$ cation is found to occur in a substitution-limited manner, accompanied by partial oxidation of Sr flux into peroxide $\mathrm{Sr}{\mathrm{O}}_2$ on the top surface. Through interface engineering, we explore the impact of substrate strain on the crystal parameters and superconductivity of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ thin films. The present study provides a time-saving, effective approach to preparing high-quality cuprate films, and further understanding the effect of surface/interface microstructures on cuprate superconductivity.

We theoretically investigate the tunneling conductance of the ($d+ip$)-wave superconductor that was recently proposed to be realized at the (110) surface of a high-$T_c$ cuprate superconductor. Utilizing the quasiclassical Eilenberger theory, we obtain the self-consistent pair potentials and the differential conductance of the normal-metal/($d+ip$)-wave superconductor junction. We demonstrate that the zero-bias peak of a $d$-wave superconductor is robust against the spin-triplet $p$-wave surface subdominant order, even though it is fragile against the spin-singlet $s$-wave one. Comparing our numerical results with experimental results, we conclude that spin-triplet $p$-wave surface subdominant order is feasible.

In this work we study the competition or coexistence between charge density wave (CDW) and superconductivity (SC) in a two-band model system in a square lattice. One of the bands has a net attractive interaction $\left(J_d\right)$ that is responsible for SC. The model includes on-site Coulomb repulsion between quasiparticles in different bands $\left(U_{dc}\right)$ and the hybridization $\left(V\right)$ between them. We are interested in describing intermetallic systems with a $d$-band of moderately correlated electrons, for which a mean-field approximation is adequate, coexisting with a large $sp$-band. For simplicity, all interactions and the hybridization $V$ are considered site-independent. We obtain the eigenvalues of the Hamiltonian numerically and minimize the free energy density with respect to the relevant parameters to obtain the phase diagrams as function of $J_d$, $U_{dc}$, $V$, band-filling $\left(n_{\mathrm{tot}}\right)$, and the relative depth of the bands $\left({\epsilon }_{d0}\right)$. We consider two types of superconducting ground states coexisting with the CDW. One is a homogeneous ground state and the other is a pair density wave where the SC order parameter has the same spatial modulation of the CDW. Our results show that the CDW and SC orders compete, but depending on the parameters of the model these phases may coexist. The model reproduces most of the experimental features of high dimensionality $(d>1)$ metals with competing CDW and SC states, including the existence of first- and second-order phase transitions in their phase diagrams.

The hexagonal Dy-based compound DyNiAl undergoes ferromagnetic and antiferromagnetic-type magnetic phase transitions at $T_{\mathrm{C}}=30$ K and $T_1=15$ K, respectively. To investigate the $4f$-electronic state and quadrupole interactions in DyNiAl, we carried out ultrasonic measurements versus temperature and applied magnetic field. The transverse elastic moduli $C_{44}$ and $C_{66}$ show a prominent elastic softening originating from an interlevel ferroquadrupolar-type interaction between the ground state and excited Kramers doublets, clarified by a crystal field analysis. In magnetic fields applied along the [100] and [001] axes, we observed a field-induced phase transition. Because the quadrupole interaction is enhanced in high magnetic fields according to our calculations, we suggest a magnetic-field-induced ferroquadrupolar ordering of the electric quadrupoles $O_{xy}$ and $O_{yz}$ for fields applied along [100] and [001], respectively, with different quadrupolar order parameters depending on the field direction.

Merging the fields of topology and magnetism expands the scope of fundamental quantum phenomena with novel functionalities for topological spintronics enormously. Here, we theoretically demonstrate that ferromagnetism provides an efficient means to achieve a topological switching between crystalline and higher-order topological insulator phases in two dimensions. Using a tight-binding model and first-principles calculations, we identify layered NpSb as a long-awaited two-dimensional topological crystalline insulator with intrinsic ferromagnetic order with a band gap which is as large as 220 meV. We show that when ${\mathcal{M}}_z$ symmetry is preserved for the out of plane magnetization of this material, it exhibits a pair of gapless edge states along all boundaries and carries a nonzero mirror Chern number ${\mathcal{C}}_{\mathcal{M}}=1$. Remarkably, when rotating the magnetization into the plane a higher-order topological insulator phase with a parity-based invariant ${\nu }_{2\text{D}}=1$ is achieved, and in-gap topological corner states emerge. Our results pave the way to understanding and engineering topological insulating states in two-dimensional ferromagnets.

The phonon Casimir effect describes the phonon-mediated interaction between defects in condensed-matter systems. Using the path-integral formalism, we derive a general method for calculating the Helmholtz free energy due to vibrational modes in systems of arbitrary dimensionality and composition. Our results make it possible to extract the defect interaction energy at any temperature for various defect configurations. We demonstrate our approach in action by performing numerical calculations for mono- and diatomic chains, as well as a diatomic molecule, at zero and finite temperatures and validate our results using exact diagonalization.

Natural and resonant oscillations of suspended circular graphene membranes (single-layer sheets lying on a flat substrate having a circular hole of radius $R$) have been simulated using full-atomic models. Substrates formed by flat surfaces of graphite and hexagonal boron nitride (h-BN) crystal, hexagonal ice, silicon carbide 6H-SiC, and nickel (111) surface have been used. The presence of the substrate leads to the forming of a gap at the bottom of the frequency spectrum of transversal vibrations of the sheet. The frequencies of natural oscillations of the membrane (oscillations localized on the suspended section of the sheet) always lie in this gap, and the frequencies of oscillations decrease by increasing radius of the membrane as ${(R+R_i)}^{-2}$ with nonzero effective increase of radius $R_i>0$. The modeling of the sheet dynamics has shown that small periodic transversal displacements of the substrate lead to resonant vibrations of the membranes at frequencies close to eigenfrequencies of nodeless vibrations of membranes with a circular symmetry. The energy distribution of resonant vibrations of the membrane has a circular symmetry and several nodal circles, whose number $i$ coincides with the number of the resonant frequency. The resonant frequencies decrease with increasing the membrane radius as ${(R+R_i)}^{-{\alpha }_i}$ with the exponent ${\alpha }_i<2$. The lower rate of resonance frequency decrease is caused by the hard anharmonicity of membrane vibrations.