${\text{CuCrO}}_2$ offers insights into the different types of spiral magnetic orderings that can form spontaneously due to frustration in triangular-lattice antiferromagnets. We explore the magnetic phase diagram up to 65 T along all the principal axes, and also use electric polarization to probe changes in the spiral order at high magnetic fields. It is known that at zero magnetic field a proper-screw spiral of the Cr S $=\frac{3}{2}$ spins forms that in turn induces electric polarization with six possible orientations in the ab plane. Applied magnetic fields in the (hard) ab plane have been shown to induce a transition to cycloidal-spiral magnetic order above 5.3 T in those domains that have spins perpendicular to the applied magnetic field. We show that the cycloidal order remains unchanged all the way up to 65 T, which is one quarter of the extrapolated saturation magnetization. On the other hand, for magnetic fields along the (easy) c axis, we observe a transition in the electric polarization near 45 T, and it is followed by a series of steps and/or oscillations in the electric polarization. The data are consistent with a proper-screw-to-cycloidal transition that is pushed from 5.3 to 45 T by easy-axis anisotropy, and is in turn followed by stretching of the magnetic spiral through commensurate and incommensurate wave vectors. This work also highlights the ability of the magnetically induced electric polarization to probe complex magnetic orders in regimes of phase space that are difficult to reach with neutron diffraction.

A magnetostatic spin-wave analog of integer quantum Hall (IQH) state is proposed in realistic patterned ferromagnetic thin films. Due to magnetic shape anisotropy, magnetic moments in a thin film lie within the plane, while all spin-wave excitations are fully gapped. Under an out-of-plane magnetic field, the film acquires a finite magnetization, where some of the gapped magnons become significantly softened near a saturation field. It is shown that, owing to a spin-orbit locking nature of the magnetic dipolar interaction, these soft spin-wave volume mode bands become chiral volume mode bands with finite topological Chern integers. A bulk-edge correspondence in IQH physics suggests that such volume mode bands are accompanied by a chiral magnetostatic spin-wave edge mode. The existence of the edge mode is justified both by micromagnetic simulations and by band calculations based on a linearized Landau-Lifshitz equation. Employing intuitive physical arguments, we introduce proper tight-binding models for these soft volume mode bands. Based on the tight-binding models, we further discuss possible applications to other systems such as magnetic ultrathin films with perpendicular magnetic anisotropy.

We report physical properties of a chromium dichalcogenide 1T-${\text{CrSe}}_2$ with tetravalent Cr cations. Interplay between orbital degrees of freedom and the valence instability of Cr is studied by means of magnetic, electrical, electron diffraction, and x-ray diffraction measurements. Associated with the phase transitions, a space group of ${\text{CrSe}}_2$ varies from trigonal $P\overline{3}m1$ to rhombohedral $R\overline{3}m$ through monoclinic $I2/m$. By considering behaviors of $\chi$ and $\rho$ in addition to possible ordering patterns of occupied $t_{2g}$ orbitals, we have concluded that the origin of successive phase transitions is simultaneous occurrence of orbital ordering and valence change of Cr through each phase transition.

Presence and significance of fivefold configurations in liquid metals are investigated by combining x-ray absorption spectroscopy and computer simulations (molecular dynamics and reverse Monte Carlo) in liquid and undercooled liquid nickel. We show that icosahedral short-range ordering (ISRO), probed by common-neighbor (CNA) and spherical invariant (${\widehat{W}}_6$) analysis, involves a limited fraction (14–18% in undercooled nickel for different structural models) of local atomic configurations. The emerging picture for the liquid structure is that of a mixture of nearly icosahedral structures embedded in a disordered network mainly composed of fragments of highly distorted icosahedra (40–45% of the total), structures reminiscent of the crystalline phase, and other configurations.

An equilibrium stability analysis of an in-plane magnetized synthetic ferrimagnetic free layer (SyF) submitted to multiple spin-transfer-torque (STT) influences and to applied field has been carried out. Analytical expressions for the frequency and linewidth of the excitations were derived, in the case of strong coupling between the two layers compared to the anisotropy field. The expression of the critical current for the onset of excitations in the SyF was then calculated. Above the critical current, the destabilized mode, acoustic or optical, is found to depend on the applied field direction, but also on the asymmetry of the two layers composing the SyF. A strong thickness asymmetry reduces the critical current at zero field, and allows to select the destabilized mode. The critical current of an in-plane SyF with mutual spin torque between the layers and without reference layer was computed. For a symmetric SyF, the optical mode is the only mode that can be destabilized by STT.

A cubic ${\text{Er}}_2$${\text{Ge}}_2$${\text{O}}_7$ pyrochlore was prepared under high-pressure and high-temperature conditions and its magnetic ground state was investigated by measurements of specific heat, dc and ac magnetic susceptibility as functions of temperature, pressure, and magnetic field. We found that ${\text{Er}}_2$${\text{Ge}}_2$${\text{O}}_7$ undergoes a long-range antiferromagnetic transition at $T_N$ ≈ 1.4 K, which can be further enhanced by applying external physical pressure. On the other hand, application of external magnetic fields suppresses the antiferromagnetic order to zero temperature around $H_c$ ≈ 2.3 T, where a magnetic-field-induced spin-flop transition was observed. $H_c$ increases accordingly with increasing $T_N$ under external pressure. A comparison of the magnetic ground states and structural variations along the isostructural series ${\text{Er}}_2$$B_2$${\text{O}}_7$ ($B$ = Sn, Ti, Ge) together with the high-pressure study on ${\text{Er}}_2$${\text{Ge}}_2$${\text{O}}_7$ indicated that the magnetic properties of these highly frustrated XY pyrochlore antiferromagnets are very sensitive to the minute structural changes that determine the anisotropic exchange interactions and the local crystal-electric-field environments of Er${}^{3+}$ ions.

We have studied the longitudinal thermal conductivity of the surface of a three-dimensional time-reversal-symmetric topological superconductor with random disorder. Majorana fermions on the surface of topological superconductors have a response to the gravitational field, which is realized as a thermal response to the temperature gradient inside of the material. Because of the presence of both time-reversal symmetry and particle-hole symmetry, disorder on the surface emerges in the Hamiltonian only as spatial deformations of the pair potential. In terms of the gravitational field, the disorder results in spatial fluctuations of the metric. We consider disorder effects on the thermal conductivity perturbatively within the self-consistent Born approximation. The density of states is calculated with the Green's function technique and the thermal conductivity of the surface modes is derived through the electronic conductivity using Wiedemann-Franz law for the Majorana fermions.

A topology-intrinsic connection between the stabilities of Fermi surfaces (FSs) and topological insulators/superconductors (TIs/TSCs) is revealed. First, through revealing the topological difference of the roles played by the time-reversal (or particle-hole) symmetry respectively on FSs and TIs/TSCs, a one-to-one relation between the topological types of FSs and TIs/TSCs is rigorously derived by two distinct methods with one relying on the direct evaluation of topological invariants and the other on K theory. Secondly, we propose and prove a general index theorem that relates the topological charge of FSs on the natural boundary of a TI/TSC to its bulk topological number. In the proof, FSs of all codimensions for all symmetry classes and topological types are systematically constructed by Dirac matrices. Moreover, implications of the general index theorem on the boundary quasiparticles are also addressed.

We propose a model to account for the observed ESR-like signal at the Larmor frequency in the current noise scanning tunnel microscope (STM) experiments identifying spin centers on various substrates. The theoretical understanding of this phenomenon, which allows for single spin detection on surfaces at room temperature, is not settled for the experimentally relevant case that the tip and substrate are not spin polarized. Our model is based on a direct tip-substrate tunneling in parallel with a current flowing via the spin states. We find a sharp signal at the Larmor frequency even at high temperatures, in good agreement with experimental data. We also evaluate the noise in presence of an ac field near resonance and predict splitting of the signal into a Mollow triplet.

We present a comprehensive study of the photon-energy ($h\nu$)-dependent line-shape evolution of molecular orbital signals of large $\pi$-conjugated molecules by resonant photoelectron spectroscopy (RPES). A comparison to RPES data on small molecules suggests that the excitation into different vibrational levels on the intermediate-state potential energy surface of the electronic excitation is responsible for the observed effect. In this simplified picture of electron-vibration coupling the character of the potential energy surfaces involved in the RPES process determines the line shape of the molecular orbital signal for a particular $h\nu$. We use the sensitivity of this effect to probe the influence of different interfaces on the electron-vibration coupling in the investigated systems. The magnitude of the variation in line shape throughout the particular $h\nu$ region allows us to reveal significant differences within the physisorptive regime.

We study the extended Su-Schrieffer-Heeger model with both nearest-neighbor and next-nearest-neighbor hopping strengths cyclically modulated and find the family of the model system exhibiting topologically nontrivial phases, which can be characterized by a nonzero Chern number defined in a two-dimensional space spanned by the momentum and a modulation parameter. It is interesting that the model has a similar phase diagram to that of the well-known Haldane model. We propose to use photonic crystal systems as the ideal systems to fabricate our model systems and probe their topological properties. Some other models with modulated on-site potentials are also found to exhibit similar phase diagrams, and their connection to the Haldane model is revealed.

We report the condensation of microcavity exciton-polaritons at $\Gamma$ points located on the boundary between the third and higher Brillouin zones in hexagonal lattices: triangular and honeycomb geometries. We collect experimental evidence that supports the finite momentum condensation: (1) the coherent Bragg peaks formed at nonzero $\Gamma$ points; (2) the nonlinear intensity increase in the exciton-polariton emission at quantum degeneracy threshold; (3) the spectral linewidth behavior: narrowing near threshold and broadening above threshold; and (4) the equivalent 4$f_{y^3-3x^{2}y}$-like orbital symmetry in real space. The $f$-orbital state at $\Gamma$ points appears as a metastable momentum valley to trap exciton-polaritons, which is explained by single-particle band-structure calculations.

We study theoretically the phonon-induced relaxation ($T_1$) and decoherence times ($T_2$) of singlet-triplet qubits in lateral GaAs double quantum dots (DQDs). When the DQD is biased, Pauli exclusion enables strong dephasing via two-phonon processes. This mechanism requires neither hyperfine nor spin-orbit interaction and yields $T_2\ll T_1$, in contrast to previous calculations of phonon-limited lifetimes. When the DQD is unbiased, we find $T_2\simeq 2T_1$ and much longer lifetimes than in the biased DQD. For typical setups, the decoherence and relaxation rates due to one-phonon processes are proportional to the temperature $T$, whereas the rates due to two-phonon processes reveal a transition from $T^2$ to higher powers as $T$ is decreased. Remarkably, both $T_1$ and $T_2$ exhibit a maximum when the external magnetic field is applied along a certain axis within the plane of the two-dimensional electron gas. We compare our results with recent experiments and analyze the dependence of $T_1$ and $T_2$ on system properties such as the detuning, the spin-orbit parameters, the hyperfine coupling, and the orientation of the DQD and the applied magnetic field with respect to the main crystallographic axes.

We study the scaling of quantum discord (a measure of quantum correlation beyond entanglement) in spin models analytically and systematically. We find that at finite temperature the block scaling of quantum discord satisfies an area law for any two-local Hamiltonian. We show that generically and heuristically the two-site scaling of quantum discord is similar to that of correlation functions. In particular, at zero temperature it decays exponentially and polynomially in gapped and gapless (critical) systems, respectively; at finite temperature it decays exponentially in both gapped and gapless systems. We compute the two-site scaling of quantum discord in the $XXZ$ chain, the $XY$ chain (in a magnetic field), and the transverse field Ising chain at zero temperature.

We investigate the behavior of a Josephson junction consisting of a ferromagnetic insulator-superconductor (FI-S) bilayer tunnel coupled to a superconducting electrode. We show that the Josephson coupling in the structure is strengthened by the presence of the spin-splitting field induced in the FI-S bilayer. Such strengthening manifests itself as an increase of the critical current $I_c$ with the amplitude of the exchange field. Furthermore, the effect can be strongly enhanced if the junction is taken out of equilibrium by a temperature bias. We propose a realistic setup to assess experimentally the magnitude of the induced exchange field, and predict a drastic deviation of the $I_c\left(T\right)$ curve ($T$ is the temperature) with respect to equilibrium.

${\text{LiZn}}_2{\text{Mo}}_3{\text{O}}_8$ has been proposed to contain $S=½$${\text{Mo}}_3{\text{O}}_{13}$ magnetic clusters arranged on a triangular lattice with antiferromagnetic nearest-neighbor interactions. Here, microwave and terahertz electron spin resonance, ${}^7\mathrm{Li}$ nuclear magnetic resonance, and muon spin rotation spectroscopies are used to characterize the local magnetic properties of ${\text{LiZn}}_2{\text{Mo}}_3{\text{O}}_8$. These results show the magnetism in ${\text{LiZn}}_2{\text{Mo}}_3{\text{O}}_8$ arises from a single isotropic $S=½$electron per cluster and that there is no static long-range magnetic ordering down to $T$ = 0.07 K. Further, there is evidence of gapless spin excitations with spin fluctuations slowing down as the temperature is lowered. These data indicate strong spin correlations, which, together with previous data, suggest a low-temperature resonating valence-bond state in ${\text{LiZn}}_2{\text{Mo}}_3{\text{O}}_8$.

Combining the Josephson effect with magnetism, or spin dependence in general, creates novel physical phenomena. The spin-asymmetric Josephson effect is a predicted phenomenon where a spin-dependent potential applied across a Josephson junction induces a spin-polarized Josephson current. Here, we propose an approach to observe the spin-asymmetric Josephson effect with spin-dependent superlattices, realizable, e.g., in ultracold atomic gases. We show that observing this effect is feasible by studying numerically the quantum dynamics of the system in one dimension. Furthermore, we show that the enhancement, or tunability, of the critical supercurrent in ferromagnetic Josephson junctions [F. S. Bergeret, A. F. Volkov, and K. B. Efetov, Phys. Rev. Lett. 86, 3140 (2001)] can be explained by the spin-asymmetric Josephson effect.

In this paper we introduce the concept of metasurfaces which are fully transparent when looking from one of the two sides of the sheet and have controllable functionalities for waves hitting the opposite side (one-way transparent sheets). We address the question on what functionalities are allowed, considering limitations due to reciprocity and passivity. In particular, we have found that it is possible to realize one-way transparent sheets which have the properties of a twist polarizer in transmission when illuminated from the other side. Also, active one-way transparent sheets with controllable copolarized reflection and transmission from the opposite side are feasible. We show that particular nonreciprocal magnetoelectric coupling inside the sheet is necessary to realize lossless nonactive transparent sheets. Furthermore, we derive the required polarizabilities of constituent dipole particles such that the layers composed of them form one-way transparent sheets. We conclude with design and simulations of an example of a nonreciprocal one-way transparent sheet functioning as an isolating twist polarizer.

We develop the density matrix renormalization group approach to systematically identify the topological order of the quantum spin liquid (QSL) through adiabatically obtaining different topological degenerate sectors of the QSL on an infinite cylinder. As an application, we study the anisotropic kagome Heisenberg model known for hosting a $Z_2$ QSL, however no numerical simulations have been able to access all four sectors before. We obtain the complete set of four topological degenerate ground states distinguished by the presence or absence of the spinon and vison quasiparticle line, which fully characterizes the topological nature of the quantum phase. We have also studied the kagome Heisenberg model, which has recently attracted a lot of attention. We find two topological sectors accurately and also estimate various properties of the other topological sectors, where the larger correlation length is found indicating the possible proximity to another phase.

Real-time Green's function simulations of molecular junctions (open quantum systems) are typically performed by solving the Kadanoff-Baym equations (KBE). The KBE, however, impose a serious limitation on the maximum propagation time due to the large memory storage needed. In this work we propose a simplified Green's function approach based on the generalized Kadanoff-Baym ansatz (GKBA) to overcome the KBE limitation on time, significantly speed up the calculations, and yet stay close to the KBE results. This is achieved through a twofold advance: First, we show how to make the GKBA work in open systems and then construct a suitable quasiparticle propagator that includes correlation effects in a diagrammatic fashion. We also provide evidence that our GKBA scheme, although already in good agreement with the KBE approach, can be further improved without increasing the computational cost.