Finite-barrier corrections to the ferromagnetic resonance (FMR) frequency of nanomagnets are obtained in closed integral form from the undamped deterministic equation of motion of the magnetization by averaging the precession frequency as expressed by elliptic functions over all possible precessional trajectories inside a well. The method is illustrated by determining the FMR frequency for nanomagnets with both uniaxial and biaxial anisotropy subjected to a uniform external field and for nanomagnets with mixed cubic and uniaxial anisotropy. The results agree with exact numerical calculations obtained from the magnetic Langevin equation via matrix continued fractions.

We use Monte Carlo (MC) methods to simulate a two-dimensional (2D) bond-diluted Ising model on the square lattice, which has frustration between nearest-neighbor interaction $J_1$ and next-nearest-neighbor interaction $J_2$. In this study, we use the parallel tempering algorithm to study thermodynamics for different diluted ratios $x$ [where $x=N{\left(J_2\right)}_{\mathrm{diluted}}/2N$, where $N$ denotes the system volume] and present a phase diagram. The presence of frustration and disorder results in a spin-glass phase, which exists between the stripe antiferromagnetic phase and Néel phase. We present a ground-state energy of $T\to 0$ and the size dependence of the Edwards-Anderson (EA) order parameter for the spin-glass phase. By scaling the mean energy and EA order parameter from the simulated annealing with the Kibble-Zurek mechanism, we obtain two different dynamic exponents, $z_E$ and $z_q$, for the spin-glass phase. Experimentally, this model has a close relationship with the $\mathrm{FeAs}$ plane of the iron-based superconductor ${\mathrm{BaFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$, where a spin-glass-like phase was found.

We study theoretically the Josephson current-phase relationship in a chaotic quantum dot coupled to superconductors by ballistic contacts. In this regime, strong proximity effect induces superconductivity in the quantum dot that leads to a significant modification in the electron density of states and formation of multiple subgaps. The magnitude of the resulting supercurrent depends on the phase difference of the superconducting order parameter in the leads and shows strongly anharmonic skewed behavior. We find that when the Thouless energy on the dot exceeds the superconducting energy gap, the second harmonic of the supercurrent becomes comparable in magnitude to the first harmonic. To address these effects on the technical level, we use the nonlinear $\sigma$-model Keldysh formalism in the framework of the circuit theory to compute dependence of the density of states, Josephson energy, and current on the superconducting phases in the leads. We analyze how these quantities change as a function of the Thouless energy and the superconducting gap. Finally, we briefly discuss subgap tail states, mesoscopic supercurrent fluctuations, weak localization correction, and also touch on anharmonicity of gatemon qubits with quantum dot Josephson junctions.

To investigate the $4f$-electronic states under a crystal electric field (CEF) and the phase transition in ${\mathrm{Dy}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$ with the antiferromagnetic transition temperature $T_{\mathrm{N}}=7$ K, we performed ultrasonic measurements on a single-crystalline sample at zero magnetic field and under fields. The transverse elastic modulus $C_{44}$ shows a characteristic elastic softening. The CEF analyses indicate that the softening is caused by an interlevel quadrupole interaction between the ground and excited Kramers doublets. Under fields, we found a magnetic-field-induced phase transition along both the [100] and [001] directions in addition to the antiferromagnetic ordering. A plausible origin of the field-induced transition is quadrupolar ordering, which is estimated from our CEF calculation. These results and the negative sign of a quadrupole-quadrupole coupling constant suggest that the effect of geometrical frustration alignment due to the kagome lattice also appears on the electric quadrupoles of the Dy ions with the antiferroquadrupolar-type interaction.

We investigate the Kondo effect in a double quantum dot which is capacitively coupled to a charge qubit. It is shown that due to this capacitive coupling, the bare interdot repulsive interaction in the double quantum dot is effectively reduced and eventually changed to an attractive interaction for strong couplings between the double quantum dot and the qubit. By deriving the low-energy effective Hamiltonian of the system, we find that the low-energy dynamics of the system corresponding to these two positive or negative effective interaction regimes can be described, respectively, by an isotropic orbital-Kondo or an anisotropic charge-Kondo Hamiltonian. Moreover, we study various thermodynamic and electronic transport properties of the system by using the numerical renormalization group method.

Excitons, the particle-hole bound states, composed of localized electron-hole states in semiconducting systems, are crucial to explaining the optical spectrum. Spectroscopic measurements can contain signatures of these two-particle bound states and can be particularly useful in determining the characteristics of these excitons. We formulate an expression for evaluating the angle-resolved photoemission spectrum arising from the ionization of excitons given their steady-state distribution in a semiconductor. We show that the spectrum contains information about the direct/indirect band gap nature of the semiconductor and is located below the conduction band minimum displaced by the exciton binding energy. The dispersive features of the spectrum contains remnants of the valence band while additional interesting features arise from different exciton distributions. Our results indicate that for most exciton probability distributions, the energy-integrated photoemission spectrum provides an estimate of the exciton Bohr radius.

Phosphorene is an anisotropic puckered two-dimensional hexagonal lattice of phosphorus atoms. The edge modes in a zigzag phosphorene nanoribbon are quasiflat in nature and fully isolated from the bulk states, which are unique in comparison to the other hexagonal lattices like graphene, silicene, etc. We theoretically investigate the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between two magnetic impurities placed on the nanoribbon and extract the signatures of the flat edge states via the behavior of it. Due to the complete separation of the edge states from the bulk, we can isolate the edge mode contribution via the RKKY interaction from that of the bulk by tuning the external gate potential when both the impurities are placed at the same edge. The bulk-induced RKKY interaction exhibits very smooth oscillation with the distance between the two impurities, whereas for edge modes it fluctuates very rapidly. We also explore the effect of tensile strain both in absence and presence of gate voltage and reveal that the RKKY interaction strength can be boosted under suitable doping, when both the impurities are within the bulk.

In analogy with light refraction at optical boundary, ballistic electrons also undergo refraction when propagating across a semiconductor junction. Establishing a negative refractive index in conventional optical materials is difficult, but the realization of negative refraction in an electronic system is conceptually straightforward, which has been verified in graphene $p\text{−}n$ junctions in recent experiments. Here, we propose a model to realize double refraction and double focusing of electric current by a normal-conductor/hexagonal-semiconductor junction. The double refraction can be either positive or negative, depending on the junction being $n\text{−}n$ type or $p\text{−}n$ type. Based on the valley-dependent negative refraction, a spin splitter (valley splitter) is designed at the $p\text{−}n$ junction system, where the spin-up and spin-down electrons are focused in different regions. These findings may be useful for the engineering of double lenses in electronic systems and have an underlying application of spin splitter in spintronics.

The surface tension $\sigma$ of free-standing graphene is studied by path-integral simulations as a function of the temperature and the in-plane stress. Even if the applied stress vanishes, the membrane displays a finite surface tension $\sigma$ due to the coupling between the bending oscillations and the real area of the membrane. Zero-point effects for $\sigma$ are significant below 100 K. Thermal cooling drives the membrane from a planar to a wrinkled morphology. Upon heating the change is reversible and shows hysteresis, in agreement with recent experiments performed on supported graphene.

Two-dimensional hexagonal and oblique lattices were investigated theoretically with the aim of observing differences in the spin expectation values between chiral and achiral systems. The spin-resolved band structures were derived from the energy eigenvalues and eigenfunctions of a Hamiltonian that includes the lattice potential and the spin-orbit interaction. The spin texture of the achiral hexagonal system was shown to have two nonzero components of the spin polarization, whereas all three components were calculated to be nonzero for the chiral system. The longitudinal component, found to be zero in the achiral lattice, was observed to invert between the enantiomorphs of the chiral lattice. A heuristic model was introduced to discern the origin of this inverting spin polarization by considering the dynamics of an electron in chiral and achiral lattices. This model was further used to demonstrate the change in magnitude of the spin polarization as a function of the lattice parameters and an electric field perpendicular to the lattice.

Using symmetry considerations, we show that nodal rings in a certain class of magnetic oxides are guaranteed to remain gapless in the presence of spin-orbital coupling (SOC). The condition is that the compound must possess a mirror plane perpendicular to its intrinsic magnetic moment. We also show by first-principles calculations that a number of widely used compounds with a cubic phase, such as $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$, $\mathrm{Cr}{\mathrm{O}}_2$, $\mathrm{L}{\mathrm{i}}_2\mathrm{V}{\mathrm{O}}_4$, $\mathrm{L}{\mathrm{i}}_2\mathrm{Ti}{\mathrm{O}}_4$, and $\mathrm{C}{\mathrm{u}}_2\mathrm{Ti}{\mathrm{O}}_4$, belonging to the symmorphic magnetic group $D_{4h}\left(C_{4h}\right)$, possess nodal rings with this guaranteed feature. Furthermore, we investigate drumhead surface states of these compounds bounded by a semi-infinite (001) surface. The drumhead surface states in some of our proposed materials (e.g., $\mathrm{Cr}{\mathrm{O}}_2$), located near the Fermi level, are easily detectable and guaranteed to be intact in the presence of SOC. The findings are important to applications of topological materials to quantum transport. This work also demonstrates the use of symmetry considerations as a powerful pathway for discovering stable node-line fermions and nontrivial surface states in the presence of SOC.

We use inelastic neutron scattering (INS) to investigate the effect of electron correlations on spin dynamics in the iron-based superconductor ${\mathrm{Ba}}_{0.75}{\mathrm{K}}_{0.25}{\mathrm{Fe}}_2{\mathrm{As}}_2$. Our INS data show a spin-wave-like dispersive feature, with a zone boundary energy of 200 meV. A first-principles analysis of dynamical spin susceptibility, incorporating the mass renormalization factor of 3, as determined by angle-resolved photoemission spectroscopy, provides a reasonable description of the observed spin excitations. This analysis shows that electron correlations in the $\mathrm{Fe}-3d$ bands yield enhanced effective electron masses, and consequently, induce substantial narrowing of the spin excitation bandwidth. Our results highlight the importance of electron correlations in an itinerant description of the spin excitations in iron-based superconductors.

We consider the scaling behavior of thermodynamic quantities in the one-dimensional transverse field Ising model near its quantum critical point (QCP). Our study has been motivated by the question about the thermodynamical signatures of this paradigmatic quantum critical system and, more generally, by the issue of how quantum criticality accumulates entropy. We find that the crossovers in the phase diagram of temperature and (the nonthermal control parameter) transverse field obey a general scaling ansatz, and so does the critical scaling behavior of the specific heat and magnetic expansion coefficient. Furthermore, the Grüneisen ratio diverges in a power-law way when the QCP is accessed as a function of the transverse field at zero temperature, which follows the prediction of quantum critical scaling. However, at the critical field, upon decreasing the temperature, the Grüneisen ratio approaches a constant instead of showing the expected divergence. We are able to understand this unusual result in terms of a peculiar form of the quantum critical scaling function for the free energy; the contribution to the Grüneisen ratio vanishes at the linear order in a suitable Taylor expansion of the scaling function. In spite of this special form of the scaling function, we show that the entropy is still maximized near the QCP, as expected from the general scaling argument. Our results establish the telltale thermodynamic signature of a transverse-field Ising chain, and will thus facilitate the experimental identification of this model quantum-critical system in real materials.

We present the theory of thermoelectric transport in metals with long-lived quasiparticles, carefully addressing the interplay of electron-electron scattering as well as electron-impurity scattering, but neglecting electron-phonon scattering. In Fermi liquids with a large Fermi surface and weak electron-impurity scattering, we provide universal and simple formulas for the behavior of the thermoelectric conductivities across the ballistic-to-hydrodynamic crossover. In this regime, the electrical conductivity is relatively unchanged by hydrodynamic effects. In contrast, the thermal conductivity can be parametrically smaller than predicted by the Wiedemann-Franz law. A less severe violation of the Mott law arises. We quantitatively compare the violations of the Wiedemann-Franz law arising from (i) momentum-conserving electron-electron scattering in the collision integral, (ii) hydrodynamic modifications of the electron-impurity scattering rate, and (iii) thermal broadening of the Fermi surface, and show that (i) is generally the largest effect. We present simple formulas for electrical and thermal magnetoconductivity across the ballistic-to-hydrodynamic limit, along with a more complicated formula for the thermoelectric magnetoconductivity. In a finite magnetic field, the Lorenz number may be smaller or larger than predicted by the Wiedemann-Franz law, and the crossover between these behaviors is a clear prediction for experiments. The arbitrarily strong violation of the Wiedemann-Franz law found in our work arises entirely from electron-electron interaction effects within the Fermi-liquid paradigm, and does not imply any non-Fermi-liquid behavior. We predict clear experimental signatures of bulk hydrodynamics in high-mobility two-dimensional GaAs semiconductor structures, where a spectacular failure of the Wiedemann-Franz law should persist down to very low temperatures in high-quality and low-density samples.

We calculate the transport relaxation time ${\tau }_{\mathrm{tr}}$ and spin transport relaxation time ${\tau }_{s,\mathrm{tr}}$ for a two-dimensional electron gas with spatially fluctuating Rashba spin-orbit interaction. These relaxation times determine the electrical and spin conductivity of the two-dimensional system, respectively. It is shown that the transport relaxation time ${\tau }_{\mathrm{tr}}$ is a nonmonotonic function of electron energy $\varepsilon$, whereas the spin transport relaxation time ${\tau }_{s,\mathrm{tr}}$ decreases with increasing $\varepsilon$, similarly to the conventional electron relaxation time $\tau$ that characterizes the decay of an electron state corresponding to certain values of the momentum and spin. Such a behavior of the relaxation times leads to unusual temperature dependence of the electrical and spin conductivity.

We explore the finite-bias DC differential conductance of a correlated quantum dot under the influence of an AC field, from the low-temperature Kondo to the finite-temperature Coulomb blockade regime. Real-time simulations are performed using a time-dependent generalization of the steady-state density functional theory (i-DFT) [Nano Lett. 15, 8020 (2015)]. The numerical simplicity of i-DFT allows for unprecedented long time evolutions. Accurate values of average current and density are obtained by integrating over several periods of the AC field. We find that (i) the zero-temperature Kondo plateau is suppressed, (ii) the photon-assisted conductance peaks are shifted due to correlations, and (iii) the Coulomb blockade is lifted with a concomitant smoothening of the sharp diamond edges.

Hexagonally structured MnRuP was studied under high pressure up to 35 GPa from 5 to 300 K using synchrotron x-ray diffraction. We observed that a partial phase transition from hexagonal to orthorhombic symmetry started at 11 GPa. The new and denser orthorhombic phase coexisted with its parent phase for an unusually long pressure range, $\mathrm{\Delta }P\approx 50\mathrm{GPa}$. We attribute this structural transformation to a magnetic origin, where a decisive criterion for the boundary of the mixed phase lies in the different distances between the Mn-Mn atoms. In addition, our theoretical study shows that the orthorhombic phase of MnRuP remains steady even at very high pressures up to $\sim 250\mathrm{GPa}$, when it should transform to a new tetragonal phase.

Through theoretical analysis and numerical calculation, it is found that the textured surface of a perfect rigid body whose groove height is sufficiently large can support multiple modes of propagating spoof surface acoustic waves (SSAWs). Dispersion curves and group velocities/refractive index versus height and width of the groove are depicted for infinitely thick perfect rigid body to make a thoroughly inquiry into the multiband transmission of SSAWs. We also give the dispersion relations of samples with finite thickness which are more practical to realize, and the corresponding experimental results are in accordance with simulated results. Theoretical, simulated, and measured acoustic pressure field distributions are given to further illustrate the characteristics of different modes. Our work could have implications for applications such as acoustic filter design and energy harvesting.

We report high-field magnetization study on the complex spin system $h\text{−}\mathrm{ErMn}{\mathrm{O}}_3$ and propose an explanation for the $c$-axis phase transitions observed at 0.8, 12, and 28 T based on an analysis of the magnetization jumps, the complete $H\text{−}T$ phase diagram and the irreducible representations of the $\mathrm{E}{\mathrm{r}}^{3+}$ ($2a$ and $4b$) spin structure. Our measurements reveal that magnetic reorientation of the representation ${\mathrm{\Gamma }}_2$ induces the magnetic transition at 12 T, i.e., from a one-third plateau phase to a fully polarized phase. The magnetic transition at 0.8 T and the anomaly at 28 T can be understood by magnetic reorientations of ${\mathrm{\Gamma }}_1\to {\mathrm{\Gamma }}_2$ and a reflection of the $\mathrm{M}{\mathrm{n}}^{3+}$ moments triggered by the $\mathrm{E}{\mathrm{r}}^{3+}$ spins in strong magnetic fields. These scenarios are in good agreement with the magnetization process perpendicular to the $c$ axis and the magnetic susceptibility results. These findings are helpful for a complete understanding of the magnetic phase transitions and the underlying physics in this family compounds.

A single-phase high-pressure (HP) modification of $\mathrm{C}{\mathrm{s}}_x\mathrm{F}{\mathrm{e}}_{2-y}\mathrm{S}{\mathrm{e}}_2$ was synthesized at 11.8 GPa at ambient temperature. Structurally this polymorph is similar to the minor low-pressure (LP) superconducting phase; namely, they both crystallize in a $\mathrm{ThC}{\mathrm{r}}_2\mathrm{S}{\mathrm{i}}_2$-type structure without ordering of the Fe vacancies within the Fe-deficient $\mathrm{FeS}{\mathrm{e}}_4$ layers. The HP $\mathrm{C}{\mathrm{s}}_x\mathrm{F}{\mathrm{e}}_{2-y}\mathrm{S}{\mathrm{e}}_2$ polymorph is found to be less crystalline and nearly twice as soft compared to the parent major and minor phases of $\mathrm{C}{\mathrm{s}}_x\mathrm{F}{\mathrm{e}}_{2-y}\mathrm{S}{\mathrm{e}}_2$. It can be quenched to low pressures and is stable at least on the scale of weeks. At ambient pressure the HP polymorph of $\mathrm{C}{\mathrm{s}}_x\mathrm{F}{\mathrm{e}}_{2-y}\mathrm{S}{\mathrm{e}}_2$ is expected to exhibit different superconducting properties compared to its LP minor phase ($T_c=27\mathrm{K}$).