We have considered relativistic soliton dynamics governed by the sine-Gordon equation and affected by short spatial inhomogeneities of the driving force and thermal noise. Developed analytical and numerical methods for calculation of soliton scattering at the inhomogeneities allowed us to examine the scattering as a measurement tool for sensitive detection of polarity of the inhomogeneities. We have considered the superconducting fluxonic ballistic detector as an example of the device in which the soliton scattering is utilized for quantum measurements of superconducting flux qubits. We optimized the soliton dynamics for the measurement process varying the starting and the stationary soliton velocity as well as configuration of the inhomogeneities. For experimentally relevant parameters we obtained the signal-to-noise ratio above 100 reflecting good practical usability of the measurement concept.

We found that a layered magnet ${\mathrm{La}}_5{\mathrm{Mo}}_4{\mathrm{O}}_{16}$ exhibits a long-time relaxation of the magnetization from a ferromagnetic to an antiferromagnetic state. This unconventional relaxation in a pure crystal magnet far below the transition temperature arises from the extremely two-dimensional nature of the magnetic interactions, by which the Mo spins are strongly coupled and form a big spin within a layer.

The $\mathrm{H}{\mathrm{o}}_2\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_7, \mathrm{E}{\mathrm{r}}_2\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_7$, and $\mathrm{Y}{\mathrm{b}}_2\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_7$ pyrochlores were studied by synchrotron x-ray diffraction to determine whether the (002) peak, forbidden in the pyrochlore space group $Fd-3m$ but observed in single crystal neutron scattering measurements, is present due to a deviation of their pyrochlore structure from $Fd-3m$ symmetry. Synchrotron diffraction measurements on precisely synthesized stoichiometric and nonstoichiometric powders and a crushed floating zone crystal of $\mathrm{H}{\mathrm{o}}_2\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_7$ revealed that the (002) reflection is absent in all cases to a sensitivity of approximately one part in 30 000 of the strongest x-ray diffraction peak. This indicates to high sensitivity that the space group of the crystal structure of these rare earth titanate pyrochlores is $Fd-3m$, and that, thus, the (002) peak observed in the neutron scattering experiments has a nonstructural origin. The cell parameters and internal strain for lightly stuffed $\mathrm{H}{\mathrm{o}}_{2+x}\mathrm{T}{\mathrm{i}}_{2-x}{\mathrm{O}}_7$ are also presented.

We study the instability of the superconducting state in a mesoscopic geometry for the low pinning material ${\text{Mo}}_3\mathrm{Ge}$ characterized by a large Ginzburg-Landau parameter. We observe that in the current-driven switching to the normal state from a nonlinear region of the Abrikosov flux flow, the mean critical vortex velocity reaches a limiting maximum velocity as a function of the applied magnetic field. Based on time-dependent Ginzburg-Landau simulations, we argue that the observed behavior is due to the high-velocity vortex dynamics confined on a mesoscopic scale. We build up a general phase diagram which includes all possible dynamic configurations of the Abrikosov lattice in a mesoscopic superconductor.

Auger recombination is an important nonradiative carrier recombination mechanism in many classes of optoelectronic devices. The microscopic Auger processes can be either direct or indirect, mediated by an additional scattering mechanism such as the electron-phonon interaction and alloy disorder scattering. Indirect Auger recombination is particularly strong in nitride materials and affects the efficiency of nitride optoelectronic devices at high powers. Here, we present a first-principles computational formalism for the study of direct and indirect Auger recombination in direct-band-gap semiconductors and apply it to the case of nitride materials. We show that direct Auger recombination is weak in the nitrides and cannot account for experimental measurements. On the other hand, carrier scattering by phonons and alloy disorder enables indirect Auger processes that can explain the observed loss in devices. We analyze the dominant phonon contributions to the Auger recombination rate and the influence of temperature and strain on the values of the Auger coefficients. Auger processes assisted by charged-defect scattering are much weaker than the phonon-assisted ones for realistic defect densities and not important for the device performance. The computational formalism is general and can be applied to the calculation of the Auger coefficient in other classes of optoelectronic materials.

We study a thermally induced spin flip of an electron spin located in a semiconductor quantum dot. This interesting effect arises from an intriguing interplay between the Zeeman coupling to an external magnetic field and the hyperfine interaction with the surrounding nuclear spins. By considering a minimal model, we explain the main mechanism driving this spin flip and analyze its dependence on the strength of the external magnetic field, the number of nuclear spins and the ratio of the electron and nuclear Zeeman energies, respectively. Finally, we show that this minimal model can be applied to experimentally relevant QDs in III-V heterostructures, where we explicitly predict the temperature at which the spin flip occurs.

Based on an analysis of the short-range chemical environment of each atom in a system, standard machine-learning-based approaches to the construction of interatomic potentials aim at determining directly the central quantity, which is the total energy. This prevents, for instance, an accurate description of the energetics of systems in which long-range charge transfer or ionization is important. We propose therefore not to target directly with machine-learning methods the total energy but an intermediate physical quantity, namely, the charge density, which then in turn allows us to determine the total energy. By allowing the electronic charge to distribute itself in an optimal way over the system, we can describe not only neutral but also ionized systems with unprecedented accuracy. We demonstrate the power of our approach for both neutral and ionized NaCl clusters where charge redistribution plays a decisive role for the energetics. We are able to obtain chemical accuracy, i.e., errors of less than a millihartree per atom compared to the reference density functional results for a huge data set of configurations with large structural variety. The introduction of physically motivated quantities which are determined by the short-range atomic environment via a neural network also leads to an increased stability of the machine-learning process and transferability of the potential.

The high-pressure behavior of natural henritermierite garnet with close to end-member composition ${\mathrm{Ca}}_3{\mathrm{Mn}}_2\left[{\mathrm{SiO}}_4{]}_2\right[{\mathrm{O}}_4{\mathrm{H}}_4]$ was studied at pressures up to 80 GPa using single-crystal synchrotron x-ray diffraction, Raman spectroscopy, and quantum-mechanical calculations based on density functional theory. An isosymmetric phase transition was observed in the pressure range between 55 and 70 GPa, which is associated with a gradual high-spin to low-spin electronic transition in ${\mathrm{Mn}}^{3+}$ and a pronounced reduction of the Jahn-Teller distortion of the ${\mathrm{Mn}}^{3+}{\mathrm{O}}_4({\mathrm{OH})}_2$ octahedra. In the high-pressure phase the Jahn-Teller distortion is totally suppressed and the ${\mathrm{Mn}}^{3+}$ is in a low-spin configuration. Experimental structural data before and after the phase transition are in excellent agreement with the theoretically predicted structural compression of the high-spin and low-spin phases, respectively. While the overall unit-cell volume is reduced by about $1.5\%$ across the phase transition, a collapse of about $4\text{–}5\%$ of the ${\mathrm{MnO}}_6$ octahedral volume is observed. The high-spin phase shows a bulk modulus $B=101$(1) GPa and its pressure derivative $B^{\prime }=4.5$(1). The bulk moduli of the coordination polyhedra are $B_{{\mathrm{MnO}}_6}=178$(2) GPa, $B_{\mathrm{Ca}1{\mathrm{O}}_8}=101.2$(5) GPa, $B_{\mathrm{Ca}2{\mathrm{O}}_8}=88.4$(8) GPa, $B_{{\mathrm{SiO}}_4}=337$(5) GPa, and $B_{{\mathrm{O}}_4{\mathrm{H}}_4}=29$(1) GPa for the high-spin phase. Mode Grüneisen parameters range between 0.34 and 0.94. The computed spin-pairing energy is $\approx 3.6$ eV at 0 GPa.

Utilizing single-crystal synchrotron x-ray scattering, we observe distorted ${\text{CuO}}_2$ planes in the electron-doped superconductor ${\mathrm{Pr}}_{1-x}{\mathrm{LaCe}}_x{\mathrm{CuO}}_{4+\delta }, x$ = 0.12. Resolution-limited rods of scattering are indicative of a long-range two-dimensional $2\sqrt{2}\times 2\sqrt{2}$ superstructure in the $a-b$ plane, adhering to planar space-group symmetry $p4gm$, which is subject to stacking disorder perpendicular to the planes. This superstructure is present only in annealed, superconducting samples, but not in the as-grown, nonsuperconducting samples. These long-range distortions of the ${\mathrm{CuO}}_2$ planes, which are generally considered to be detrimental to superconductivity, have avoided detection to date due to the challenges of observing and interpreting subtle diffuse-scattering features.

We have employed x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) at the Ni and Mn $L_{3,2}$ edges in ${\mathrm{LaMnO}}_3/{\mathrm{LaNiO}}_3$ superlattices grown along (001) and (111) orientations. Our results show a significant XMCD signal and thus the presence of magnetic moments localized on both Ni and Mn, which are ferromagnetically coupled to each other. X-ray absorption experiments reveal a charge transfer between Ni and Mn which is larger for (111) than for (001) superlattices. These results are compared to theoretical predictions for such superlattices.

Motivated by experiments on the double perovskites ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$ and ${\mathrm{La}}_2{\mathrm{MgIrO}}_6$, we study the magnetism of spin-orbit coupled $j_{\mathrm{eff}}=1/2$ iridium moments on the three-dimensional, geometrically frustrated, face-centered cubic lattice. The symmetry-allowed nearest-neighbor interaction includes Heisenberg, Kitaev, and symmetric off-diagonal exchange. A Luttinger-Tisza analysis shows a rich variety of orders, including collinear $A$-type antiferromagnetism, stripe order with moments along the $\left\{111\right\}$ direction, and incommensurate noncoplanar spirals, and we use Monte Carlo simulations to determine their magnetic ordering temperatures. We argue that existing thermodynamic data on these iridates underscores the presence of a dominant Kitaev exchange, and also suggest a resolution to the puzzle of why ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$, but not ${\mathrm{La}}_2{\mathrm{MgIrO}}_6$, exhibits “weak” ferromagnetism.

Chromium is a $3d$ spin-density wave antiferromagnetic (AF) metal with a Néel temperature of 311 K. By using the thermal spin injection method, we have observed a large inverse spin Hall voltage in Cr. With a negligible magnetic proximity effect and a large spin Hall angle, Cr is an intrinsic pure spin current detector that is superior to many $5d$ metals. Even more strikingly, the temperature dependent thermal spin injection measurements show that the large spin Hall angle in Cr is independent of the AF ordering, and indicating that the inverse spin Hall effect is unrelated to AF ordering in Cr.

Taking advantage of the new opportunities provided by x-ray free electron laser (FEL) sources when coupled to a long laser pulse as available at the Linear Coherent Light Source (LCLS), we have performed x-ray absorption near-edge spectroscopy (XANES) of laser shock compressed iron up to 420 GPa ($\pm 50$) and 10 800 K ($\pm 1390$). Visible diagnostics coupled with hydrodynamic simulations were used to infer the thermodynamical conditions along the Hugoniot and the release adiabat. A modification of the pre-edge feature at 7.12 keV in the XANES spectra is observed above pressures of 260 GPa along the Hugoniot. Comparing with ab initio calculations and with previous laser-heated diamond cell data, we propose that such changes in the XANES pre-edge could be a signature of molten iron. This interpretation then suggests that iron is molten at pressures and temperatures higher than 260 GPa ($\pm 29$) and 5680 K ($\pm 700$) along the principal Fe Hugoniot.

We develop a formalism to study indirect resonant inelastic x-ray scattering (RIXS) in systems with itinerant electrons, accounting for the attraction between valence electrons and the positively charged core hole exactly, and apply this formalism to the hole-doped cuprate superconductors. We focus on the relationship between RIXS line shapes and band structure, including broken symmetries. We show that RIXS is capable of distinguishing between competing order parameters, establishing it as a useful probe of the pseudogap phase.

The projected entangled pair state (PEPS) ansatz can represent a thermal state in a strongly correlated system. We introduce a variational algorithm to optimize this tensor network whose essential ingredient is an auxiliary tree tensor network (TTN). Since the full tensor environment is taken into account, with increasing bond dimension the PEPS-TTN ansatz provides the exact Gibbs state. Our presentation opens with a 1D version for a matrix product state (MPS-TTN) and then generalizes to PEPS-TTN in 2D. Benchmark results in the quantum Ising model are presented.

A tight-binding potential model which goes beyond the Slater-Koster two-center approximation and includes explicit three-center and crystal field expressions is presented. Using carbon and silicon as examples, we show that various bulk structures, surface reconstructions, and the structures of clusters and liquids of C and Si can be well described by the present three-center tight-binding model. These results demonstrate that three-center interaction and crystal field effect are very important for improving the transferability of tight-binding models in describing the structures and properties of materials over a broad range of bonding configurations.

We reexamine the tunneling density of states in quantum dots and nanoparticles within the model which is an extension of the universal Hamiltonian to the case of uniaxial anisotropic exchange. We derive the exact analytical result for the tunneling density of states in the case of an arbitrary single-particle energy spectrum. We find that, similar to the case of the isotropic exchange, the tunneling density of states as a function of energy has the maximum due to a finite value of the total spin of the ground state near the Stoner instability. We demonstrate that there are no additional extrema which have been predicted on the basis of perturbative analysis [M. N. Kiselev and Y. Gefen, Phys. Rev. Lett. 96, 066805 (2006).].

We present a calculation of the effective geometry-induced quantum potential for the carriers in graphene shaped as a helicoidal nanoribbon. In this geometry the twist of the nanoribbon plays the role of an effective transverse electric field in graphene and this is reminiscent of the Hall effect. However, this effective electric field has a different sign for the two isospin states and translates into a mechanism to separate the two chiral species on the opposing rims of the nanoribbon. Isospin transitions are expected with the emission or absorption of microwave radiation which could be adjusted to be in the THz region.