We theoretically investigate magnetic properties in the low-temperature phase with the formation of eight-site clusters, octamers, in the spinel compound $\mathrm{CuIr}{}_2$$\mathrm{S}{}_4$. The octamer state was considered to be a spin-singlet state induced by a Peierls instability through the strong anisotropy of $d$ orbitals, the so-called orbital Peierls state. We reexamine this picture by taking into account the spin-orbit coupling, which was ignored in the previous study. We derive a low-energy effective model between $j_{\mathrm{eff}}=1/2$ quasispins on $\mathrm{Ir}{}^{4+}$ cations in an octamer from the multiorbital Hubbard model with the strong spin-orbit coupling by performing the perturbation expansion from the strong correlation limit. The effective Hamiltonian is in the form of the Kitaev-Heisenberg model but with an additional interaction, a symmetric off-diagonal exchange interaction originating from the perturbation process including both $d\text{−}d$ and $d\text{−}p\text{−}d$ hoppings. Analyzing the effective Hamiltonian on two sites and the octamer by the exact diagonalization, we find that there is competition between a spin-singlet state and a quadrupolar state. The former singlet state is a conventional one, adiabatically connected to the orbital Peierls state. On the other hand, the latter quadrupolar state is stabilized by the additional interaction, which consists of a linear combination of different total spin momenta along the spin quantization axis. In the competing region, the model exhibits paramagnetic behavior with a renormalized small effective moment at low temperature. This peculiar remnant paramagnetism is not obtained in the Kitaev-Heisenberg model without the additional interaction. Our results renew the picture of the octamer state and provide a scenario for the intrinsic paramagnetic behavior recently observed in a muon spin rotation experiment [K. M. Kojima et al., Phys. Rev. Lett. 112, 087203 (2014)].

Under the application of a force a material will deform and, hence, the crystal lattice will experience strain. This induced strain will alter the electronic properties of the material. In particular, strain in graphene generates an artificial vector potential that, if spatially varying, admits a pseudomagnetic field. Current theories for spatially varying strain use linear or finite strain theory, whose derivation is based on small displacements of infinitesimal length vectors. Here we apply a differential geometry method to derive a strain theory for large displacements of finite length vectors. This method gives a finite displacement term whose contribution is comparable to that of the linear strain term. Furthermore, we show that a “domain-wall”-like pseudomagnetic-field profile can be generated when a wide graphene ribbon is subjected to a pair of opposing point forces (point stretch). The resulting field is a function of the new finite displacement term only and displays a maximum strength of over three times that which is predicted by the linear strain theory. These results extend the current theories of strain, which are based on the transformation of infinitesimal length vectors, to finite length vectors, thus providing an accurate description of pseudomagnetic-field structures in strained materials.

We present nonlocal resistance measurements in an ultrahigh-mobility two-dimensional electron gas. Our experiments show that even at weak magnetic fields, classical guiding along edges leads to a strong nonlocal resistance on macroscopic distances. In this high Landau level regime, the transport along edges is dissipative and can be controlled by the amplitude of the voltage drop along the edge. We report resonances in the nonlocal transport as a function of this voltage that are interpreted as escape and formation of edge channels, and the formation of zero-differential resistance states when the nonlocal voltage is measured on length scales much larger than the mean free path.

A device enabling mechanically controlled spin and electric transport in mesoscopic structures is proposed. It is based on the transfer of electrons through weak links formed by suspended nanowires, on which the charge carriers experience a strong Rashba spin-orbit interaction that twists their spins. It is demonstrated that when the weak link bridges two magnetically polarized electrodes, a significant spintro-voltaic effect takes place. Then, by monitoring the generated voltage, one is able to measure electronic spins accumulated in the electrodes, induced, e.g., by circularly polarized light or, alternatively, the amount of spin twisting. Mechanically tuning the device by bending the nanowire allows one to achieve full control over the spin orientations of the charge carriers.

One intriguing finding in graphene is the vacancy-induced magnetism that highlights the interesting interaction between local magnetic moments and conduction electrons. Within density functional theory, the current understanding of the ground state is that a Stoner instability gives rise to ferromagnetism of $\pi$-electrons aligned with the localized moment of a $\sigma$ dangling bond and the induced $\pi$ magnetic moments vanish at low vacancy concentrations. However, the observed Kondo effect suggests that $\pi$-electrons around the vacancy should antiferromagnetically couple to the local moment and should carry nonvanishing moments. Here we propose that a phase possessing both significant out-of-plane displacements and $\pi$ bands with antiferromagnetic coupling to the localized $\sigma$ moment is the ground state. With the features we provide, it is possible for spin-resolved scanning tunneling microscopy, scanning tunneling spectroscopy, and angle-resolved photoelectron spectroscopy measurements to verify the proposed phase.

Within the same failure mode, iron has the lowest ideal tensile strength among the transition metals crystallizing in the body-centered cubic structure. Here, we demonstrate that this anomalously low strength of Fe originates partly from magnetism and is reflected in unexpected alloying effects in dilute Fe($M$) ($M=\mathrm{Al}$, V, Cr, Mn, Co, Ni) binaries. We employ the structural energy difference and the magnetic pressure to disentangle the magnetic effect on the ideal tensile strength from the chemical effect. We find that the investigated solutes strongly alter the magnetic response of the Fe host from the weak towards a stronger ferromagnetic behavior, which is explained based on single-particle band energies.

Motivated by the novel electronic behaviors seen in transition-metal oxides, we look for physical insight into disordered, strongly correlated systems by exploring the atomic limit. In recent work, the atomic limit has provided a useful reference point in systems with strong local interactions. For comparison with experiments, the exploration of nonlocal interactions is of interest. In the atomic limit, both the case of on-site interactions alone and the case of infinite-range $\left(1/r\right)$ interactions are well understood; however, not so the intervening possibilities. Here we study the atomic limit of the extended Anderson-Hubbard model using classical Monte Carlo to calculate the single-particle density of states. We show that the combination of nearest-neighbor interactions and site disorder produces a zero-bias anomaly caused by residual charge ordering, and the addition of on-site interactions has a nonmonotonic effect on the depth of this zero-bias anomaly. A key conclusion is that the form of the density of states in this classical system strongly resembles density of states results obtained for the full extended Anderson-Hubbard model when $U<4V$.

We consider the problem of converting a product state to a ground state of a topologically ordered system through a locally generated open-system dynamic. Employing quantum-information tools, we show that such a conversion takes an amount of time proportional to the diameter of the system. Our result applies to typical two-dimensional topologically ordered systems as well as, for example, the three-dimensional and four-dimensional toric codes. It is tight for the toric code, giving a scaling with the linear system size. Our results have immediate operational implications for the preparation of topologically ordered states, a crucial ingredient for topological quantum computation: Dissipation cannot provide any significant speedup compared to unitary evolution.

Epitaxial heterostructures consisting of a variable number of ${\mathrm{SrTiO}}_3/{\mathrm{DyScO}}_3$ bilayers deposited on ${\mathrm{DyScO}}_3$ substrates were investigated using time-domain terahertz spectroscopy down to helium temperatures. Interdigitated electrodes deposited on top of the structures allowed probing of the terahertz response upon an applied electric field. The phase transition into a ferroelectric state is observed in ${\mathrm{SrTiO}}_3$ films in all samples close to room temperature (between 250 and 310 K) due to in-plane epitaxial strain induced by the substrate and intercalated layers of ${\mathrm{DyScO}}_3$. Evolution of the dielectric spectra with temperature and external electric field is described by a general model which involves a damped harmonic oscillator (soft mode) coupled to a Debye relaxation (central mode). Both modes are connected with the soft mode eigenvector, as recently shown by molecular dynamics simulations, and they reflect a strong anharmonicity of the soft-mode potential. At high temperatures the soft-mode frequency variation drives all the changes observed in the spectra with temperature and applied field. At low temperatures, deep in the ferroelectric phase, the soft mode significantly hardens and loses its importance for the terahertz dynamics; the central mode becomes stronger and it almost completely determines the shape of the measured spectra. The observed variation of phase transition temperature and of the dielectric response among the structures is ascribed to a partial epitaxial strain relaxation confirmed also by x-ray diffraction.

Material properties controlled by evolving defect structures, such as mechanical response, often involve processes spanning many length and time scales which can not be modeled using a single approach. We present a variety of results that demonstrate the ability of phase field crystal (PFC) models to describe complex defect evolution phenomena on atomistic length scales and over long, diffusive time scales. Primary emphasis is given to the unification of conservative and nonconservative dislocation creation mechanisms in three-dimensional fcc and bcc materials. These include Frank-Read–type glide mechanisms involving closed dislocation loops or grain boundaries as well as Bardeen-Herring–type climb mechanisms involving precipitates, inclusions, and/or voids. Both source classes are naturally and simultaneously captured at the atomistic level by PFC descriptions, with arbitrarily complex defect configurations, types, and environments. An unexpected dipole-to-quadrupole source transformation is identified, as well as various complex geometrical features of loop nucleation via climb from spherical particles. Results for the strain required to nucleate a dislocation loop from such a particle are in agreement with analytic continuum theories. Other basic features of fcc and bcc dislocation structure and dynamics are also outlined, and initial results for dislocation-stacking fault tetrahedron interactions are presented. These findings together highlight various capabilities of the PFC approach as a coarse-grained atomistic tool for the study of three-dimensional crystal plasticity.

Acoustic avalanches are a general feature of solids under stress, e.g., evoked by external compression or arising from internal processes like martensitic phase transformations. From integral measurements, it is usually concluded that nucleation, phase boundary pinning, or interface incompatibilities during this first-order phase transition all may generate acoustic emission. This paper studies the local sources of acoustic emission to enlight the microscopic mechanisms. From two-dimensional spatially resolved acoustic emission measurement and simultaneous optical observation of the surface, we can identify microstructural events at the phase boundary that lead to acoustic emission. A resolution in the $100\text{−}\mu \mathrm{m}$ range was reached for the location of acoustic emission sources on a coarse-grained Ni-Mn-Ga polycrystal. Both, the acoustic activity and the size distribution of the microstructural transformation events, exhibit power-law behavior. The origin of the acoustic emission are elastically incompatible areas, such as differently oriented martensitic plates that meet each other, lamellae growing up to grain boundaries, and grain boundaries in proximity to transforming grains. Using this result, we propose a model to explain the decrease of the critical exponent under a mechanical stress or magnetic field.

We consider a quantum quench in a noninteracting fermionic one-dimensional field theory. The system of size $L$ is initially prepared into two halves $\mathcal{L}$ $\left(\right[-L/2,0])$ and $\mathcal{R}$ $\left(\right[0,L/2])$, each of them thermalized at two different temperatures $T_{\mathcal{L}}$ and $T_{\mathcal{R}}$, respectively. At a given time, the two halves are joined together by a local coupling and the whole system is left to evolve unitarily. For an infinitely extended system $(L\to \infty )$, we show that the time evolution of the particle and energy densities is well described via a hydrodynamic approach which allows us to evaluate the correspondent stationary currents. We show, in such a case, that the two-point correlation functions are deduced, at large times, from a simple nonequilibrium steady state. Otherwise, whenever the boundary conditions are retained (in a properly defined thermodynamic limit), any current is suppressed at large times, and the stationary state is described by a generalized Gibbs ensemble, which is diagonal and depends only on the post-quench mode occupation.

We have developed and measured a high-gain quantum-limited microwave parametric amplifier based on a superconducting lumped LC resonator with the inductor L including an array of eight superconducting quantum interference devices (SQUIDs). This amplifier is parametrically pumped by modulating the flux threading the SQUIDs at twice the resonator frequency. Around 5 GHz, a maximum gain of 31 dB, a product amplitude gain $\times$ bandwidth above 60 MHz, and a 1 dB compression point of $-123$ dBm at 20 dB gain are obtained in the nondegenerate mode of operation. Phase-sensitive amplification-deamplification is also measured in the degenerate mode and yields a maximum gain of 37 dB. The compression point obtained is 18 dB above what would be obtained with a single SQUID of the same inductance, due to the smaller nonlinearity of the SQUID array.

We report on the temperature dependence of the phonons and crystallographic parameters in ${\mathrm{BiMnO}}_3$ single crystals grown under high pressure and high temperature. The crystallographic structure of the sample was refined from room temperature to liquid helium temperature in the centrosymmetric $C2/c$ space group, i.e., a group which does not allow ferroelectricity. In addition, the lattice dynamics was probed by Raman spectroscopy down to liquid nitrogen temperature, i.e., below the ferromagnetic transition at $T_C=100\pm 2\mathrm{K}$. Both crystallographic and Raman data indicate the absence of a structural phase transition at the ferromagnetic ordering or any other temperature. The Raman signature around $T_C$ shows a significant spin-phonon coupling for the high-frequency bands.

We show that enantiomers are extremely sensitive to the quality of the light polarization in resonant elastic x-ray scattering experiments. In $\alpha \text{-quartz}$, the (001) reflection shows a very strong sensitivity to a small fraction of linear component included in nominally circularly polarized light. This fraction induces an important phase difference between the angular dependence of the recorded intensity from each enantiomer. We are thus able to model all aspects of the resonant scattering and determine the dominant physics, extract the crystal chirality, and obtain very precise values for the Stokes polarization parameters. Effects of birefringence and of higher-order electric transitions are also quantified and it is shown that in comparison they are negligible.

We present a 5-phase equation of state for elemental carbon which addresses a wide range of density and temperature conditions: $3\mathrm{g}/\mathrm{cc}<\rho <20\mathrm{g}/\mathrm{cc},0\mathrm{K}<T<\infty$. The phases considered are diamond, BC8, simple cubic, simple hexagonal, and the liquid/plasma state. The solid phase free energies are constrained by density functional theory (DFT) calculations. Vibrational contributions to the free energy of each solid phase are treated within the quasiharmonic framework. The liquid free energy model is constrained by fitting to a combination of DFT molecular dynamics performed over the range $\mathrm{10\hspace{0.17em}000}\text{K}<T<\mathrm{100\hspace{0.17em}000}\text{K}$, and path integral quantum Monte Carlo calculations for $T>\mathrm{100\hspace{0.17em}000}\text{K}$ (both for $\rho$ between 3 and 12 g/cc, with select higher-$\rho$ DFT calculations as well). The liquid free energy model includes an atom-in-jellium approach to account for the effects of ionization due to temperature and pressure in the plasma state, and an ion-thermal model which includes the approach to the ideal gas limit. The precise manner in which the ideal gas limit is reached is greatly constrained by both the highest-temperature DFT data and the path integral data, forcing us to discard an ion-thermal model we had used previously in favor of a new one. Predictions are made for the principal Hugoniot and the room-temperature isotherm, and comparisons are made to recent experimental results.

We propose an efficient basis expansion for electron orbitals to describe real-time electron dynamics in crystalline solids. Although a conventional grid representation in the three-dimensional Cartesian coordinates works robustly, it requires a large amount of computational resources. To reduce computational costs, we consider basis expansion methods employing eigenstates of the ground state Hamiltonian with a truncation. We have found that adding occupied eigenstates of nearby $k$ points to the truncated basis functions composed of eigenstates of the original $k$ point is crucially important. We demonstrate the usefulness of the method for linear and nonlinear electron dynamics calculations in crystalline ${\mathrm{SiO}}_2$.

Inelastic neutron scattering experiments on powder samples of GeTe together with density functional theory investigations of the phonon dynamics in the low- and high-temperature phases of GeTe crystal are reported. The dispersion of phonons in the high-temperature rocksalt phase show soft branches with the lowest one at the $\Gamma$ high-symmetry point. The structural phase transition in GeTe is reconsidered and shown to be driven by the con-densation of exactly three components of the triply degenerate optical transverse soft-phonon mode at the Brillouin zone center. The mechanism proposed allows us to explain the formation of structural distortions in the low-temperature ferroelectric phase of GeTe revealed by various experiments. A displacive nature of the phase change in crystalline GeTe is supported by the results of the present theoretical studies.