In this work, using zero kelvin ab initio calculations, we revisit the structure and ferroelectric phase transition in antiphase domain boundaries (APBs) in ${\text{SrTiO}}_3$ (STO), which has been previously addressed in terms of a phenomenological approach. We confirmed the main qualitative conclusion of the phenomenological results that APBs normal to the rotation axis of the oxygen octahedra (“easy” walls) do not exhibit the transition while those parallel to the rotation axis (“hard” walls) do. However, we found the structure of the hard walls to be close to the Ising type in contrast to the phenomenological prediction of the nearly Néel type. We simulated a pressure-induced phase transition in the hard wall. Combining the results of simulation and experimental data on STO, we evaluated the pressure sensitivity of the ferroelectricity in the hard wall at low temperatures to show that it can be suppressed with very small pressure (a few kbar). We also roughly estimated the ferroelectric transition temperature in the hard wall corroborating the result of the phenomenological treatment.

Uranium monocarbide, a potential fuel material for the generation IV reactors, is investigated within density functional theory. Its electronic, magnetic, elastic, and phonon properties are analyzed and discussed in terms of spin-orbit interaction and localized versus itinerant behavior of the $5f$ electrons. The localization of the $5f$ states is tuned by varying the local Coulomb repulsion interaction parameter. We demonstrate that the theoretical electronic structure, elastic constants, phonon dispersions, and their densities of states can reproduce accurately the results of x-ray photoemission and bremsstrahlung isochromat measurements as well as inelastic neutron scattering experiments only when the $5f$ states experience the spin-orbit interaction and simultaneously remain partially localized. The partial localization of the $5f$ electrons could be represented by a moderate value of the on-site Coulomb interaction parameter of about 2 eV. The results of the present studies indicate that both strong electron correlations and spin-orbit effects are crucial for realistic theoretical description of the ground-state properties of uranium carbide.

The length dependence of thermal conductivity over more than two orders of magnitude has been systematically studied for a range of materials, interatomic potentials, and temperatures using the atomistic approach-to-equilibrium molecular dynamics (AEMD) method. By comparing the values of conductivity obtained for a given supercell length and maximum phonon mean free path (MFP), we find that such values are strongly correlated, demonstrating that the AEMD calculation with a supercell of finite length actually probes the thermal conductivity corresponding to a maximum phonon MFP. As a consequence, the less pronounced length dependence usually observed for poorer thermal conductors, such as amorphous silica, is physically justified by their shorter average phonon MFP. Finally, we compare different analytical extrapolations of the conductivity to infinite length and demonstrate that the frequently used Matthiessen rule is not applicable in AEMD. An alternative extrapolation more suitable for transient-time, finite-supercell simulations is derived. This approximation scheme can also be used to classify the quality of different interatomic potential models with respect to their capability of predicting the experimental thermal conductivity.

A hydrodynamic theory of transport in quantum mechanically phase-disordered superconductors is possible when supercurrent relaxation can be treated as a slow process. We obtain general results for the frequency-dependent conductivity of such a regime. With time-reversal invariance, the conductivity is characterized by a Drude-type peak, with width given by the supercurrent relaxation rate. Using the memory matrix formalism, we obtain a formula for this width (and hence also the dc resistivity) when the supercurrent is relaxed by short-range density-density interactions. This leads to an effective field theoretic and fully quantum derivation of a classic result on flux flow resistance. With strong breaking of time-reversal invariance, the optical conductivity exhibits what we call a “hydrodynamic supercyclotron” resonance. We obtain the frequency and decay rate of this resonance for the case of supercurrent relaxation due to an emergent Chern-Simons gauge field. The supercurrent decay rate in this “topologically ordered superfluid vortex liquid” is determined by the conductivities of the normal fluid component, rather than the vortex core.

The nitrogen-vacancy (NV) center in diamond has attractive properties for a number of quantum technologies that rely on the spin angular momentum of the electron and the nuclei adjacent to the center. The nucleus with the strongest interaction is the ${}^{13}\mathrm{C}$ nuclear spin of the first shell. Using this degree of freedom effectively hinges on precise data on the hyperfine interaction between the electronic and the nuclear spin. Here, we present detailed experimental data on this interaction, together with an analysis that yields all parameters of the hyperfine tensor, as well as its orientation with respect to the atomic structure of the center.

$A_2\mathrm{S}{\mathrm{n}}_2{\mathrm{O}}_7$ $(A=\mathrm{Nd},\text{Sm},\text{Gd},\text{Er},\text{Yb},\text{and Y})$ materials with the pyrochlore structure were irradiated with 2.2 GeV Au ions to systematically investigate disordering of this system in response to dense electronic excitation. Structural modifications were characterized, over multiple length scales, by transmission electron microscopy, x-ray diffraction, and Raman spectroscopy. Transformations to amorphous and disordered phases were observed, with disordering dominating the structural response of materials with small $A$-site cation ionic radii. Both the disordered and amorphous phases were found to possess weberite-type local ordering, differing only in that the disordered phase exhibits a long-range, modulated arrangement of weberite-type structural units into an average defect-fluorite structure, while the amorphous phase remains fully aperiodic. Comparison with the behavior of titanate and zirconate pyrochlores showed minimal influence of the high covalency of the Sn-O bond on this phase behavior. An analytical model of damage accumulation was developed to account for simultaneous amorphization and recrystallization of the disordered phase during irradiation.

We study the low-energy physics of a broad class of time-reversal invariant and SU(2)-symmetric one-dimensional spin-$S$ systems in the presence of quenched disorder via a strong-disorder renormalization-group technique. We show that, in general, there is an antiferromagnetic phase with an emergent $\mathrm{SU}(2S+1)$ symmetry. The ground state of this phase is a random singlet state in which the singlets are formed by pairs of spins. For integer spins, there is an additional antiferromagnetic phase which does not exhibit any emergent symmetry (except for $S=1)$. The corresponding ground state is a random singlet one but the singlets are formed mostly by trios of spins. In each case the corresponding low-energy dynamics is activated, i.e., with a formally infinite dynamical exponent, and related to distinct infinite-randomness fixed points. The phase diagram has two other phases with ferromagnetic tendencies: a disordered ferromagnetic phase and a large spin phase in which the effective disorder is asymptotically finite. In the latter case, the dynamical scaling is governed by a conventional power law with a finite dynamical exponent.

A magnetic skyrmion is a topological magnetization structure with a nanometric size and a well-defined swirling spin distribution, which is anticipated to be an essential building block for novel skyrmion-based device applications. We study the motion of magnetic skyrmions in multilayer synthetic antiferromagnetic (SAF) racetracks as well as in conventional monolayer ferromagnetic (FM) racetracks at finite temperature. There is an odd-even effect of the constituent FM layer number on the skyrmion Hall effect (SkHE). Namely, due to the suppression of the SkHE, the magnetic skyrmion has no transverse motion in multilayer SAF racetracks packed with even FM layers. It is shown that a moving magnetic skyrmion is stable even at room temperature ($T=300$ K) in a bilayer SAF racetrack but it is destructed at $T=100$ K in a monolayer FM racetrack. Our results indicate that the SAF structures are reliable and promising candidates for future applications in skyrmion electronics and skyrmion spintronics.

The controversy regarding the precise nature of the high-temperature phase of $1T\text{−}\mathrm{TiS}{\mathrm{e}}_2$ lasts for decades. It has intensified in recent times when new evidence for the excitonic origin of the low-temperature charge-density wave state started to unveil. Here we address the problem of the high-temperature phase through precise measurements and detailed analysis of the optical response of $1T\text{−}\mathrm{TiS}{\mathrm{e}}_2$ single crystals. The separate responses of electron and hole subsystems are identified and followed in temperature. We show that neither semiconductor nor semimetal pictures can be applied in their generic forms as the scattering for both types of carriers is in the vicinity of the Ioffe-Regel limit with decay rates being comparable to or larger than the offsets of band extrema. The nonmetallic temperature dependence of transport properties comes from the anomalous temperature dependence of scattering rates. Near the transition temperature the heavy electrons and the light holes contribute equally to the conductivity; this surprising coincidence is regarded as the consequence of dominant intervalley scattering that precedes the transition. The low-frequency peak in the optical spectra is identified and attributed to the critical softening of the $L$-point collective mode.

Searching for room-temperature ferromagnetic semiconductors has evolved into a broad field of material science and spintronics for decades, nevertheless, these novel states remain rare. Phosphorene, a monolayer black phosphorus with a puckered honeycomb lattice structure possessing a finite band gap and high carrier mobility, has been synthesized recently. Here we show, by means of two different large-scale quantum Monte Carlo methods, that relatively weak interactions can lead to remarkable edge magnetism in the phosphorene nanoribbons. The ground state constrained path quantum Monte Carlo simulations reveal strong ferromagnetic correlations along the zigzag edges, and the finite temperature determinant quantum Monte Carlo calculations show a high Curie temperature up to room temperature.

We solve the orbitally degenerate two-band Hubbard model within dynamical mean field theory and map out the instabilities to various symmetry-broken phases based on an analysis of the corresponding lattice susceptibilities. Phase diagrams as a function of the Hund coupling parameter $J$ are obtained both for the model with rotationally invariant interaction and for the model with Ising-type anisotropy. For negative $J$, an intraorbital spin-singlet superconducting phase appears at low temperatures, while the normal state properties are characterized by an orbital-freezing phenomenon. This is the $\text{negative-}J$ analog of the recently discovered fluctuating-moment induced $s\text{-wave}$ spin-triplet superconductivity in the spin-freezing regime of multiorbital models with $J>0$.

We consider quantum point contact (QPC) defined within a disordered two-dimensional electron gas as studied by scanning gate microscopy. We evaluate the conductance maps in the Landauer approach with a wave-function picture of electron transport for samples with both low and high electron mobility at finite temperatures. We discuss the spatial distribution of the impurities in the context of the branched electron flow. We reproduce the surprising temperature stability of the experimental interference fringes far from the QPC. Next, we discuss funnel-shaped features that accompany splitting of the branches visible in previous experiments. Finally, we study elliptical interference fringes formed by an interplay of scattering by the pointlike impurities and by the scanning probe. We discuss the details of the elliptical features as functions of the tip voltage and the temperature, showing that the first interference fringe is very robust against the thermal widening of the Fermi level. We present a simple analytical model that allows for extraction of the impurity positions and the electron-gas depletion radius induced by the negatively charged tip of the atomic force microscope, and apply this model on experimental scanning gate images showing such elliptical fringes.

Due to their low surface mass density, two-dimensional materials with a strong piezoelectric response are interesting for nanoelectromechanical systems with high force sensitivity. Unlike graphene, the two sublattices in a monolayer of hexagonal boron nitride (hBN) are occupied by different elements, which breaks inversion symmetry and allows for piezoelectricity. This has been confirmed with density functional theory calculations of the piezoelectric constant of hBN. Here, we formulate an entirely analytical derivation of the electronic contribution to the piezoelectric response in this system based on the concepts of strain-induced pseudomagnetic vector potential and the modern theory of polarization that relates the polar moment to the Berry curvature. Our findings agree with the symmetry restrictions expected for the hBN lattice and reproduce well the magnitude of the piezoelectric effect previously obtained ab initio.

Single-layer ${\mathrm{TaS}}_2$ is epitaxially grown on Au(111) substrates. The resulting two-dimensional crystals adopt the $1H$ polymorph. The electronic structure is determined by angle-resolved photoemission spectroscopy and found to be in excellent agreement with density functional theory calculations. The single-layer ${\mathrm{TaS}}_2$ is found to be strongly $n$ doped, with a carrier concentration of $0.3\left(1\right)$ extra electrons per unit cell. No superconducting or charge density wave state is observed by scanning tunneling microscopy at temperatures down to 4.7 K.

We present in detail the fluorescence properties of single thick-shell CdSe/CdS colloidal nanocrystals coupled to a linear one-dimensional gold grating. In addition to the photoluminescence decay rate increase, we point out that the polarization ratio of the emission is dramatically enhanced. It overcomes 80% and can reach values close to unity. Experimental results are successfully compared to theoretical predictions. Blinking suppression is also reported.

We investigate the influence of the chemical potential mismatch $\mathrm{\Delta }$ (different electron densities) on Coulomb drag between two parallel ballistic quantum wires. For pair collisions, the drag resistivity ${\rho }_{\mathrm{D}}\left(\mathrm{\Delta }\right)$ shows a peculiar anomaly at $\mathrm{\Delta }=0$ with ${\rho }_{\mathrm{D}}$ being finite at $\mathrm{\Delta }=0$ and vanishing at any nonzero $\mathrm{\Delta }$. The “bodyless” resonance in ${\rho }_{\mathrm{D}}\left(\mathrm{\Delta }\right)$ at zero $\mathrm{\Delta }$ is only broadened by processes of multiparticle scattering. We analyze Coulomb drag for finite $\mathrm{\Delta }$ in the presence of both two- and three-particle scattering within the kinetic equation framework, focusing on a Fokker-Planck picture of the interaction-induced diffusion in momentum space of the double-wire system. We describe the dependence of ${\rho }_{\mathrm{D}}$ on $\mathrm{\Delta }$ for both weak and strong intrawire equilibration due to three-particle scattering.

We suggest a phenomenological theory of dynamical phase transitions and the subsequent spaciotemporal evolution induced by a short optical pulse in a system which is already prone to a thermodynamic instability. We address the case of pumping to excitons whose density contributes additively to the thermodynamic order parameter like for charge-transfer excitons in electronic charge-ordering transitions. To describe both thermodynamic and dynamical effects on equal footing, we adopt for the phase transition a view of the “excitonic insulator” (EI) and suggest a formation of the macroscopic quantum state for the pumped excitons. The double nature of the ensemble of excitons leads to an intricate time evolution: the dynamical transition between number-preserved and phase-locked regimes, macroscopic quantum oscillations from interference between the Bose condensate of excitons, and the ground state of the EI. Modeling for an extended sample shows also stratification in domains of low and high densities which evolve through local dynamical phase transitions and a sequence of domain merges.

Very large magnetoresistance discovered in single crystals of the ferromagnetic Fe-intercalated transition-metal dichalcogenide ${\mathrm{Fe}}_{0.28}{\mathrm{TaS}}_2$ was attributed to the deviation of the Fe concentration from commensurate values ($x=1/4$ or 1/3), which caused magnetic moment misalignments. Here we report a study of ${\mathrm{Fe}}_x{\mathrm{TaS}}_2$ crystals with $0.23\le x\le 0.35$, demonstrating that crystallographic defects lead to spin disorder, which correlates with magnetotransport properties, such as switching magnetic field $H_S$, magnetoresistance (MR), and even zero-field resistivity ${\rho }_0$ and temperature coefficient $A$ in $\rho \left(T\right)={\rho }_0+A{T}^2$: The ordering temperature $T_C$ and Weiss temperature ${\theta }_W$ are maximized at the superstructure composition $x=1/4$, whereas $H_s$, MR, ${\rho }_0$, and $A$ are minimum. Conversely, at a composition intermediate between the superstructure compositions $x=1/4$ and 1/3, the corresponding magnetotransport properties reach local maxima.

We study spin excitation spectra of one-, two-, and three-dimensional magnets featuring nonmagnetic defects at a wide range of concentrations. Taking the Heisenberg model as the starting point, we tackle the problem by both direct numerical simulations in large supercells and using a semianalytic coherent-potential approximation. We consider the properties of the excitations in both direct and reciprocal spaces. In the limits of the concentration $c$ of the magnetic atoms tending to 0 or 1 the properties of the spin excitations are similar in all three dimensions. In the case of a low concentration of magnetic atoms the spin excitation spectra are dominated by the modes confined in the real space to single atoms or small clusters and delocalized in the reciprocal space. In the limit of $c$ tending to 1, we obtain the spin-wave excitations delocalized in the real space and localized in the reciprocal space. However, for the intermediate concentrations the properties of the spin excitations are strongly dimensionality dependent. We pay particular attention to the formation, with increase of $c$, of the Lorentzian-shaped peaks in the spectral densities of the spin excitations, which can be regarded as magnon states with a finite lifetime given by the width of the peaks. In general, low-dimensional magnets are more strongly affected by the presence of nonmagnetic impurities than their bulk counterparts. The details of the electronic structure, varying with the dimensionality and the concentration, substantially influence the spin excitation spectra of real materials, as we show in the example of the FeAl alloy.