Ultraviolet Raman spectroscopy revealed the existence of an unusual large-frequency shift occurring to a nonsoft mode of $E({\mathrm{TO}}_4$) when ${\mathrm{BaTiO}}_3$ is strained to a ${\mathrm{SrTiO}}_3$ substrate [D. Tenne et al., Science 313, 1614 (2006)]. It raised two interesting questions: (i) whether there are other nonsoft modes that possess similar or even larger strain-induced frequency shifts and (ii) how the mode sequence is altered by these shifts in frequency. Note that mode sequence is also pivotal in correctly indexing and assigning the spectroscopy peaks observed in all Raman experiments. By mapping out the evolutions of individual phonon modes as a function of strain using first-principles density functional perturbation calculations, we determine the mode sequence and strain-induced phonon frequency shifts in prototypical ${\mathrm{BaTiO}}_3$. Our study reveals that the mode sequence is drastically different when ${\mathrm{BaTiO}}_3$ is strained to ${\mathrm{SrTiO}}_3$ compared to that in the unstrained structure, caused by multiple mode crossings. Furthermore, we predict that three other nonsoft modes, $A_1$(${\mathrm{TO}}_2$), $E({\mathrm{LO}}_4$), and $A_1$(${\mathrm{TO}}_3$), display even larger strain-induced frequency shifts than $E({\mathrm{TO}}_4$). The strain responses of individual modes are found to be highly mode specific, and a mechanism that regulates the magnitude of the frequency shift is provided. As another key outcome of this study, we tackle a long-standing problem of LO-TO splitting in ferroelectrics. A rigorous definition for the LO-TO splitting is formulated, which allows this critical quantity to be calculated quantitatively. The definition immediately reveals a new finding; that is, a large LO-TO splitting not only exists for $E({\mathrm{LO}}_4$), which is previously known and originates from a soft mode, it also occurs for a nonsoft $A_1$(${\mathrm{LO}}_3$) mode. The LO-TO splitting is shown to decrease drastically with compressive strain, and this decrease cannot be explained by the Born effective charges and high-frequency dielectric constants.

We study the low-temperature thermal conductivity $\left(\kappa \right)$ of ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ single crystals in magnetic fields up to 14 T along the [111], [100], and [110] directions. For ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ we find experimentally that (i) the low-$T\kappa \left(H\right)$ isotherms exhibit not only the steplike decreases at the low-field $(<2\mathrm{T})$ magnetic transitions but also obvious field dependencies in high fields $(>7\mathrm{T})$; (ii) at $T\le 0.5$ K, the $\kappa \left(H\right)$ curves show anisotropic irreversibility in low fields, that is, the $\kappa \left(H\right)$ hysteresis is located at the first-order transition with $H\parallel$ [100] and [110], while it is located between two successive transitions with $H\parallel$ [111]; (iii) the $\kappa$ in the hysteresis loops for $H\parallel$ [100] and [110] show an extremely slow relaxation with the time constant of $\sim 1000$ min. For ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ we find that (i) the zero-field $\kappa \left(T\right)$ show a kinklike decrease at the first-order transition $(\sim 200\mathrm{mK})$ with decreasing temperature, (ii) the low-$T\kappa \left(H\right)$ isotherms show a decrease in low field and a large enhancement in high fields, and (iii) the low-$T\kappa \left(H\right)$ curves show a sharp minimum at 0.5 T for $H\parallel$ [110] and [111]. The roles of monopole excitations, field-induced transitions, spin fluctuations, and magnetoelastic coupling are discussed.

Two ubiquitous features of frustrated spin systems stand out: massive degeneracy of their ground states and flat, or dispersionless, excitation branches. In real materials, the former is frequently lifted by secondary interactions or quantum fluctuations, in favor of an ordered or spin-liquid state, but the latter often survive. We demonstrate that flat modes may precipitate remarkably strong quantum effects even in the systems that are otherwise written off as almost entirely classical. The resultant spectral features should be reminiscent of the quasiparticle breakdown in quantum systems, only here the effect is strongly amplified by the flatness of spin-excitation branches, leading to the damping that is not vanishingly small even at $S\gg 1$. We provide a theoretical analysis of the excitation spectrum of the $S=5/2$ iron jarosite to illustrate our findings and to suggest further studies of this and other frustrated spin systems.

Gilbert-like damping in magnetization dynamics is commonly attributed to the interplay of the spin, the electron, and the phonon reservoirs. Spatial correlations within the spin reservoir itself, for example magnons, mediate damping as well. We show theoretically that temporal correlations within the spin reservoir cause a similar effect. We investigate the role of time retardation in the atomistic Landau-Lifshitz-Gilbert equation using two different retardation kernels. Although viscous damping is explicitly excluded, we find both analytically and numerically that damping and higher-order effects emerge due to time retardation. Thus, our results establish a mechanism for damping and inertia in magnetic systems.

We use an external magnetic field to probe the detection mechanism of a superconducting nanowire single-photon detector. We argue that the hot belt model (which assumes partial suppression of the superconducting order parameter $\mathrm{\Delta }$ across the whole width of the superconducting nanowire after absorption of the photon) does not explain observed weak-field dependence of the photon count rate (PCR) for photons with $\lambda =450\mathrm{nm}$ and noticeable decrease of PCR (with increasing the magnetic field) in a range of the currents for photons with wavelengths $\lambda =450–1200\mathrm{nm}$. Found experimental results for all studied wavelengths can be explained by the vortex hot spot model (which assumes partial suppression of $\mathrm{\Delta }$ in the area with size smaller than the width of the nanowire) if one takes into account nucleation and entrance of the vortices to the photon induced hot spot and their pinning by the hot spot with relatively large size and strongly suppressed $\mathrm{\Delta }$.

We study the electronic instabilities of doped bilayer silicene using the random phase approximation. In contrast to the singlet $d+i{d}^{\prime }$ superconductivity at the low doping region, we find that the system is an itinerant ferromagnet in the narrow doping regions around the Van Hove singularities, and a triplet $p+i{p}^{\prime }$ superconductor in the vicinity of these regions. Adding a weak Kane-Mele spin-orbit coupling to the system further singles out the time-reversal invariant equal-spin helical $p+i{p}^{\prime }$ pairing as the leading instability. The triplet pairing identified here is driven by the ferromagnetic fluctuations, which become strong and enhance the superconducting critical temperature remarkably near the phase boundaries between ferromagnetism and superconductivity.

The recent conjecture of a topologically protected surface state in ${\mathrm{SmB}}_6$ and the verification of robust surface conduction below 4 K have prompted a large effort to understand surface states. Conventional Hall transport measurements allow current to flow on all surfaces of a topological insulator, so such measurements are influenced by contributions from multiple surfaces of varying transport character. Instead, we study magnetotransport of ${\mathrm{SmB}}_6$ using a Corbino geometry, which can directly measure the conductivity of a single, independent surface. Both (011) and (001) crystal surfaces show a strong negative magnetoresistance at all magnetic field angles measured. The (011) surface has a carrier mobility of $122{\text{cm}}^2/\text{V}·\text{s}$ with a carrier density of $2.5\times {10}^{13}{\text{cm}}^{-2}$, which are significantly lower than indicated by Hall transport studies. This mobility value can explain the failure so far to observe Shubnikov–de Haas oscillations. Analysis of the angle dependence of conductivity on the (011) surface suggests a combination of a field-dependent enhancement of the carrier density and a suppression of Kondo scattering from native oxide layer magnetic moments as the likely origin of the negative magnetoresistance. Our results also reveal a hysteretic behavior whose magnitude depends on the magnetic field sweep rate and temperature. Although this feature becomes smaller when the field sweep is slower, it does not disappear or saturate during our slowest sweep-rate measurements, which is much slower than a typical magnetotransport trace. These observations cannot be explained by quantum interference corrections such as weak antilocalization but are more likely due to an extrinsic magnetic effect such as the magnetocaloric effect or glassy ordering.

We study a family of frustrated antiferromagnetic $\text{spin}-S$ systems with a fully dimerized ground state. Starting from the simplest case of the frustrated zigzag spin ladder, we generalize the family to more complex geometries like tetrahedral ladders and spin tubes. After presenting some numerical results about the phase diagram of these systems, we show that the ground state is robust against the inclusion of weak disorder in the couplings as well as several kinds of perturbations, allowing to study some other interesting models as a perturbative expansion of the exact one. A discussion on how to determine the dimerization region in terms of quantum information estimators is also presented. Finally, we explore the relation of these results with the case of a four-leg spin tube, which recently was proposed as a model for the description of the compound ${\mathrm{Cu}}_2{\mathrm{Cl}}_4{\mathrm{D}}_8{\mathrm{C}}_4{\mathrm{SO}}_2$, delimiting the region of the parameter space where this model presents dimerization in its ground state.

Exploring spin-orbit coupling (SOC) in single-layer materials is important for potential spintronics applications. Using first-principles calculations, we show that single-layer antimony telluroiodide SbTeI behaves as a two-dimensional semiconductor exhibiting a $G_0{W}_0$ band gap of 1.82 eV. More importantly, we observe the Rashba spin splitting in the SOC band structure of single-layer SbTeI with a sizable Rashba coupling parameter of 1.39 $\mathrm{eV}\text{Å}$, which is significantly larger than that of a number of two-dimensional systems including surfaces and interfaces. The low formation energy and real phonon modes of single-layer SbTeI imply that it is stable. Our study suggests that single-layer SbTeI is a candidate single-layer material for applications in spintronics devices.

We propose a hybrid system composed of a Majorana qubit and a nanomechanical resonator, implemented by a spin-orbit-coupled superconducting nanowire, using a set of static and oscillating ferromagnetic gates. The ferromagnetic gates induce Majorana bound states in the nanowire, which hybridize and constitute a Majorana qubit. Due to the oscillation of one of these gates, the Majorana qubit can be coherently rotated. By tuning the gate voltage to modulate the local spin-orbit coupling, it is possible to reach the resonance of the qubit-oscillator system for relatively strong couplings.

We derive the frequency-resolved heat current expression in the linear-response regime for a setup composed of a reservoir, an interacting central site, and a tunneling barrier under the action of a time-dependent electrical signal. We exploit the frequency parity properties of response functions to obtain the heat current expression for interacting quantum conductors. Importantly, the corresponding heat formula, valid for arbitrary ac frequencies, can describe photon-assisted heat transport. In particular, we analyze the heat transfer for an interacting multilevel conductor (a carbon nanotube quantum dot) coupled to a single reservoir. We show that the electrothermal admittance can reverse its sign by properly tuning the ac frequency.

We show that a dangling bond (DB) dimer on Ge(001):H exhibits a dynamical behavior when the empty states are imaged with scanning tunneling microscopy (STM) at liquid helium temperature. Large amplitude Ge atom vibrations are decisive in facilitating a specific appearance of the structure in the STM images. The underlying mechanism is unraveled using a theoretical model and calculations within the density functional theory framework. Furthermore, we demonstrate the ability to induce controlled switching of the DB dimer with noncontact atomic force microscope and the stabilizing role of the dimer-dimer interaction.

The generation and control of the valley-polarized current play the key roles in valleytronics applications, but still remain as tough challenges. We propose a general idea of switchable valley injection, and predict theoretically that a ferromagnet-covered graphene junction would take the role of a valley injector, whose on/off state can be easily switched only by changing the magnetic direction. This scheme can be extended to other two-dimensional crystals and may lead to an alternative path for valleytronic practical applications.

Recent experimental observations have reported that, rather than randomly distributed, nitrogen atoms may prefer to be located on one of the two sublattices of graphene. It has been suggested that such a preference may present a possible avenue to tailor the band gap of graphene while maintaining its excellent electronic transport properties. Among the proposed mechanisms to explain this effect is the suggestion that long-range interimpurity interactions mediated by the conduction electrons of graphene may give rise to the asymmetry between sublattice occupations. Electron-mediated interactions are known to be prevalent not only between N atoms but also between impurities that are adsorbed to a specific location within the hexagonal structure, namely to the top of the carbon atom. Furthermore, this interaction is known to become more long ranged as the dimensionality of the system is lowered. For that reason, in this paper we investigate whether a similar sublattice asymmetry appears in the case of metallic carbon nanotubes doped with hydrogen adatoms. Our results indicate that similar sublattice asymmetries are observable and even more pronounced in small-diameter CNTs with a dilute concentration of adsorbates, diminishing with increasing diameter or impurity concentrations.

In this work, we present an analytical theory of strongly correlated Wigner crystals (WCs) in the lowest Landau level (LLL) by constructing an approximate, but accurate effective two-body interaction for composite fermions (CFs) participating in the WCs. This requires integrating out the degrees of freedom of all surrounding CFs, which we accomplish analytically by approximating their wave functions by delta functions. This method produces energies of various strongly correlated WCs that are in excellent agreement with those obtained from the Monte Carlo simulation of the full CF crystal wave functions. We compute the compressibility of the strongly correlated WCs in the LLL and predict discontinuous changes at the phase boundaries separating different crystal phases.

We analyze superconductivity in the cuprates near the onset of an incommensurate charge-density-wave (CDW) order with momentum $\mathbf{Q}=(Q,0)/(0,Q)$, as observed in experiments. We first consider a semiphenomenological charge-fermion model in which hot fermions, separated by $\mathbf{Q}$, attract each other by exchanging soft CDW fluctuations. We find that in a quantum-critical region near the CDW transition, $T_c=A{\overline{g}}_c$, where ${\overline{g}}_c$ is charge-fermion coupling and $A$ is the prefactor, which we explicitly compute. We then consider the particular microscopic scenario in which the CDW order parameter emerges as a composite field made of primary spin-density-wave fields. We show that charge-fermion coupling ${\overline{g}}_c$ is of the order of spin-fermion coupling ${\overline{g}}_s$. As a consequence, superconducting $T_c$ is substantially enhanced near the onset of CDW order. Finally, we analyze the effect of an external magnetic field $H$. We show that, as $H$ increases, the optimal $T_c$ decreases and the superconducting dome becomes progressively more confined to the CDW quantum-critical point. These results are consistent with experiments.

${\mathrm{BaFe}}_2{\left({\mathrm{PO}}_4\right)}_2$ is an unusual Ising insulating ferromagnet based on the ${\mathrm{Fe}}^{2+}$ spin $S=2$ ion, the susceptibility of which suggests a large orbital component to the Fe local moment. We apply density functional theory based methods to obtain a microscopic picture of the competing interactions and the critical role of spin-orbit coupling (SOC) in this honeycomb lattice system. The low-temperature ferromagnetic phase displays a half-semimetallic Dirac point pinning the Fermi level and preventing gap opening before consideration of SOC, presenting a case in which correlation effects modeled by a repulsive Hubbard $U$ fail to open a gap. Simultaneous inclusion of both correlation and SOC drives a large orbital moment in excess of 0.7 ${\mu }_B$ (essentially $L=1$) for spin aligned along the $\widehat{c}$ axis, with a gap comparable with the inferred experimental value. The large orbital moment accounts for the large Ising anisotropy, in spite of the small magnitude of the SOC strength on the $3d$ (Fe) ion. Ultimately, the Mott-Hubbard gap is enabled by degeneracy lifting by SOC and the large Fe moments, rather than by standard Hubbard interactions alone. We suggest that competing orbital occupations are responsible for the structural transitions involved in the observed reentrant rhombohedral-triclinic-rhombohedral sequence.

Nuclear spins are an important source of dephasing for electron spin qubits in GaAs quantum dots. Most studies of their dynamics have focused on the relatively slow longitudinal polarization. We present a semiclassical model and experimental data showing that the dynamics of the transverse hyperfine field can be probed by correlating individual Landau-Zener sweeps across the $\mathrm{S}-{\mathrm{T}}_{+}$ transition of a two-electron spin qubit. The relative Larmor precession of different nuclear spin species leads to oscillations in these correlations, which decay due to dephasing of the nuclei. In the presence of spin-orbit coupling, oscillations with the absolute Larmor frequencies whose amplitude depends on the spin-orbit coupling strength are expected. These oscillations reflect rapid dynamics of the transverse hyperfine field, which are relevant for several qubit control schemes.

We present a systematic experimental study along with theoretical modeling of the energy transfer in an ensemble of closely packed CdTe colloidal nanocrystals identified as the Förster resonant energy transfer (FRET). We prove that at low temperature of 4.2 K, mainly the ground dark exciton states in the initially excited small-size (donor) nanocrystals participate in the dipole-dipole FRET leading to additional excitation of the large-size (acceptor) nanocrystals. The FRET becomes possible due to the weak admixture of the bright exciton states to the dark states. The admixture takes place even in zero magnetic field and allows the radiative recombination of the dark excitons. An external magnetic field considerably enhances this admixture, thus increasing the energy transfer rate by a factor of 2–3 in a field of 15 T, as well as the radiative rates of the dark excitons in the donor and acceptor nanocrystals. The theoretical modeling allows us to determine the spectral dependence of the probability for the NC to serve as a donor for larger nanocrystals, to evaluate the energy transfer rates as well as to predict their dependencies on the magnetic field, to describe the spectral shift of the photoluminescence maximum due to the energy transfer, and to reproduce the experimentally observed spectral dependencies of the photoluminescence recombination dynamics in the magnetic field.

Epitaxial $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3\left(\mathrm{LSMO}\right)/\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Fe}{\mathrm{O}}_3$ (LSFO) superlattices serve as model systems to explore the magnetic structure and exchange coupling at (111)-oriented perovskite oxide interfaces. The (111) orientation possesses a buckled honeycomb structure resembling that of graphene with the stacking of highly polar layers. Furthermore, the bulk LSFO magnetic structure suggests that an ideal (111) interface should have fully uncompensated antiferromagnetic (AF) moments leading to exchange bias interactions. Detailed soft x-ray magnetic spectroscopy and microscopy reveal that interfacial effects and ultrathin superlattice sublayers can stabilize orientations of the LSFO AF spin axis, which differ from that of LSFO films and LSMO/LSFO bilayers. A portion of the AF moments can be reoriented to an arbitrary direction by a moderate external magnetic field through spin-flop coupling with the ferromagnetic LSMO sublayers that have low magnetocrystalline anisotropy in the (111) plane.