The shandite family of solids, with hexagonal structure and composition $A_3{M}_2{X}_2$ (A = Ni, Co, Rh, Pd; M = Pb, In, Sn, Tl; X = S, Se), has attracted recent research attention due to promising applications as thermoelectric materials. Herein we discuss the electron and phonon transport properties of shandite-structured ${\mathrm{Ni}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$, based on a combination of density functional theory, Boltzmann transport theory, and experimental measurements. ${\mathrm{Ni}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ exhibits a metallic and nonmagnetic ground state with ${\mathrm{Ni}}^0$ oxidation state and very low charge on Sn and S atoms. Seebeck coefficients obtained from theoretical calculations are in excellent agreement with those measured experimentally between 100 and 600 K. From the calculation of the ratio $\sigma /\tau$ between the electronic conductivity and relaxation time, and the experimental determination of electron conductivity, we extract the variation of the scattering rate ($1/\tau$) with temperature between 300 and 600 K, which turns out to be almost linear, thus implying that the dominant electron-scattering mechanism in this temperature range is via phonons. The electronic thermal conductivity, which deviates only slightly from the Wiedemann-Franz law, provides the main contribution to thermal transport. The small lattice contribution to the thermal conductivity is calculated from the phonon structure and third-order force constants, and is only $\sim 2\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 300 K (less than 10% of the total thermal conductivity), which is confirmed by experimental measurements. Overall, ${\mathrm{Ni}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ is a poor thermoelectric material ($ZT\sim$ 0.01 at 300 K), principally due to the low absolute value of the Seebeck coefficient. However, the understanding of its transport properties will be useful for the rationalization of the thermoelectric behavior of other, more promising members of the shandite family.

We present a study of charge density wave (CDW) ordering in ${\mathrm{CeTe}}_3$ at room temperature using a scanning tunneling microscope and Raman spectroscopy. Two characteristic CDW ordering wave vectors obtained from the Fourier analysis are assessed to be $|{\mathbf{c}}^{*}-\mathbf{q}|=4.19{\mathrm{nm}}^{-1}$ and $\left|\mathbf{q}\right|=10.26{\mathrm{nm}}^{-1}$ where $|{\mathbf{c}}^{*}|=2\pi /c$ is the reciprocal lattice vector. The scanning tunneling spectroscopy measurements, along with inelastic light (Raman) scattering measurements, show a CDW gap ${\mathrm{\Delta }}_{\max }$ of approximately 0.37 eV. In addition to the CDW modulation, we observe an organization of the Te sheet atoms in an array of alternating V- and N-shaped groups along the CDW modulation, as predicted in the literature.

Single crystals of the ternary iron selenide ${\mathrm{RbFeSe}}_2$ have been investigated by means of x-ray diffraction, magnetic susceptibility, magnetization, and specific-heat measurements as well as by Mössbauer spectroscopy. Built up from linear chains of edge-sharing ${\mathrm{FeSe}}_4$ tetrahedra, ${\mathrm{RbFeSe}}_2$ represents a quasi-one-dimensional antiferromagnet. Below $T_{\mathrm{N}}=248$ K three-dimensional antiferromagnetic collinear magnetic order sets in, with the magnetic moments oriented perpendicularly to the chain direction. The hyperfine fields determined from our Mössbauer studies reveal strongly reduced magnetic moments. The high-temperature susceptibility data of ${\mathrm{RbFeSe}}_2$ suggest a one-dimensional metallic character along the chains.

We study the quantum mechanics of magnetic skyrmions in the vicinity of the skyrmion-crystal to ferromagnet phase boundary in two-dimensional magnets. We show that the skyrmion excitation has an energy dispersion that splits into multiple bands due to the combination of magnus force and the underlying lattice. Condensation of the skyrmions can give rise to an intermediate phase between the skyrmion crystal and ferromagnet: a quantum liquid, in which skyrmions are not spatially localized. We show that the critical behavior depends on the spin size $S$ and the topological number of the skyrmion. Experimental signatures of quantum skyrmions in inelastic neutron-scattering measurements are also discussed.

We present a comprehensive study of the magnetic properties of the long-range-ordered quasi-one-dimensional $J_1-J_2$ systems with a newly developed torque equilibrium spin-wave expansion approach, which can describe the spin Casimir and magnon decay effects in a unified framework. While the framework does not lose generality, our discussion will be restricted to two representative systems, each of which has only one type of interchain coupling ($J_3$ or $J_4$) and is referred to as the $J_3$ or $J_4$ system, respectively. In spite of the long-range spiral order, the dynamical properties of these systems turn out to be highly nontrivial due to the incommensurate noncollinear spin configuration and the strong quantum fluctuation effects enhanced by the frustration and low dimensionality. Both the systems show prominent spin Casimir effects induced by the vacuum fluctuation of the spin waves and related modification of the ordering vector, Lifshitz point position, and sublattice magnetization. In addition to these static properties, the dynamical behaviors of these systems are also remarkable. Significant and spontaneous magnon decay effects are manifested in the quantum corrections to the excitation spectrum, including the broadening of the spectrum linewidth and downward renormalization of the excitation energy. Furthermore, the excitation spectrum appears to be very sensitive to the types of the interchain coupling and manifests three distinct features: (i) the magnon decay patterns between the $J_3$ and $J_4$ systems are very different, (ii) the renormalized spectrum and the overall decay rate of the $J_3$ and $J_4$ systems show very different sensitivity to the magnetic anisotropy, and (iii) there is a nearly flat mode in the renormalized magnon spectrum of the $J_4$ system along the X-M direction. By adjusting the strength of magnetic anisotropy and varying the approximation scheme, it is revealed that these striking distinct features are quite robust and have deep connection with both the spin Casimir and the magnon decay effects. Thus these special consequences of the interchain coupling on the spin-wave dynamics may serve as a set of probes for different types of interchain couplings in experiments. Finally, to guide experimental measurements such as inelastic neutron scattering in realistic materials and complement our theoretical framework, we develop an analytical theory of the dynamical structure factor within the torque equilibrium formulism and provide explicit results of the quasi-one-dimensional $J_1-J_2$ systems.

Experimental data on the phase sound speed of metallic glasses show anomalies in the terahertz range, reflecting an underlying complex behavior of their phonon dispersion spectrum not yet explained. We determine the phonon dispersion curve of metallic glasses by means of massive molecular dynamics simulations, allowing us to obtain the $\text{low-}q$ region behavior with unprecedented detail. Results confirm that the sound speed is constant below the THz range, down to the macroscopic limit. On the contrary, a hardening of the sound speed, more notable in the transverse case, is found in the THz range. This behavior is modeled in terms of a relaxation model. The model gives quantitative agreement and allows us to determine a new threshold frequency ${\omega }_h$, at the end of the boson-peak region. Above ${\omega }_h$ the shear modulus increases dramatically, reflecting the end of the amorphous-like acoustic propagation region characterized by the excess density of vibrational states.

A theoretical description of spin current injection from a nonmagnetic layer into a magnetic one is presented, with the main emphasis on the description and determination of the penetration depth of spin current component transverse to the magnetization. This penetration depth also determines the depth of spin transfer torque generation. Physically, the spin current may be driven by an external electric field or by a temperature gradient. To determine the penetration depth we used ab initio calculations of channel and mixing conductances as well as of mixing transmission. The results are then used to determine the second harmonic voltage response, which in turn can be used to determine the penetration depth experimentally.

Physical properties of a pyrohafnate compound ${\mathrm{Pr}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$ have been investigated by ac magnetic susceptibility ${\chi }_{\mathrm{ac}}\left(T\right)$, dc magnetic susceptibility $\chi \left(T\right)$, isothermal magnetization $M\left(H\right)$, and heat-capacity $C_{\mathrm{p}}\left(T\right)$ measurements on polycrystalline as well as single-crystal samples combined with high-resolution synchrotron x-ray diffraction (XRD) for structural characterization and inelastic neutron scattering (INS) to determine the crystal-field energy-level scheme and wave functions. Synchrotron XRD data confirm the ordered cubic pyrochlore $\left(Fd\overline{3}m\right)$ structure without any noticeable site mixing or oxygen deficiency. No clear evidence of long-range magnetic ordering is observed down to 90 mK, however the ${\chi }_{\mathrm{ac}}\left(T\right)$ evinces slow spin dynamics revealed by a frequency dependent broad peak associated with spin freezing. The INS data reveal the expected five well-defined magnetic excitations due to crystal-field splitting of the $J=4$ ground-state multiplet of the ${\mathrm{Pr}}^{3+}$. The crystal-field parameters and ground-state wave function of ${\mathrm{Pr}}^{3+}$ have been determined. The Ising anisotropic nature of the magnetic ground state is inferred from the INS as well as $\chi \left(T\right)$ and $M\left(H\right)$ data. Together these properties make ${\mathrm{Pr}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$ a candidate compound for quantum spin-ice behavior.

The pyrochlore lattice involves corner-sharing tetrahedra and the resulting geometric frustration is believed to suppress any antiferromagnetic order for Mott insulators on this structure. There are nevertheless short-range correlations which could be vital near the Mott–Hubbard insulator-metal transition. We use a static auxiliary-field-based Monte Carlo to study this problem in real space on reasonably large lattices. The method reduces to unrestricted Hartree–Fock at zero temperature but captures the key magnetic fluctuations at finite temperature. Our results reveal that increasing interaction drives the nonmagnetic (semi) metal to a “spin disordered” metal with small local moments, at some critical coupling, and then, through a small pseudogap window, to a large moment, gapped, Mott insulating phase at a larger coupling. The spin disordered metal has a finite residual resistivity which grows with interaction strength, diverging at the upper coupling. We present the resistivity, optical conductivity, and density of states across the metal-insulator transition and for varying temperature. These results set the stage for the more complex cases of Mott transition in the pyrochlore iridates and molybdates.

We show that the entangled-plaquette variational Ansatz can be adapted to study the two-dimensional t-J model in the presence of two mobile holes. Specifically, we focus on a square lattice comprising up to $N=256$ sites in the parameter range $0.4\le J/t\le 2.0$. Ground state energies are obtained via the optimization of a wave function in which the weight of a given configuration is expressed in terms of variational coefficients associated with square and linear entangled plaquettes. Our estimates are in excellent agreement with exact results available for the $N=16$ lattice. By extending our study to considerably larger systems we find, based on the analysis of the long-distance tail of the probability of finding two holes at spatial separation $r$, and on our computed two-hole binding energies, the existence of a two-hole bound state for all the values of $J/t$ explored here. It is estimated that $d$-wave binding of the two holes does not occur for $J/t<J_c/t\simeq 0.19$.

By first-principles calculations, we find that $\mathrm{B}{\mathrm{a}}_{2}X(X=\mathrm{Si},\mathrm{Ge})$ hosts a topological semimetal phase with one nodal ring in the $k_x=0$ plane, which is protected by the glide mirror symmetry when spin-orbit coupling (SOC) is ignored. The corresponding drumheadlike surface flat band appears on the (100) surface in surface Green's function calculation. Furthermore, a topological-semimetal-to-insulator transition is found. The nodal line semimetal would evolve into a topological insulator as SOC is turned on. The topologically protected metallic surface states emerge around the $\overline{\mathrm{\Gamma }}$ point, which could be tuned into the topologically trivial insulator state by more than 3% hydrostatic strain. These results reveal a category of materials showing quantum phase transition between topological semimetal and insulator, and tunability through elastic strain engineering.

We present generic conditions for phase band crossings for a class of periodically driven integrable systems represented by free fermionic models subjected to arbitrary periodic drive protocols characterized by a frequency ${\omega }_D$. These models provide a representation for the Ising and $XY$ models in $d=1$, the Kitaev model in $d=2$, several kinds of superconductors, and Dirac fermions in graphene and atop topological insulator surfaces. Our results demonstrate that the presence of a critical point/region in the system Hamiltonian (which is traversed at a finite rate during the dynamics) may change the conditions for phase band crossings that occur at the critical modes. We also show that for $d>1$, phase band crossings leave their imprint on the equal-time off-diagonal fermionic correlation functions of these models; the Fourier transforms of such correlation functions, $F_{{\vec{k}}_0}\left({\omega }_0\right)$, have maxima and minima at specific frequencies which can be directly related to ${\omega }_D$ and the time at which the phase bands cross at $\vec{k}={\vec{k}}_0$. We discuss the significance of our results in the contexts of generic Hamiltonians with $N>2$ phase bands and the underlying symmetry of the driven Hamiltonian.

Subjecting a nanohelix to a transverse electric field gives rise to superlattice behavior with tunable electronic properties. We theoretically investigate such a system and find Bloch oscillations and negative differential conductance when a longitudinal electric field (along the nanohelix axis) is also applied. Furthermore, we study dipole transitions across the transverse-electric-field-induced energy gap, which can be tuned to the eulogized terahertz frequency range by experimentally attainable external fields. We also reveal a photogalvanic effect by shining circularly polarized light onto our helical quantum wire.

Strong and ultrastrong light-matter coupling are remarkable phenomena of quantum electrodynamics occurring when the interaction between matter excitation and an electromagnetic field cannot be described by usual perturbation theory. This is generally achieved by coupling an excitation with large oscillator strength to the confined electromagnetic mode of an optical microcavity. In this work, we demonstrate that strong/ultrastrong coupling can also take place in the absence of optical confinement. We have studied the nonperturbative spontaneous emission of collective excitations in a dense two-dimensional electron gas that superradiantly decays into free space. By using a quantum model based on the input-output formalism, we have derived the linear optical properties of the coupled system, and we demonstrated that its eigenstates are mixed light-matter particles, as in any system displaying strong or ultrastrong light-matter interaction. Moreover, we have shown that in the ultrastrong coupling regime, i.e., when the radiative broadening is comparable to the matter excitation energy, the commonly used rotating-wave and Markov approximations yield unphysical results. Finally, the input-output formalism has allowed us to prove that Kirchhoff's law, describing thermal emission properties, applies to our system in all the light-matter coupling regimes considered in this work.

Electron spins in quantum dots can interact with impurity spins located in an adjacent region. This interaction may be controllable using external electric fields and it can involve an appreciable spin-orbit interaction (SOI) part affecting the expected operation of the dot spins. In this work we propose a method to quantify the interaction between a dot spin and a nearby spin by calculating the electrical current through the dot. We demonstrate some interesting regimes where the SOI can be detected, and find regimes of negative differential conductance which are sensitive to the strength of the SOI. The present study could be useful for spin-orbit qubits formed in quantum dot devices.

Using first-principles calculations, we investigate the tunneling electroresistance (TER) of ferroelectric tunnel junctions [$\mathrm{Pt}/{\mathrm{BaTiO}}_3\text{(BTO)}/{\mathrm{SrTiO}}_3\left(\mathrm{STO}\right)/\mathrm{Pt}$]. It is found that the TER of Pt/BTO/STO/Pt junctions can be greatly increased with increasing thickness of STO layers. The underlying physics of this giant TER is the switchable two-dimensional electron gas (2DEG) at a nonpolar BTO/STO interface induced by the ferroelectric polarization. Our calculations show that when the ferroelectric polarization is pointing from BTO to STO, a 2DEG forms at the interface and acts as bridge for electrons to tunnel through the junctions. Nevertheless, there is no 2DEG at the interface under the opposite direction of the ferroelectric polarization, which results in a large tunnel resistance. More importantly, this ferroelectrically switchable 2DEG leads to a low resistance area product for Pt/BTO/STO/Pt junctions, which offers good compatibility with other components in an integrated circuit and is highly desired for industrial applications.

We report on magnetospectroscopy of HgTe quantum wells in magnetic fields up to 45 T in a temperature range from 4.2 up to 185 K. We observe intra- and inter-band transitions from zero-mode Landau levels, which split from the bottom conduction and upper valence sub-bands and merge under the applied magnetic field. To describe experimental results, realistic temperature-dependent calculations of Landau levels have been performed. We show that although our samples are topological insulators at low temperatures only, the signature of such a phase persists in optical transitions at high temperatures and high magnetic fields. Our results demonstrate that temperature-dependent magnetospectroscopy is a powerful tool to discriminate trivial and topological insulator phases in HgTe quantum wells.

We investigated the mechanisms of secondary electron (SE) emission from Be(0001) by impact of 100 and 150 eV electrons. We made use of (e, 2e) spectroscopy to disentangle the different SE production mechanisms. We observed a large increase in the SE yield when the energy loss of the primary electron equals the characteristic energy of volume and surface plasmons. The line shape of the SE spectrum associated with plasmon excitation reveals that one relevant emission mechanism corresponds to direct single-particle excitation in which the plasmon energy and momentum are transferred to a valence band electron of the solid. The contributions to the SE yield associated with surface and volume plasmon excitation are comparable in the case of specular geometry, where the projectile momentum is mainly transferred perpendicular to the surface. On the contrary, the emission of SEs associated with surface plasmon excitation is significantly enhanced when the exchanged momentum lies close to the surface plane and electrons are emitted from Be surface state. This reflects the increased sensitivity to surface modes of the latter geometry. Finally, the coupling between the direct ionization channel and the plasmon-assisted one results in a resonant increase of the secondary emission.

First-principles calculations of the essential spin-orbit and spin relaxation properties of phosphorene are performed. Intrinsic spin-orbit coupling induces spin mixing with the probability of $b^2\approx {10}^{-4}$, exhibiting a large anisotropy, following the anisotropic crystalline structure of phosphorene. For realistic values of the momentum relaxation times, the intrinsic (Elliott-Yafet) spin relaxation times are hundreds of picoseconds to nanoseconds. Applying a transverse electric field (simulating gating and substrates) generates extrinsic $C_{2v}$ symmetric spin-orbit fields in phosphorene, which activate the D'yakonov-Perel' mechanism for spin relaxation. It is shown that this extrinsic spin relaxation also has a strong anisotropy and can dominate over the Elliott-Yafet one for strong enough electric fields. Phosphorene on substrates can thus exhibit an interesting interplay of both spin-relaxation mechanisms, whose individual roles could be deciphered using our results.