Using a combination of first-principles calculations and experimental transport measurements, we study the electronic and magnetic structure of the unfilled skutterudite ${\mathrm{FeSb}}_3$. We employ the hybrid functional approach for exchange correlation. The ground state is determined to be antiferromagnetic with an atomic magnetic moment of $1.6{\mu }_B/\mathrm{Fe}$. The Néel temperature $T_N$ is estimated at 6 K, in agreement with experiments which found a paramagnetic state down to 10 K. The ground state is semiconducting, with a small electronic gap of $33\mathrm{meV}$, also consistent with previous experiments on films. Charge carrier concentrations are estimated from Hall resistance measurements. The Seebeck coefficient is measured and mapped using a scanning probe at room temperature that yields an average value of $38.6\mu \mathrm{V}{\mathrm{K}}^{-1}$, slightly lower than the theoretical result. The theoretical conductivity is analyzed as a function of temperature and concentration of charge carriers.

To understand the seemingly absent temperature dependence in the conductance of two-dimensional topological insulator edge states, we perform a numerical study which identifies the quantitative influence of the combined effect of dephasing and elastic scattering in charge puddles close to the edges. We show that this mechanism may be responsible for the experimental signatures in HgTe/CdTe quantum wells if the puddles in the samples are large and weakly coupled to the sample edges. We propose experiments on artificial puddles which allow one to verify this hypothesis and to extract the real dephasing time scale using our predictions. In addition, we present a method to include the effect of dephasing in wave-packet-time-evolution algorithms.

We study the conductance of a single electron transistor (SET) with a ferroelectric (or dielectric) layer placed in the gate capacitor. We assume that the ferroelectric (FE) has a retarded response with arbitrary relaxation time. We show that in the case of “fast” but still retarded response of the FE (dielectric) layer an additional contribution to the Coulomb blockade effect appears leading to the suppression of the SET conductance. We take into account fluctuations of the FE (dielectric) polarization using Monte Carlo simulations. For “fast” FE, these fluctuations partially suppress the additional Coulomb blockade effect. Using Monte Carlo simulations, we study the transition from “fast” to “slow” FE. For high temperatures, the peak value of the SET conductance is almost independent of the FE relaxation time. For temperatures close to the FE Curie temperature, the conductance peak value nonmonotonically depends on the FE relaxation time. A maximum appears when the FE relaxation time is of the order of the SET discharging time. Below the Curie point the conductance peak value decreases with increasing the FE relaxation time. The conductance shows the hysteresis behavior for any FE relaxation time at temperatures below the FE transition point. We show that conductance hysteresis is robust against FE internal fluctuations.

Solid-solid transformations in Pd-Pt nanoalloys in the size range 32–38 atoms and for different compositions are computationally studied by the superposition approximation to the partition function, and by molecular dynamics simulations. A broad spectrum of transition types is shown to take place. These transition types are: (i) one-to-one type, in which the global minimum, which is dominant at low temperatures, transforms into another single isomer with increasing temperature; (ii) one-to-many type, in which the transition is from a single isomer to a family of other isomers; (iii) many-to-many type, in which the transition is between two different families of isomers; (iv) many-to-one type, in which the effect of vibrational entropy is to greatly reduce the number of relevant structures with increasing temperatures. We provide a rationale for these behaviors, which stem from the interplay between energetics and vibrational entropy effects. The vibrational entropy is explained by analyzing the vibrational density of states and the specific features of the normal modes. Quantum effects on the structural transitions are also discussed.

Excitons, trions, biexcitons, and exciton-trion complexes in two-dimensional transition metal dichalcogenide sheets of ${\mathrm{MoS}}_2, {\mathrm{MoSe}}_2, {\mathrm{MoTe}}_2, {\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$ are studied by means of density functional theory and path-integral Monte Carlo method in order to accurately account for the particle-particle correlations. In addition, the effect of dielectric environment on the properties of these exciton complexes is studied by modifying the effective interaction potential between particles. Calculated exciton and trion binding energies are consistent with previous experimental and computational studies, and larger systems such as biexciton and exciton-trion complex are found highly stable. Binding energies of biexcitons are similar to or higher than those of trions, but the binding energy of the trion depends significantly stronger on the dielectric environment than that of biexciton. Therefore, as a function of an increasing dielectric constant of the environment the exciton-trion complex “dissociates” to a biexciton rather than to an exciton and a trion.

The spin-resolved transport properties of a single-wall carbon nanotube quantum dot, with an attached single molecular magnet, are studied theoretically. With the aid of the real-time diagrammatic technique in the lowest-order perturbation expansion with respect to the tunnel coupling, the current, differential conductance, and the tunnel magnetoresistance (TMR) are determined in both the linear and nonlinear response regimes. It is shown that transport properties depend greatly on both the shell filling sequence of the carbon nanotube and the type of exchange interaction between the molecular magnet and nanotube. This results in highly nontrivial behavior of the TMR, which is especially visible in the low bias voltage regime. Depending on the gate voltage and parameters of the system, we find transport regimes where either a greatly enhanced or negative TMR develops. The mechanism leading to such behavior is associated with nonequilibrium spin accumulation, which builds up in the antiparallel magnetic configuration of the device. We show that it is crucial whether the spin accumulation occurs in the highest-weight spin states or in states with lower spin values. While in the former case it leads to enhanced TMR, in the latter case it may result in negative tunnel magnetoresistance. In addition, we analyze how the above effects depend on the magnitude of the molecular magnet's spin, and show that this dependence is generally nonmonotonic.

Ultrafast laser irradiation can trigger anisotropically structured nanoscaled gratinglike arrangements of matter, the laser-induced periodic surface structures (LIPSSs). We demonstrate here that the formation of LIPSS is intrinsically related to the coherence of the laser field. Employing several test materials that allow large optical excursions, we observe the effect of randomizing spatial phase in generating finite domains of ripples. Using three-dimensional finite-difference time-domain methods, we evaluate energy deposition patterns below a material's rough surface and show that modulated pattern, i.e., a spatially ordered electromagnetic solution, results from the coherent superposition of waves. By separating the field scattered from a surface rough topography from the total field, the inhomogeneous energy absorption problem is reduced to a simple interference equation. We further distinguish the contribution of the scattered near field and scattered far field on various types of inhomogeneous energy absorption features. It is found that the inhomogeneous energy absorption which could trigger the low-spatial-frequency LIPSSs (LSFLs) and high-spatial-frequency LIPSSs (HSFLs) of periodicity $\mathrm{\Lambda }>\lambda /\text{Re}\left(\tilde{n}\right)$ are due to coherent superposition between the scattered far field (propagation) and the refracted field, while HSFLs of $\mathrm{\Lambda }<\lambda /\text{Re}\left(\tilde{n}\right)$ are triggered by coherent superposition between the scattered near field (evanescent) and the refracted field. This is a general scenario that involves a topography-induced scattering phenomenon and stationary evanescent fields, being applied to two model case materials that exhibit large optical excursions upon excitation (W, Si) and nonplasmonic to plasmonic transitions. We indicate the occurrence of a general light interference phenomenon that does not necessarily involve wavelike surface plasmonic excitation. Finally, we discuss the role of interference field and scattered field on the enhancement of LIPSSs by simulating interpulse feedback effects and provide the electromagnetic origin of grooves $(\mathrm{\Lambda }>\lambda )$ related to a feedback-driven topography evolution. Those results strongly suggest the electromagnetic interpretation of LIPSSs in interplay with an evolving surface topography.

We present a model for wave propagation in a monolayer of spheres on an elastic substrate. The model, which considers sagittally polarized waves, includes: horizontal, vertical, and rotational degrees of freedom; normal and shear coupling between the spheres and substrate, as well as between adjacent spheres; and the effects of wave propagation in the elastic substrate. For a monolayer of interacting spheres, we find three contact resonances, whose frequencies are given by simple closed-form expressions. For a monolayer of isolated spheres, only two resonances are present. The contact resonances couple to surface acoustic waves in the substrate, leading to mode hybridization and “avoided crossing” phenomena. We present dispersion curves for a monolayer of silica microspheres on a silica substrate, assuming adhesive Hertzian interactions, and compare calculations using an effective medium approximation (including elasticity of the substrate) to a discrete model of a monolayer on a rigid substrate. While the effective medium model does not describe discrete lattice effects occurring at short wavelengths, we find that it is well suited for describing the interaction between the monolayer and substrate in the long wavelength limit. We suggest that a complete picture of the dynamics of a monolayer adhered to an elastic substrate can be found by combining the dispersion curves generated with the effective medium model for the elastic substrate and the discrete model for the rigid substrate. This model is potentially scalable for use with nano- to macroscale systems, and offers the prospect of experimentally extracting contact stiffnesses from measurements of acoustic dispersion.

The nonlinear ac stationary response of uniaxial paramagnets and superparamagnets—nanoscale solids or clusters with spin number $S\sim {10}^0-{10}^4$—in superimposed uniform ac and dc bias magnetic fields of arbitrary strength, each applied along the easy axis of magnetization, is determined by solving the evolution equation for the reduced density matrix represented as a finite set of three-term differential-recurrence relations for its diagonal matrix elements. The various harmonic components arising from the nonlinear response of the magnetization, dynamic magnetic hysteresis loops, etc., are then evaluated via matrix continued fractions indicating a pronounced dependence of the response on $S$ arising from the quantum spin dynamics, which differ markedly from the magnetization dynamics of classical nanomagnets. In the linear response approximation, the results concur with existing solutions.

We investigate anomalous Hall effect in a magnet coupled to a triplet superconductor under phase gradient. It is found that the anomalous Hall supercurrent arises from the nontrivial structure of the magnetization. The magnetic structure manifested in the Hall supercurrent is characterized by even order terms of the exchange coupling, essentially different from that discussed in the context of anomalous Hall effect, reflecting the dissipationless nature of the supercurrent. We also discuss a possible candidate for magnetic structure to verify our prediction.

We demonstrate quantum critical scaling for an $S=1/2$ Heisenberg antiferromagnetic chain compound $\mathrm{Cu}\left({\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2\right){\left({\mathrm{NO}}_3\right)}_2$ in a magnetic field around saturation, by analyzing previously reported magnetization [Y. Kono et al., Phys. Rev. Lett. 114, 037202 (2015)], thermal expansion [J. Rohrkamp et al., J. Phys.: Conf. Ser. 200, 012169 (2010)], and NMR relaxation data [H. Kühne et al., Phys. Rev. B 80, 045110 (2009)]. The scaling of magnetization is demonstrated through collapsing the data for a range of both temperature and field onto a single curve without making any assumption for a theoretical form. The data collapse is subsequently shown to closely follow the theoretically predicted scaling function without any adjustable parameters. Experimental boundaries for the quantum critical region could be drawn from the variable range beyond which the scaled data deviate from the theoretical function. Similarly to the magnetization, quantum critical scaling of the thermal expansion is also demonstrated. Further, the spin dynamics probed via NMR relaxation rate $1/T_1$ close to the saturation is shown to follow the theoretically predicted quantum critical behavior as $1/T_1\propto T^{-0.5}$ persisting up to temperatures as high as $k_{\mathrm{B}}T\simeq J$, where $J$ is the exchange coupling constant.

We report the superconductivity evolution of one unit cell (1-UC) and 2-UC FeSe films on $\mathrm{SrTi}{\mathrm{O}}_3$(001) substrates with potassium (K) adsorption. By in situ scanning tunneling spectroscopy measurement, we find that the superconductivity in 1-UC FeSe films is continuously suppressed with increasing K coverage, whereas nonsuperconducting 2-UC FeSe films become superconducting with a gap of $\sim 17\mathrm{meV}$ or $\sim 11\mathrm{meV}$ depending on whether the underlying 1-UC films are superconducting or not. This work explicitly reveals that the FeSe/STO interface plays a vital role in enhancing Cooper pairing in both 1-UC and 2-UC FeSe films.

Using first-principles evolutionary simulations, we have systematically investigated phase stability in the Hf-O system at pressure up to 120 GPa. New compounds ${\mathrm{Hf}}_5{\mathrm{O}}_2,{\mathrm{Hf}}_3{\mathrm{O}}_2$, HfO, and ${\mathrm{HfO}}_3$ are discovered to be thermodynamically stable at certain pressure ranges. Two new high-pressure phases are found for ${\mathrm{Hf}}_2\mathrm{O}$: one with space group Pnnm and anti-${\mathrm{CaCl}}_2$-type structure, another with space group $I{4}_1$/amd. Pnnm-${\mathrm{HfO}}_3$ shows interesting structure, simultaneously containing oxide ${\mathrm{O}}^{2-}$ and peroxide [O-${\mathrm{O]}}^{2-}$ anions. Remarkably, it is $P\overline{6}2m$-HfO rather than OII-${\mathrm{HfO}}_2$ that exhibits the highest mechanical characteristics among Hf-O compounds. Pnnm-${\mathrm{Hf}}_2\mathrm{O}$, Imm2-${\mathrm{Hf}}_5{\mathrm{O}}_2,P\overline{3}1m\text{−}{\mathrm{Hf}}_2\mathrm{O}$, and $P\overline{4}m2\text{−}{\mathrm{Hf}}_2{\mathrm{O}}_3$ phases also show superior mechanical properties; theoretically these phases become metastable phases to ambient pressure and their properties can be exploited.

One of the most fascinating and counterintuitive recent effects in multiferroics is directional anisotropy, the asymmetry of light propagation with respect to the direction of propagation. In such case the absorption in a material can be different for opposite directions. Besides absorption, different velocities of light for different directions of propagation may be also expected, which is termed directional birefringence. In this work, we demonstrate large directional anisotropy in multiferroic samarium ferroborate. The effect is observed for linear polarization of light in the range of millimeter wavelengths, and it survives down to low frequencies. The dispersion and absorption close to the electromagnon resonance can be controlled by external magnetic field and are fully suppressed in one direction. By changing the geometry of the external field, samarium ferroborate shows giant optical activity, which makes this material a universal tool for optical control: with a magnetic field as an external parameter it allows switching between two functionalities: polarization rotation and directional anisotropy.

The nonadiabatic contribution of environmental degrees of freedom yields an effective inertia of spin in the effective spin dynamics. In this paper, we study several aspects of the inertia of spin in metallic ferromagnets: (i) a concrete expression of the spin inertia $m_s$: $m_s=\hslash S_c/\left(2g_{\mathrm{sd}}\right)$, where $S_c$ is the spin polarization of conduction electrons and $g_{\mathrm{sd}}$ is the $sd$ coupling constant; (ii) a dynamical behavior of spin with inertia, discussed from the viewpoints of a spinning top and of a particle on a sphere; (iii) the behavior of spin waves and domain walls in the presence of inertia and the behavior of spin with inertia under a time-dependent magnetic field.

We have carried out an extensive investigation on the resistance fluctuations (noise) in an epitaxial thin film of ${\mathrm{VO}}_2$ encompassing the metal-insulator transition (MIT) region to investigate the dynamic phase coexistence of metal and insulating phases. Both flicker noise as well as the Nyquist noise (thermal noise) were measured. The experiments showed that flicker noise, which has a $1/f$ spectral power dependence, evolves with temperature in the transition region following the evolution of the phase fractions and is governed by activated kinetics. Importantly, closer to the insulating end of the transition, when the metallic phase fraction is low, the magnitude of the noise shows an anomaly and a strong non-Gaussian component of noise develops. In this region, the local electron temperature (as measured through the Nyquist noise thermometry) shows a deviation from the equilibrium bath temperature. It is proposed that this behavior arises due to current crowding where a substantial amount of the current is carried through well separated small metallic islands leading to a dynamic correlated current path redistribution and an enhanced effective local current density. This leads to a non-Gaussian component to the resistance fluctuation and an associated local deviation of the electron temperature from the bath. Our experiment establishes that phase coexistence leads to a strong inhomogeneity in the region of MIT that makes the current transport strongly inhomogeneous and correlated.

The negative sign problem in quantum Monte Carlo (QMC) simulations of cluster impurity problems is the major bottleneck in cluster dynamical mean-field calculations. In this paper, we systematically investigate the dependence of the sign problem on the single-particle basis. We explore both the hybridization-expansion and interaction-expansion variants of continuous-time QMC simulations for three-site and four-site impurity models with baths that are diagonal in the orbital degrees of freedom. We find that the sign problem in these models can be substantially reduced by using a nontrivial single-particle basis. Such bases can be generated by diagonalizing a subset of the intracluster hoppings.

We investigate theoretically in detail the nonlinear effects in the response of an optical/microwave cavity coupled to a Duffing mechanical resonator. The cavity is driven by a laser at a red or blue mechanical subband, and a probe laser measures the reflection close to the cavity resonance. Under these conditions, we find that the cavity exhibits optomechanically induced reflection (OMIR) or absorption (OMIA) and investigate the optomechanical response in the limit of nonlinear driving of the mechanics. Similar to linear mechanical drive, in an overcoupled cavity the red sideband drive may lead to both OMIA and OMIR depending on the strength of the drive, whereas the blue sideband drive only leads to OMIR. The dynamics of the phase of the mechanical resonator leads to the difference between the shapes of the response of the cavity and the amplitude response of the driven Duffing oscillator, for example, at weak red sideband drive the OMIA dip has no inflection point. We also verify that mechanical nonlinearities beyond Duffing model have little effect on the size of the OMIA dip though they affect the width of the dip.

In order to study quantum effects in a two-dimensional crystal lattice of a free-standing monolayer graphene, we have performed both path-integral Monte Carlo (PIMC) and classical Monte Carlo (MC) simulations for temperatures up to 2000 K. The REBO potential is used for the interatomic interaction. The total energy, interatomic distance, root-mean-square displacement of the atom vibrations, and the free energy of the graphene layer are calculated. The obtained lattice vibrational energy per atom from the classical MC simulation is very close to the energy of a three-dimensional harmonic oscillator $3k_{B}T$. The PIMC simulation shows that quantum effects due to zero-point vibrations are significant for temperatures $T<1000$ K. The quantum contribution to the lattice vibrational energy becomes larger than that of the classical lattice for $T<400$ K. The lattice expansion due to the zero-point motion causes an increase of 0.53% in the lattice parameter. A minimum in the lattice parameter appears at $T\simeq 500$ K. Quantum effects on the atomic vibration amplitude of the graphene lattice and its free energy are investigated.