$\mathrm{C}{\mathrm{u}}^{+}$ ion behavior is one of the heavily discussed topics in physical and material fields because of its unclear behavior and high potential in materials application. To provide a breakthrough in this field, comprehensive research that connects the real space and the reciprocal lattice (momentum) space is effective. Here, we investigated the $\mathrm{C}{\mathrm{u}}^{+}$ ion behavior in two-dimensional layered system $\mathrm{C}{\mathrm{u}}_{0.33}\mathrm{TiS}{\mathrm{e}}_2$, which shows the $\mathrm{C}{\mathrm{u}}^{+}$ disorder-order phase transition at 200 K. Inelastic x-ray scattering revealed a large change of the $\mathrm{C}{\mathrm{u}}^{+}$ ion dynamics at the phase transition. It is expected that the complex interactions are involved in the behavior of $\mathrm{C}{\mathrm{u}}^{+}$ ions intercalated in the negatively charged $\mathrm{TiS}{\mathrm{e}}_2$ layers by charge transfer. However, Monte Carlo simulations using a simple combination of repulsive forces between $\mathrm{C}{\mathrm{u}}^{+}$ ions well reproduce the characteristic temperature dependence of the anisotropic diffuse scattering. These insights provide a unified understanding in controlling the intercalated system.

The magnetic properties of iron chalcogenides and pnictides have found much interest as magnetic fluctuations are suggested to drive the formation of Cooper pairs in iron-based superconductors. Here, we have studied the pressure dependence of the magnetic and electrical properties of the iron-selenide compound $\left[\mathrm{C}{\mathrm{s}}_6\mathrm{Cl}\right]\left[\mathrm{F}{\mathrm{e}}_{24}\mathrm{S}{\mathrm{e}}_{26}\right]$ by energy-domain synchrotron Mössbauer spectroscopy up to 17 GPa and by resistivity measurements up to 45 GPa. Similar to the binary superconductor $\mathrm{F}{\mathrm{e}}_{1+x}\mathrm{Se}$, the host-guest-type crystal structure of $\left[\mathrm{C}{\mathrm{s}}_6\mathrm{Cl}\right]\left[\mathrm{F}{\mathrm{e}}_{24}\mathrm{S}{\mathrm{e}}_{26}\right]$ contains edge-sharing $\mathrm{FeS}{\mathrm{e}}_4$ units, but its ground state is antiferromagnetically ordered. A complex hyperfine pattern suggests a nontrivial spin structure like a spin spiral with large magnetic moments in the range 2 to $3{\mu }_{\mathrm{B}}$ at ambient pressure. High pressure drastically suppresses the Néel temperature from 220 K at ambient pressure to below 30 K at 12 GPa. Between 5 and 10 GPa the complex magnetic hyperfine pattern collapses and a low-moment magnetic ground state emerges at higher pressures. A concomitant decrease of the resistivity by several orders of magnitude indicates that the electronic system becomes more itinerant. Full metallization occurs near 25 GPa, but no sign of superconductivity down to 1.5 K was found at any pressure. It is suggested that the insulating ground state of $\left[\mathrm{C}{\mathrm{s}}_6\mathrm{Cl}\right]\left[\mathrm{F}{\mathrm{e}}_{24}\mathrm{S}{\mathrm{e}}_{26}\right]$ corresponds to an orbitally selective Mott phase, which due to an increased bandwidth, is transformed into a Hund's metal state under pressure.

We investigate the structural, magnetic, and spin-state transitions, and magnetocaloric properties of ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CoNbO}}_6$ ($x=0$–1) double perovskites. The structural transition from tetragonal to monoclinic phase at $x⩾0.6$, and an evolution of (101)/(103) superlattice reflections and Raman active modes indicate the enhancement in the B-site ordering with $x$. The magnetic susceptibility data reveal the transition from weak ferromagnetic (FM) to antiferromagnetic (AFM) ordering for $x⩾0.6$ with $T_{\mathrm{N}}\approx 9$–15 K. Interestingly, the La substitution drives towards a more insulating state due to an increase in high-spin ${\mathrm{Co}}^{2+}$, whereas a spin-state crossover is observed in ${\mathrm{Co}}^{3+}$ from high-spin to intermediate-/low-spin states with $x$. We discuss the correlation between complex magnetic interactions and the presence of various Co spin states in the system. Moreover, the emergence of metamagnetic nature due to the competition between FM and AFM interactions as well as crossover from conventional to inverse magnetocaloric effect have been demonstrated by a detailed analysis of temperature and field dependent change in magnetic entropy.

Silicon's weak intrinsic spin-orbit coupling and centrosymmetric crystal structure are a critical bottleneck to the development of Si spintronics, because they lead to an insignificant spin Hall effect (spin current generation) and inverse spin Hall effect (spin current detection). Here, we undertake current, magnetic field, crystallography dependent magnetoresistance, and magnetothermal transport measurements to study the spin transport behavior in freestanding Si thin films. We observe a large spin Hall magnetoresistance in both $p\text{−}\mathrm{Si}$ and $n\text{−}\mathrm{Si}$ at room temperature and it is an order of magnitude larger than that of Pt. One explanation of the unexpectedly large and efficient spin Hall effect is spin-phonon coupling instead of spin-orbit coupling. The macroscopic origin of the spin-phonon coupling can be large strain gradients that can exist in the freestanding Si films. This discovery in a light, earth abundant and centrosymmetric material opens a new path of strain engineering to achieve spin dependent properties in technologically highly developed materials.

Critical properties of the fully frustrated XY models on square and triangular lattices are investigated by means of the nonequilibrium relaxation (NER) method. We examine the validity of the conclusion on the universality class of the chiral transition previously obtained by the NER method, in which that belongs to a non-Ising-type one. To clarify it, we analyze the NER of fluctuations for longer Monte Carlo steps on lager lattices than those performed previously. The calculation is made at the chiral transition temperatures estimated carefully by the use of recently improved dynamical scaling analysis. The result indicates that the asymptotic behavior of the time-dependent exponents, which should converge to the critical exponents, show the same tendency as those obtained previously in shorter times. We also apply the improved dynamical scaling analysis to the estimation of Kosterlitz-Thouless (KT) transitions and confirm the existence of the double transitions for the chiral and KT phases with more reliable estimations.

We investigated the spin-orbit (SO) effective magnetic-field-induced spin precession of a high-mobility two-dimensional electron gas (2DEG) in a modulation-doped (001) GaAs/AlGaAs quantum well close to the persistent spin helix (PSH) state. The oscillating spin signal induced by the SO fields in zero external magnetic fields was clearly observed. When the photoexcited carrier density is sufficiently small compared with the 2DEG density, the spin precession length in the space domain was obtained by analyzing the precession frequency in the PSH regime. The excitation density dependence of the spin dynamics was reproduced well using the scattering time-dependent spin dynamics simulation. We revealed that the increase in the photoexcited carriers results in a transition from ballistic motion to diffusive motion, leading to a decrease in the spin-diffusion coefficient.

It has been technically challenging to concurrently probe the electrons and the lattices in materials during nonequilibrium processes, allowing their correlations to be determined. Here, in a single set of ultrafast electron diffraction patterns taken on the charge-density-wave (CDW) material $1T$-TaSeTe, we discover a temporal shift in the diffraction intensity measurements as a function of scattering angle. With the help of dynamic models and theoretical calculations, we show that the ultrafast electrons probe both the valence-electron and lattice dynamic processes, resulting in the temporal shift measurements. Our results demonstrate unambiguously that the CDW is not merely a result of the periodic lattice deformation ever present in $1T$-TaSeTe but has significant electronic origin. This method demonstrates an approach for studying many quantum effects that arise from electron-lattice dephasing in molecules and crystals for next-generation devices.

A bond-directional anisotropic exchange interaction, called the Kitaev interaction, is a promising route to realize quantum spin liquids. The Kitaev interactions were found in Mott insulators with the strong spin-orbit coupling, in the presence of quantum interference between indirect electron transfers. Here we theoretically propose a different scenario by introducing a polar structural asymmetry that unbalances the quantum interference. We show that the imbalance activates additional exchange processes and gives rise to a dominant antiferromagnetic Kitaev interaction, in stark contrast to the conventional ferromagnetic ones. We demonstrate by ab initio calculations that polar Ru trihalides with multiple anions, $\alpha \text{−}{\mathrm{RuH}}_{3/2}{X}_{3/2}$ ($X=\text{Cl}$ and Br), exhibit dominant antiferromagnetic Kitaev interactions by this mechanism. Our proposal opens the way for materializing the Kitaev spin liquids in unexplored parameter regions.

Superconducting materials exhibiting topological properties are emerging as an exciting platform to realize fundamentally new excitations from topological quantum states of matter. In this letter, we explore the possibility of a field-free platform for generating Majorana zero energy excitations by depositing magnetic Fe impurities on the surface of candidate topological superconductors, LiFeAs and $\mathrm{PbTaS}{\mathrm{e}}_2$. We use scanning tunneling microscopy to probe localized states induced at the Fe adatoms on the atomic scale and at sub-Kelvin temperatures. We find that each Fe adatom generates a striking zero-energy bound state inside the superconducting gap, which do not split in magnetic fields up to 8 T, underlining a nontrivial topological origin. Our findings point to magnetic Fe adatoms evaporated on bulk superconductors with topological surface states for exploring Majorana zero modes and quantum information science under field-free conditions.

A stable suspension of nanoscale particles due to the Casimir force is of great interest for many applications such as sensing, noncontract nanomachines. However, the suspension properties are difficult to change once the devices are fabricated. Vanadium dioxide (${\mathrm{VO}}_2$) is a phase change material, which undergoes a transition from a low-temperature insulating phase to a high-temperature metallic phase around a temperature of 340 K. In this work, we study Casimir forces between a nanoplate (gold or Teflon) and a layered structure containing a ${\mathrm{VO}}_2$ film. It is found that stable Casimir suspensions of nanoplates can be realized in a liquid environment, and the equilibrium distances are determined, not only by the layer thicknesses but also by the matter phases of ${\mathrm{VO}}_2$. Under proper designs, a switch from quantum trapping of the gold nanoplate (“on” state) to its release (“off” state) as a result of the metal-to-insulator transition of ${VO}_2$, is revealed. On the other hand, the quantum trapping and release of a Teflon nanoplate is found under the insulator-to-metal transition of ${\mathrm{VO}}_2$ . Our findings offer the possibility of designing switchable devices for applications in micro and nanoelectromechanical systems.

In this paper, we study the emergence of a Landauer transport regime from the quantum-mechanical dynamics of free electrons in a disordered tight-binding chain, which is coupled to finite leads with open boundaries. Both partitioned and partition-free initial conditions are analyzed and seen to give rise, for large enough leads, to the same spatially uniform quasisteady-state current, which agrees with the Landauer value. The quasisteady-state regime is preceded by a transient regime, which lasts for a time proportional to the length of the disordered sample, and followed by recursions, after a time that is proportional to the lead size. These theoretical predictions may be of interest to future experiments on transport of fermionic ultracold atoms across optical lattices. We also observe finite-size current oscillations, superimposed on the quasisteady state, whose behavior depends crucially on the conditions initially imposed on the system. Finally, we show how a time-resolved Kubo formula is able to reproduce this Landauer transport regime, as the leads grow bigger.

We investigate the nonequilibrium dynamics of a class of isolated one-dimensional systems possessing two degenerate ground states, initialized in a low-energy symmetric phase. We report the emergence of a timescale separation between fast (radiation) and slow (kink or domain wall) degrees of freedom. We find a universal long-time dynamics, largely independent of the microscopic details of the system, in which the kinks control the relaxation of relevant observables and correlations. The resulting late-time dynamics can be described by a set of phenomenological equations, which yield results in excellent agreement with the numerical tests.

The theory of photon drag of superconducting fluctuations in two-dimensional electron gas is developed. It is shown that the frequency dependence of the induced current is qualitatively similar to the case of photon drag of conventional two-dimensional degenerate electron gas. With decreasing temperature the magnitude of the effect increases dramatically and the current of superconducting fluctuations carries an additional power of reduced temperature in comparison with the Aslamazov-Larkin contribution. The developed effect is expected to be sufficient to be visible against the conventional photocurrent background.

We study the properties of the Luttinger-Ward functional (LWF) in a simplified Hubbard-type model without time or spatial dimensions, but with $N$ identical replicas located on a single site. The simplicity of this $(0+0)d$ model permits an exact solution for all $N$ and for both bosonic and fermionic statistics. We show that fermionic statistics are directly linked to the fact that multiple values of the noninteracting Green's function $G_0$ map to the same value of the interacting Green's function $G$; that is, the mapping $G_0\mapsto G$ is noninjective. This implies that with fermionic statistics the $(0+0)dN$-replica model has a multiply valued LWF. The number of LWF values in the fermionic model increases proportionally to the number of replicas $N$, while in the bosonic model the LWF has a single value regardless of $N$. We also discuss the formal connection between the $N$-replica model and the $(0+1)d$ Hubbard atom which was used in previous studies of LWF's multivaluedness.

Two-dimensional (2D) ferroelectrics are a special class of materials that fit the need of miniaturization of devices, which can lead to a plethora of applications. Up to now, albeit the breakthrough in the discovery of a handful of 2D ferroelectrics, the mechanism of which still requires better understanding. In this work, we clarify that the ferroelectric ground state in monolayer (ML) ${\mathrm{In}}_2{\mathrm{Se}}_3$ is strongly affected by in-plane strain, and the strain-induced phase transition is of a first-order transition. We further propose that strain is an effective approach to tune the ferroelectric ground state in 2D materials, which can even intrinsically introduce out-of-plane polarizations. By performing first-principles calculations, we show that the epitaxial strain is able to tune the structural transition between polar and nonpolar layer groups. According to this, the epitaxial strain can not only fully suppress the polarization in ML-${\mathrm{In}}_2{\mathrm{Se}}_3$, but also cause out-of-plane polarizations in a group of 2D nonpolar materials, i.e., ML-${\mathrm{Bi}}_2{\mathrm{Se}}_3$, ML-${\mathrm{Bi}}_2{\mathrm{Te}}_3$, ML-${\mathrm{Sb}}_2{\mathrm{Te}}_3$, and ML-${\mathrm{Sb}}_2{\mathrm{Se}}_3$. The coupling mechanism between strain and polarization in these compounds is further illustrated.

The dynamical encirclement around a second-order exceptional point (EP) and corresponding chirality driven nonadiabatic modal dynamics have attracted enormous attention in the topological study of various non-Hermitian systems. However, dynamical encirclement around multiple second-order EPs in a multistate system is yet to be explored. Here, exploiting an exclusive design of a planar gain-loss assisted three-mode supported optical waveguide with local Kerr nonlinearity, we encounter multiple second-order EPs. Judiciously, choosing a specific parameter space by varying the unbalanced gain-loss profile, we encircle multiple EPs simultaneously, and explore the beam dynamics toward corresponding chiral or nonchiral aspects of the device. While propagating through the designed waveguide, three coupled modes are collapsed into a specific dominating mode, owing to corresponding nonadiabatic corrections around multiple EPs. We reveal that in the absence of chirality the same amount of focusing- and defocusing-type nonlinearity gives different dominating output, irrespective of the choice of inputs, for the same topological structure of the waveguide. An analytical model to describe this anomalous mode collapsing phenomenon due to interplay between nonlinearity and the EP has been developed. Our findings should provide a promising opportunity to switch or retrieve a selective mode from a multimode signal in integrated devices.

We propose a realization of higher-order topological phases in a spring-mass model with a breathing kagome structure. To demonstrate the existence of the higher-order topological phases, we characterize the topological properties and show that the corner states appear under the fixed boundary condition. To characterize the topological properties, we introduce a formula for the ${\mathbb{Z}}_3$ Berry phases in the Brillouin zone. From the numerical result of this ${\mathbb{Z}}_3$ Berry phase, we have elucidated that coupling between the longitudinal and transverse modes yields a state characterized by the Berry phase $\frac{2\pi }{3}$ for our mechanical breathing kagome model. In addition, we suggest that the corner states can be detected experimentally through a forced vibration.

We present a detailed magnetic study of the triangular antiferromagnet ${\mathrm{ErMgGaO}}_4$. A point charge calculation under the single ion approximation reveals a crystal field ground state doublet with a strong Ising-like behavior of the ${\mathrm{Er}}^{3+}$ moment along the $c$ axis. Magnetic susceptibility and specific heat measurements indicate no presence of magnetic transitions above 0.1 K and no evidence of residual entropy as the temperature approaches zero. Zero field (ZF) $\mu \mathrm{SR}$ measurements shows no sign of static uniform or random field and longitudinal field (LF) $\mu \mathrm{SR}$ measurements exhibit persistent spin fluctuations down to our lowest temperature of 25 mK. Our results provide evidence of a quantum spin liquid state in the triangular antiferromagnet ${\mathrm{ErMgGaO}}_4$.