Strong magnetocrystalline anisotropy (MA) is a well-known property of uranium compounds. The almost isotropic ferromagnetism in ${\mathrm{U}}_4{\mathrm{Ru}}_7{\mathrm{Ge}}_6$ reported in this paper represents a striking exception. We present results for magnetization, ac susceptibility, thermal expansion, specific heat, and electrical resistivity measurements performed on a ${\mathrm{U}}_4{\mathrm{Ru}}_7{\mathrm{Ge}}_6$ single crystal at various temperatures and magnetic fields; we discuss the results in relation to first-principles electronic structure calculations. ${\mathrm{U}}_4{\mathrm{Ru}}_7{\mathrm{Ge}}_6$ behaves as an itinerant $5f$-electron ferromagnet ($T_{\mathrm{C}}=10.7\mathrm{K}, {\mu }_{\mathrm{S}}=0.85{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ at $1.9\mathrm{K}$). The ground-state easy magnetization direction is along the [111] axis of the cubic lattice. The anisotropy field ${\mu }_0{H}_{\mathrm{a}}$ along the [001] direction is only about $0.3\mathrm{T}$, which is at least three orders of magnitude smaller than for other U ferromagnets. At $T_r=5.9\mathrm{K}$ the easy magnetization direction changes to [001], and remains [001] up to $T_{\mathrm{C}}$. This transition is due to a change in magnetic symmetry, and is quite apparent in the low-field magnetization, ac susceptibility, and thermal expansion data, whereas only weak anomalies are observed at $T_r$ in the temperature dependence of the specific heat and electrical resistivity. The magnetoelastic interaction induces a rhombohedral (tetragonal) distortion of the paramagnetic cubic crystal lattice in the case of the [111] ([001]) easy magnetization direction. The rhombohedral distortion is connected with two crystallographically inequivalent U sites. Our density functional theory calculations, including spin-orbit interaction (SOI) of the U $5f$ electrons, also produces two inequivalent U sites, because SOI leads to a reduction of the symmetry of the former cubic structure. The calculated ground state is in agreement with the experimentally observed [111] easy magnetization direction. The first excited state has moments along the [001] direction, which agrees with the moment orientation for $T>T_{\mathrm{r}}$. The energy of the first excited state is $0.9\mathrm{meV}$ above the ground state, which is comparable to the value of $0.51\mathrm{meV}$, corresponding to $k_{\mathrm{B}}{T}_{\mathrm{r}}$. We propose that weak MA of the ${\mathrm{U}}_4{\mathrm{Ru}}_7{\mathrm{Ge}}_6$ compound is due to the lack of direct overlap of the $5f$ orbitals of the nearest U ions, which is screened out by the closed Ru and Ge cuboctahedra coordinating each U ion.

We investigate the quantum many-body instabilities for electrons on the honeycomb lattice at half filling with extended interactions, motivated by a description of graphene and related materials. We employ a recently developed fermionic functional renormalization group scheme, which allows for highly resolved calculations of wave-vector dependences in the low-energy effective interactions. We encounter the expected anti-ferromagnetic spin density wave for a dominant on-site repulsion between electrons, and charge order with different modulations for dominant pure $n^{\text{th}}$ nearest-neighbor repulsive interactions. Novel instabilities towards incommensurate charge density waves take place when nonlocal density interactions among several bond distances are included simultaneously. Moreover, for more realistic Coulomb potentials in graphene including enough nonlocal terms there is a suppression of charge order due to competition effects between the different charge ordering tendencies, and if the on-site term fails to dominate, the semimetallic state is rendered stable. The possibility of a topological Mott insulator being the favored tendency for dominating second-nearest-neighbor interactions is not realized in our results with high momentum resolution.

We generalize the recently developed inchworm quantum Monte Carlo method to the full Keldysh contour with forward, backward, and equilibrium branches to describe the dynamics of strongly correlated impurity problems with time-dependent parameters. We introduce a method to compute Green's functions, spectral functions, and currents for inchworm Monte Carlo and show how systematic error assessments in real time can be obtained. We then illustrate the capabilities of the algorithm with a study of the behavior of quantum impurities after an instantaneous voltage quench from a thermal equilibrium state.

We have investigated electron-electron interaction effects on gapless edge states in the antiferromagnetic phase of zigzag-edge bilayer graphene nanoribbons under a voltage bias between the layers by using a tight-binding model with on-site Coulomb interactions. We found that a zigzag-edge bilayer graphene nanoribbon can have gapless edge states corresponding to peculiar topologically nontrivial insulator phases, such as a half-metallic quantum valley Hall phase. The half-metallicity was found to be due to the on-site Coulomb interactions through which excess charges produced by the voltage bias between the layers give rise to the potential difference between the opposite edges. A topological phase diagram in the antiferromagnetic phase of a zigzag-edge bilayer graphene nanoribbon was determined as a function of the bias voltage and the ribbon width. The quantum confinement effect, which was found to be graphene-like for narrow ribbons and to be bilayer graphene-like for wide ribbons, was also confirmed to play an important role in determining the topologically nontrivial insulator phases.

We introduce a general framework to construct many-body lattice wave functions starting from the conformal blocks (CBs) of rational conformal field theories (RCFTs). We discuss the different ways of encoding the physical degrees of freedom of the lattice system using both the internal symmetries of the theory and the fusion channels of the CBs. We illustrate this construction both by revisiting the known Haldane-Shastry model and by providing a novel implementation for the Ising RCFT. In the latter case, we find a connection to the Ising transverse field (ITF) spin chain via the Kramers-Wannier duality and the Temperley-Lieb-Jones algebra. We also find evidence that the ground state of the finite-size critical ITF Hamiltonian corresponds exactly to the wave function obtained from CBs of spin fields.

We assess the performance of nonempirical, truly nonlocal, and semilocal functionals with regard to structural and thermal properties of $3d,4d$, and $5d$ nonmagnetic transition metals. We focus on constraint- based functionals and consider the consistent-exchange van der Waals density-functional version vdW-DF-cx [Phys. Rev. B 89, 035412 (2014)], the semilocal PBE functional [Phys. Rev. Lett. 77, 3865 (1996)], and the PBEsol functional [Phys. Rev. Lett. 100, 136406 (2008)], as well as the AM05 metafunctional [Phys. Rev. B 72, 085108 (2005)]. Using the quasiharmonic approximation, the structural parameters, elastic response, and thermal expansion at finite temperatures are computed and compared to experimental data. We also compute cohesive energies explicitly including zero-point vibrations. It is shown that overall vdW-DF-cx provides an accurate description of thermal properties and retains a level of transferability and accuracy that is comparable to or better than some of the best constraint-based semilocal functionals. Especially, with regard to the cohesive energies, the consistent inclusion of spin-polarization effects in the atoms turns out to be crucial, and it is important to use the rigorous spin-vdW-DF-cx formulation [Phys. Rev. Lett. 115, 136402 (2015)]. This demonstrates that vdW-DF-cx has general-purpose character and can be used to study systems that have both sparse and dense electron distributions.

Thermal transport in most carbon allotropes is determined by phonons. The properties of the atomic bonds will influence the phonon transport process directly. In this paper we studied two novel carbon allotropes as examples, one novel allotrope phase is topological semimetal in an $s{p}^2$ bonding network with a 16-atom body-centered orthorhombic unit cell (BCO-${\mathrm{C}}_{16}$) [Phys. Rev. Lett. 116, 195501 (2016)] and the other novel allotrope is derived by substituting each atom in diamond with a carbon tetrahedron (T-carbon) [Phys. Rev. Lett. 106, 155703 (2011)], which possesses an $s{p}^3$ bonding network. Graphene and diamond with standard $s{p}^2$ and $s{p}^3$ hybridization, respectively, are also examined for comparison. We explored the related properties of the atomic bonds of these allotropes with the density functional theory, i.e., the atomic orbital hybridization, effective spring constants of atomic bonds, the anharmonicity of atomic bonds, etc. By comparing the results, we unveiled the veil behind different lattice thermal conductivities of these allotropes at atomic bond levels (BCO-${\mathrm{C}}_{16}$ vs graphene and T-carbon vs diamond), despite their similar hybridization. In addition, within the framework of a phonon Boltzmann transport equation, the mode level phonon transport properties of the four carbon allotropes are also studied in detail, which are well consistent with the information from atomic bonds. We expect that the method of analyzing the strength and anharmonicity of atomic bonds here will be helpful for studying the thermal transport in crystalline materials in the future.

We report the transport properties of monolayer and bilayer graphene from first-principles calculations and Boltzmann transport theory (BTE). Our resistivity studies on monolayer graphene show Bloch-Grüneisen behavior in a certain range of chemical potentials. By substituting boron nitride in place of a carbon dimer of graphene, we predict a twofold increase in the Seebeck coefficient. A similar increase in the Seebeck coefficient for bilayer graphene under the influence of a small electric field $\sim 0.3$ eV has been observed in our calculations. Graphene with impurities shows a systematic decrease of electrical conductivity and mobility. We have also calculated the lattice thermal conductivities of monolayer graphene and bilayer graphene using phonon BTE which show excellent agreement with experimental data available in the temperature range 300–700 K.

We numerically examine the high-harmonic generation in graphene caused by an intense few-cycle terahertz laser field by solving the time-dependent Dirac equation. The observed spectra feature a complex interplay of interband and intraband electron dynamics. At high harmonic frequencies, we observe a plateau region with a cut-off frequency linearly proportional to the laser field. The linear dependence of the cutoff on the field resembles the behavior for electrons driven in bulk crystals but differs from the case of atoms or molecules. The unique features of the graphene band structure allow for a transparent decomposition of interband and intraband contributions and for an analytic estimate of the harmonic cut-off frequency in excellent agreement with the simulations.

As the grain size decreases to the nanometer range, the characteristics of the ferroelectric nanoceramic can be ultimately determined by the competition between two effects: the intrinsic effect that is associated with the local properties of the grain boundary and the extrinsic effect that arises from the dynamics of domain structure which is highly influenced by the depolarization field caused by the grain boundary. In this work we investigate such a competition with a phase-field simulation based on the time-dependent Ginzburg-Landau kinetic equation. The study is performed on poled/unpoled nanoceramics under high- and low-amplitude bipolar alternating electric field with selected grain size and loading frequency. Our calculations for poled $\mathrm{BaTi}{\mathrm{O}}_3$ at 100 Hz show that, for the grain size from 170 to 50 nm, its properties are dominated by the extrinsic effect, and from 50 to 10 nm, they are dominated by the intrinsic one. As the grain size decreases, the dielectric and piezoelectric constants at the remnant state continuously rise in the extrinsic-dominated region and then drop sharply in the intrinsic-dominated region. Our frequency calculations from 10 to 2500 Hz at the grain size of 100 nm indicate that the high-frequency behavior is very similar to that of the small grain-size, intrinsic-dominated one, whereas the low-frequency behavior is closely related to that of the large grain-size, extrinsic-dominated part, with the demarcation line occurring around 400 Hz. For the unpoled ceramics under small-signal loading, the intrinsic effect is dominant over the entire range of grain size and frequency.

We use the first-principles approach to clarify the thermodynamic stability as a function of pressure and temperature of three different $\alpha$-rhombohedral-boron-like boron subnitrides, with the compositions of ${\mathrm{B}}_6\mathrm{N}, {\mathrm{B}}_{13}{\mathrm{N}}_2$, and ${\mathrm{B}}_{38}{\mathrm{N}}_6$, proposed in the literature. We find that, out of these subnitrides with the structural units of ${\mathrm{B}}_{12}$(N-N), ${\mathrm{B}}_{12}$(NBN), and ${\left[{\mathrm{B}}_{12}\left(\mathrm{N}-\mathrm{N}\right)\right]}_{0.33}{\left[{\mathrm{B}}_{12}\left(\mathrm{NBN}\right)\right]}_{0.67}$, respectively, only ${\mathrm{B}}_{38}{\mathrm{N}}_6$, represented by ${\left[{\mathrm{B}}_{12}\left(\mathrm{N}-\mathrm{N}\right)\right]}_{0.33}{\left[{\mathrm{B}}_{12}\left(\mathrm{NBN}\right)\right]}_{0.67}$, is thermodynamically stable. Beyond a pressure of about 7.5 GPa depending on the temperature, also ${\mathrm{B}}_{38}{\mathrm{N}}_6$ becomes unstable, and decomposes into cubic boron nitride and $\alpha$-tetragonal-boron-like boron subnitride ${\mathrm{B}}_{50}{\mathrm{N}}_2$. The thermodynamic stability of boron subnitrides and relevant competing phases is determined by the Gibbs free energy, in which the contributions from the lattice vibrations and the configurational disorder are obtained within the quasiharmonic and the mean-field approximations, respectively. We calculate lattice parameters, elastic constants, phonon and electronic density of states, and demonstrate that ${\left[{\mathrm{B}}_{12}\left(\mathrm{N}-\mathrm{N}\right)\right]}_{0.33}{\left[{\mathrm{B}}_{12}\left(\mathrm{NBN}\right)\right]}_{0.67}$ is both mechanically and dynamically stable, and is an electrical semiconductor. The simulated x-ray powder-diffraction pattern as well as the calculated lattice parameters of ${\left[{\mathrm{B}}_{12}\left(\mathrm{N}-\mathrm{N}\right)\right]}_{0.33}{\left[{\mathrm{B}}_{12}\left(\mathrm{NBN}\right)\right]}_{0.67}$ are found to be in good agreement with those of the experimentally synthesized boron subnitrides reported in the literature, verifying that ${\mathrm{B}}_{38}{\mathrm{N}}_6$ is the stable composition of $\alpha$-rhombohedral-boron-like boron subnitride.

We report the first observation of Rabi oscillations in the spin-7/2 ensemble of trivalent gadolinium ions hosted in $\mathrm{CaW}{\mathrm{O}}_4$ single crystal. A number of transitions within the lowest electronic multiplet ${}^8{S}_{7/\phantom{72}2}$ of $\mathrm{G}{\mathrm{d}}^{3+}$ ion are studied using a combination of continuous-wave and pulsed electron paramagnetic resonance spectroscopy. The corresponding Rabi damping curves and the spin coherence times are detected at varying strengths of the microwave field. These data are well reproduced by a theoretical model which accounts for the intrinsic inhomogeneity of the microwave field within the microwave resonator and the magnetic dipole interactions in the diluted spin ensemble. The results indicate that the studied 8-level ground manifold of $\mathrm{G}{\mathrm{d}}^{3+}$ ion can represent an effective three-qubit quantum system.

With the help of the exact solution of the Kitaev model the parametric pumping of the two-dimensional quantum spin liquid under the action of the ac magnetic field is studied. In the dynamical regime the field produces oscillations of the magnetization with the field's frequency, modulated by the Rabi-like oscillations. In the steady-state regime, the Rabi-like oscillations are damped. The absorption of the ac field by the Kitaev spin model is finite and manifests resonance features. Such a behavior is generic for quantum spin liquids with fermionic excitations, and it is different from the linear spin-wave response of magnetically ordered systems to such a parametric pumping.

Monolayer transition metal dichalcogenides (TMDCs) offer a tantalizing platform for control of both spin and valley degrees of freedom, which may enable future optoelectronic devices with enhanced and novel functionalities. Here, we investigate the valley dynamics of two prototypical members of TMDCs, namely $\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{WS}{\mathrm{e}}_2$, using time-resolved Kerr rotation (TRKR) at temperatures from 10 K to 300 K. This pump-probe technique enables sub-picosecond temporal resolution, providing insight into ultrafast valley dynamics, which is inaccessible by polarized and time-resolved photoluminescence spectroscopy. Bi-exponential decay dynamics were observed for both materials at low temperatures, and the fast decay component indicated a rapid exciton valley depolarization time $(<10\mathrm{ps})$ due to strong Coulomb exchange interactions between the $K$ valleys. However, the slow decay components (several tens of picoseconds) were attributed to different origins in the two materials, which were further elucidated by temperature-dependent TRKR measurements. Moreover, the spatial dependence of the TRKR intensity across $\mathrm{Mo}{\mathrm{S}}_2$ monolayer flakes indicated a weaker valley polarization near the edges, which is likely associated with quenched excitons near the grain boundaries or a disordered edge region in chemical vapor deposition–grown monolayers. These temporal and spatial TRKR measurements reveal insight into the complex dynamics of valley excitonic states, which will be critical for valleytronic applications of monolayer TMDCs.

By using Fourier-transform scanning tunneling spectroscopy we measure the interference patterns produced by the impurity scattering of confined Dirac quasiparticles in epitaxial graphene nanoflakes. Upon comparison of the experimental results with tight-binding calculations of realistic model flakes, we show that the characteristic features observed in the Fourier-transformed local density of states are related to scattering between different transverse modes (subbands) of a graphene nanoflake and allow direct insight into the gapped electronic spectrum of graphene. We also observe a strong reduction of quasiparticle lifetime which is attributed to the interaction with the underlying substrate. In addition, we show that the distribution of the on-site energies at flower defects leads to an effectively broken pseudospin selection rule, where intravalley backscattering is allowed.

First-principles studies of the electron-phonon coupling in graphene predict a high coupling strength for the $\sigma$ band with $\lambda$ values of up to 0.9. Near the top of the $\sigma$ band, $\lambda$ is found to be $\approx 0.7$. This value is consistent with the recently observed kinks in the $\sigma$ band dispersion by angle-resolved photoemission. While the photoemission intensity from the $\sigma$ band is strongly influenced by matrix elements due to sublattice interference, these effects differ significantly for data taken in the first and neighboring Brillouin zones. This can be exploited to disentangle the influence of matrix elements and electron-phonon coupling. A rigorous analysis of the experimentally determined complex self-energy using Kramers-Kronig transformations further supports the assignment of the observed kinks to strong electron-phonon coupling and yields a coupling constant of $0.6\left(1\right)$, in excellent agreement with the calculations.

We consider a one-dimensional spin-$\frac{1}{2}$ chain with Heisenberg interaction in a disordered parallel magnetic field. This system is known to exhibit the many-body localization (MBL) transition at critical strength of disorder. We analyze the response of the chain when additional perpendicular magnetic field is applied to an individual spin and propose a method for accurate determination of the mobility edge via local spin measurements. We further demonstrate that the exponential decrease of the spin response with the distance between perturbed spin and measured spin can be used to characterize the localization length in the MBL phase. We also studied the effect of the quench on statistics of the entire energy spectrum. We demonstrate that the quench introduces level repulsion between energy states corresponding to different eigenvalues of the total spin in the delocalized regime, while level spacing is described by the Poisson statistics in the localized regime even at strong quench.

One-dimensional organo-metal halide perovskite ($\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$) nanorods whose diameter and length are dictated by the inner size of porous silicon nanotube templates have been grown, characterized, and compared to bulk perovskites in the form of microwires. We have observed a structural phase transition for bulk perovskites, where the crystal structure changes from tetragonal to orthorhombic at about 160 K, as opposed to small diameter one-dimensional perovskite nanorods, of the order of 30–70 nm in diameter, where the phase transition is inhibited and the dominant phase remains tetragonal. Two major experimental techniques, infrared absorption spectroscopy and photoluminescence, were utilized to probe the temperature dependence of the perovskite phases over the 4–300 K temperature range. Yet, different characteristics of the phase transition were measured by the two spectroscopic methods and explained by the presence of small, tetragonal inclusions embedded in the orthorhombic phase. The inhibition of the phase transition is attributed to the large surface area of these one-dimensional perovskite nanorods, which gives rise to a large stress that, in turn, prevents the formation of the orthorhombic phase. The absence of phase transition enables the measurement of the tetragonal bandgap energy down to low temperatures.