The possibility of magnetization reversal via a bias-mediated perpendicular magnetic anisotropy is examined theoretically in an antiferromagnet. The numerical analyses based on a Néel vector formulation as well as the micromagnetic Landau-Lifshitz-Gilbert simulation reveal that the desired switching can be achieved through dynamical responses that are significantly different from their ferromagnetic counterparts. Instead of the usual precessional trajectories around the applied effective magnetic field, their motions are rather pendulum-like due to the layered magnetic sublattices with a strong antiparallel exchange interaction, where the inertial behavior plays a crucial role. The absence of spiral damping can also lead to faster relaxation by orders of magnitude. With no reliance on the current driven processes, the investigated mechanism is predicted with a low energy requirement of only a few aJ per switching operation in the antiferromagnets.

We present the results of electrical transport measurements of ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{Cu}}_{1-y}{\mathrm{Ni}}_y{\mathrm{O}}_4$ thin single-crystal films at magnetic fields up to 9 T. Adding Ni impurity with strong Coulomb scattering potential to a slightly underdoped cuprate makes the signs of resistivity saturation at ${\rho }_{\mathrm{sat}}$ visible in the measurement temperature window up to 350 K. Employing the parallel-resistor formalism reveals that ${\rho }_{\mathrm{sat}}$ is consistent with the classical Ioffe-Regel-Mott limit and changes with carrier concentration $n$ as ${\rho }_{\mathrm{sat}}\propto 1/\sqrt{n}$. Thermopower measurements show that Ni tends to localize mobile carriers, decreasing their effective concentration as $n\cong 0.15-y$. The classical unmodified Kohler's rule is fulfilled for magnetoresistance in the nonsuperconducting part of the phase diagram when applied to the ideal branch in the parallel-resistor model.

We study theoretically the current-voltage characteristics (IVCs) of the Josephson field effect transistor—a ballistic SNINS junction with superconducting (S) electrodes confining a planar normal-metal region (N), which is controlled by the gate-induced potential barrier (I). Using the computation technique developed earlier for long single-channel junctions in the coherent multiple Andreev reflection (MAR) regime, we find a significant difference of the subgap current structure compared to the subharmonic gap structure in tunnel junctions and atomic-size point contacts. For long junctions, whose lengths significantly exceed the coherence length, the IVC exhibits current peaks at multiples (harmonics) of the distance ${\delta }_m$ between the static Andreev levels $e{V}_n=n{\delta }_m$. Moreover, the averaged IVC follows the powerlike behavior rather than the exponential one and has a universal scaling with the junction transparency. This result is qualitatively understood using an analytical approach based on the concept of resonant MAR trajectories. In shorter junctions having lengths comparable to the coherence length, the IVC has an exponential form common for point contacts, however the current structures appear at the subharmonics of the interlevel distance $e{V}_n={\delta }_m/n$ rather than the gap subharmonics $2\mathrm{\Delta }/n$.

By use of ultrasound studies we resolved magnetoacoustic quantum oscillation deep into the mixed state of the multiband nonmagnetic superconductor ${\mathrm{YNi}}_2{\mathrm{B}}_2\mathrm{C}$. Below the upper critical field, only a very weak additional damping appears that can be well explained by the field inhomogeneity caused by the flux-line lattice in the mixed state. This is clear evidence for no or a vanishingly small gap for one of the bands, namely, the spheroidal $\alpha$ band. This contrasts de Haas–van Alphen data obtained by use of torque magnetometry for the same sample, with a rapidly vanishing oscillation signal in the mixed state. This points to a strongly distorted flux-line lattice in the latter case that, in general, can hamper a reliable extraction of gap parameters by use of such techniques.

We have developed a field-effect transistor (FET) device using a solid ion conductor (SIC) as the gate dielectric, which can tune the carrier density of FeSe by driving lithium ions in and out of the FeSe thin flakes and consequently control the physical properties and phase transitions. A dome-shaped superconducting phase diagram was mapped out with increasing Li content, with $T_c\sim 46.6$ K for optimal doping, and an insulating phase was reached at the extremely overdoped regime. Our study suggests that, by using a solid ion conductor as the gate dielectric, the SIC-FET device is able to induce much higher carrier doping in the bulk, suit many surface-sensitive experimental probes, and stabilize structural phases that are inaccessible in ordinary conditions.

$\mathrm{SrCo}{\mathrm{O}}_{2.5}$ has a long-range oxygen vacancy ordering that makes it a promising energy material and catalyst carrier. The study of its electronic properties is vital for its practical applications. Here, we investigate its electronic behavior and lattice structural evolution under high pressure up to 22 GPa using synchrotron infrared spectroscopy and x-ray diffraction. A clear electronic transition from a semiconducting state to another semiconducting state is observed around 7.3 GPa upon compression, based on infrared results. Detailed structural examination shows that this electronic transition is accompanied by a structural phase transition, which occurs between 5.3 and 8.6 GPa, as confirmed by x-ray diffraction. The band gap reduces by $\sim 40\%$ at high pressure compared to ambient conditions. This work demonstrates that the oxygen vacancy ordering in $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ can be sustained up to $\sim 8.6\mathrm{GPa}$ and pressure can narrow the band gap, forcing this unique material to enter into another electronic state with a new crystal structure.

To understand how the constituent atoms participate in the hydrogenation of Pd-based alloys at $\sim 0.1$ MPa of hydrogen pressure $\left(P_{{\mathrm{H}}_2}\right)$, we investigated the electronic states in Pd-$M$ $(M=\text{Ru},\text{Rh},\text{Ag},\text{and Au})$ alloys and their hydrides element-selectively by using x-ray absorption spectroscopy at the $L_{2,3}$ edges. Spectral changes near the absorption edge demonstrate that both Pd and $M$ atoms form bonds with H atoms in the Pd-$M$ $(M=\text{Ru} \text{and Rh})$ alloys even at $P_{{\mathrm{H}}_2}\sim 0.1$ MPa. This is a striking result because high pressures of more than 1 GPa are required for the hydrogenation of Rh and Ru pure metals. In contrast, only Pd atoms bond with H atoms and the $M$-H bond is absent in the case of Pd-$M$ $(M=\text{Ag} \text{and Au})$ alloys. Therefore, the hydrogen-induced changes in the electronic states differ between $M\mathrm{s}$ with fully occupied $d$ shells and $M\mathrm{s}$ with partially occupied $d$ shells. This study reveals that the thermodynamic hydrogenation properties of Pd-$M$ alloys can be determined by a combination of the formation of the $M$-H bond and lattice expansion or compression by alloying Pd metal with $M$.

It has been shown that acoustic waves with helical wavefronts can carry angular momentum, which can be transmitted towards a propagating medium. Such a wave field can be achieved by using a planar array of electroacoustic transducers, forming a given spatial distribution of phased sound sources which produce the desired helical wavefronts. Here, we introduce a technique to generate acoustic vortices, based on the passive acoustic metasurface concept. The proposed metasurface is composed of space-coiled cylindrical unit cells transmitting sound pressure with a controllable phase shift, which are arranged in a discretized circular configuration, and thus passively transforming an incident plane wavefront into the desired helical wavefront. This method presents the advantage of overcoming the restrictions on using many acoustic sources, and it is implemented with a transmitting metasurface which can be easily three-dimensionally printed. The proposed straightforward design principle can be adopted for easy production of acoustic angular momentum with minimum complexity and using a single source.

We introduce a theoretical scheme for the analog quantum simulation of long-range XYZ models using current trapped-ion technology. In order to achieve fully tunable Heisenberg-type interactions, our proposal requires a state-dependent dipole force along a single vibrational axis, together with a combination of standard resonant and detuned carrier drivings. We discuss how this quantum simulator could explore the effect of long-range interactions on the phase diagram by combining an adiabatic protocol with the quasiperiodic drivings, and test the validity of our scheme numerically. At the isotropic Heisenberg point, we show that the long-range Hamiltonian can be mapped onto a nonlinear sigma model with a topological term that is responsible for its low-energy properties, and we benchmark our predictions with matrix-product-state numerical simulations.

We have investigated the structural and physical properties of double perovskite (DP) $\mathrm{La}A{\mathrm{VMoO}}_6$ ($A={\mathrm{Ca}}^{2+}$, ${\mathrm{Ba}}^{2+}$; abbreviated as LCVMO and LBVMO from now on) compounds, proposed to be possible half-metallic antiferromagnets (HMAFMs). Here we show that within $\mathrm{La}A{\mathrm{VMoO}}_6$ double perovskite structure, La-O covalency competes against $B$-site as well $A$-site cationic order and this competition critically influences their physical properties. Evidently, the presence of ${\mathrm{Ca}}^{2+}$ or ${\mathrm{Ba}}^{2+}$ at the $A$ site along with ${\mathrm{La}}^{3+}$ would offer a tool to modify the $A$-site ordering and consequently influence the La-O covalency as well. Our experimental results reveal that LCVMO lies at the extreme end of this family and accommodates large scale phase separation in terms of La, V, and Ca, Mo-rich phases as a result of dominant La-O covalency. On the other hand, LBVMO is more correctly described as a layered $A$-site ordered and nearly complete $B$-site disordered double perovskite where cationic order dominates the La-O covalency. The general trend of our experimental findings is in agreement with the ab initio electronic structure calculations, carried out on realistic structures based on local coordination obtained from extended x-ray-absorption fine-structure study.

Single crystals of the $R$-type ferrite $\mathrm{SrNiR}{\mathrm{u}}_5{\mathrm{O}}_{11}$ were grown from a chloride flux. The hexagonal crystal structure contains ruthenium located on distorted kagome nets. The low-temperature dc magnetic susceptibilities (${\chi }_{\perp }$ and ${\chi }_{\parallel }$, perpendicular and parallel to the $\mathit{c}$ axis, respectively) diverge as $T^{-0.3}$, and do not exhibit any indication of long-range magnetic order down to 4.5 K. The electrical resistivity varies as $T^{1.6}$ below 40 K, which is typical of non-Fermi liquids, and may originate from a competition between residual magnetic interactions among $\mathrm{N}{\mathrm{i}}^{2+}$ ($S=1$) spins and geometrical frustration on the two-dimensional kagome lattice of $\mathrm{R}{\mathrm{u}}^{3+}$ ($S=½$) spins. The transverse magnetoresistivity ${\rho }_{xy}$ at constant temperature $T=5\mathrm{K}$ for current ($\mathit{J}$) -magnetic field ($\mathit{H}$) configurations, $\mathit{J}\perp \mathit{H}\parallel \mathit{c}$ axis and $\mathit{J}\parallel \mathit{H}\perp \mathit{c}$ axis, reveals no anomalous contribution, which is consistent with the absence of magnetic order. Fits of the specific heat data below 10 K require a dominant, but unusual electronic term of the form $C_{el}=\gamma T^{1.2}$, which is expected for massless Dirac fermion states in topological insulators, or spin-liquid phases.

Motivated by the spin-triplet superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, the thermal Hall conductivity is investigated for several pairing symmetries with broken time-reversal symmetry. In the chiral $p$-wave phase with a fully opened quasiparticle excitation gap, the temperature dependence of the thermal Hall conductivity has a temperature linear term associated with the topological property directly and an exponential term, which shows a drastic change around the Lifshitz transition. Examining $f$-wave states as alternative candidates with $\mathit{d}={\mathrm{\Delta }}_0\widehat{z}(k_x^2-k_y^2)(k_x\pm i{k}_y)$ and ${\mathrm{\Delta }}_0\widehat{z}{k}_x{k}_y(k_x\pm i{k}_y)$ with gapless quasiparticle excitations, we study the temperature dependence of the thermal Hall conductivity, where for the former state the thermal Hall conductivity has a quadratic dependence on temperature, originating from the linear dispersions, in addition to linear and exponential behavior. The obtained result may enable us to distinguish between the chiral $p$-wave and $f$-wave states in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

We investigate the phase diagram of dirty two-band superconductors. This paper primarily focuses on the properties and observability of the time-reversal symmetry-breaking $s+is$ superconducting states, which can be generated in two-band superconductors by interband impurity scattering. We show that such states can appear in two distinct ways. First, according to a previously discussed scenario, the $s+is$ state can form as an intermediate phase at the impurity-driven crossover between $s_{\pm }$ and $s_{++}$ states. We show that there is a second scenario where domains of the $s+is$ state exists in the form of an isolated dome inside the $s_{\pm }$ domain, completely detached from the transition between $s_{\pm }$ and $s_{++}$ states. We demonstrate that in both cases the $s+is$ state generated by impurity scattering exists in an extremely small interval of impurity concentrations. Although this likely precludes direct experimental observation of the $s+is$ state formation due to this mechanism, this physics leads to the appearance of a region inside both the $s_{\pm }$ and $s_{++}$ domains with unusual properties due to softening of normal modes.

We have studied the electronic structure of ${\mathrm{Eu}}_3{\mathrm{F}}_4{\mathrm{Bi}}_2{\mathrm{S}}_4$ using a combination of Eu $L_3$-edge x-ray absorption spectroscopy (XAS) and space-resolved angle-resolved photoemission spectroscopy (ARPES). From the Eu $L_3$-edge XAS, we have found that the Eu in this system is in mixed valence state with coexistence of ${\mathrm{Eu}}^{2+}/{\mathrm{Eu}}^{3+}$. The bulk charge doping was estimated to be $\sim 0.3$ per Bi site in ${\mathrm{Eu}}_3{\mathrm{F}}_4{\mathrm{Bi}}_2{\mathrm{S}}_4$, which corresponds to the nominal $x$ in a typical RE${\mathrm{O}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$ system (RE: rare-earth elements). From the space-resolved ARPES, we have ruled out the possibility of any microscale phase separation of Eu valence in the system. Using a microfocused beam we have observed the band structure as well as the Fermi surface that appeared similar to other compounds of this family with disconnected rectangular electronlike pockets around the $X$ point. The Luttinger volume analysis gives the effective carrier to be 0.23 electrons per Bi site in ${\mathrm{Eu}}_3{\mathrm{F}}_4{\mathrm{Bi}}_2{\mathrm{S}}_4$, indicating that the system is likely to be in the underdoped region of its superconducting phase diagram.

We investigate the noise correlations of spin and charge currents through an electron spin resonance (ESR)-pumped quantum dot, which is tunnel coupled to three electrodes maintained at an equivalent chemical potential. A recursive scheme is employed with inclusion of the spin degrees of freedom to account for the spin-resolved counting statistics in the presence of non-Markovian effects due to coupling with a dissipative heat bath. For symmetric spin-up and spin-down tunneling rates, an ESR-induced spin flip mechanism generates a pure spin current without an accompanying net charge current. The stochastic tunneling of spin carriers, however, produces universal shot noises of both charge and spin currents, revealing the effective charge and spin units of quasiparticles in transport. In the case of very asymmetric tunneling rates for opposite spins, an anomalous relationship between noise autocorrelations and cross correlations is revealed, where super-Poissonian autocorrelation is observed in spite of a negative cross correlation. Remarkably, with strong dissipation strength, non-Markovian memory effects give rise to a positive cross correlation of the charge current in the absence of a super-Poissonian autocorrelation. These unique noise features may offer essential methods for exploiting internal spin dynamics and various quasiparticle tunneling processes in mesoscopic transport.

We present first-principles total-energy electronic-structure calculations that provide the microscopic mechanism of Ag atom diffusion between the half unit cells (HUCs) on the Si(111)-$\left(7\times 7\right)$ surface with and without the tip of the atomic force microscope (AFM). We find that, without the presence of the AFM tip, there are three pathways of inter-HUC diffusion. The pathway, in which the diffusing atom passes over the nanohole of the surface, has the lowest energy barrier. The diffusion along this pathway between the two HUCs is almost symmetric with the energy barrier of about 0.8 eV in both directions. In the other two pathways, the adatom diffuses along the edge of the nanohole. The diffusion along these two pathways have an energy barrier of about 1 eV. With the presence of the tip, we find that the reaction pathways are essentially the same, but the diffusion along the edge of the nanohole has a lower energy barrier than the diffusion over the nanohole. Thus the diffusion channel is changed by the presence of the tip. In the diffusion along the edge of the nanohole, the energy barrier in one direction is substantially reduced to be 0.2–0.4 eV by the tip, while that of the diffusion in the reverse direction remains larger than 1 eV. The Si tip reduces the energy barrier more than the Pt tip due to the flexibility of the tip apex structure. In addition to the reduction of the barrier, the tip traps the diffusing adatom preventing diffusion in the reverse direction. Also we find that the shape of the tip apex structure is important for the adatom's trapping ability. The bond formation between the AFM tip atom and the surface adatom is essential for atom manipulation using the AFM tip. Our results show that atom manipulation is possible with both the metallic and semiconducting AFM tips.

In this paper we investigate theoretically the influence of piezoelectric coupling on phonon dispersion relations. Specifically we solve dispersion relations for a fully coupled zinc-blende freestanding quantum well for different orientations of the crystal unit cell. It is shown that the phonon mode density in GaAs can change by a factor of approximately 2–3 at $q_{x}a=1$ for different crystal-growth directions relative to the slab thickness direction. In particular, it is found that optical and acoustic phonon modes are always piezoelectrically coupled, independent of the crystal-growth direction, and will be jointly excited by electrical stimulus. We demonstrate this for an electrically excited freestanding slab for two cases of high-symmetry crystal-growth directions and finally show the impact of the Drude model for permittivity on the phonon dispersion. In particular, it is verified that the piezoelectric effect leads to a drastically enhanced coupling of acoustic and optical phonon modes and increase in the local phonon density of states near the plasma frequency where the permittivity approaches zero.

Optical properties of transition metal dichalcogenides monolayers are controlled by Wannier-Mott excitons forming a series of $1s,2s,2p,...$ hydrogen-like states. We develop the theory of the excited excitonic states energy spectrum fine structure. We predict that $p$- and $s$-shell excitons are mixed due to the specific $D_{3h}$ point symmetry of the transition metal dichalcogenide monolayers. Hence, both $s$- and $p$-shell excitons are active in both single- and two-photon processes, providing an efficient mechanism of second harmonic generation. The corresponding contribution to the nonlinear susceptibility is calculated.