The frustrated Heisenberg $J_1\text{−}{J}_2$ model on a square lattice is numerically investigated by variational Monte Carlo simulations. We propose an antiferromagnetic fermion resonating-valence-bond (AF-fRVB) state that has the ability to examine the entire phase diagram in the $J_1\text{−}{J}_2$ model. Two phase transition points, the second order around $J_2/J_1=0.45$ and the first order around $J_2/J_1=0.6$, can be extracted more clearly than the conventional bosonic RVB state. At the maximally frustrated point $\left(J_2/J_1=0.5\right)$, the AF-fRVB state shows the variational ground-state energy in the thermodynamic limit very close to the one estimated by the projected entangled pair state at the largest bond dimension available. On the other hand, in the frustrated regime $0.4\lesssim J_2/J_1\le 0.5$, AF-fRVB states with $\text{ext}{s}_2$ (using the terminology in the field of iron-based superconductors) and $d_{xy}$ pairing symmetries are degenerate in the thermodynamic limit, implying the existence of gapless Dirac excitations in the spinon spectrum.

The dynamic behavior of single bistable Si dopants in the $\mathrm{GaAs}\left(110\right)$ surface, which switch between a positive and a negative charge configuration, was investigated using a scanning tunneling microscope (STM) and noise analysis electronics. The dopant atom switching frequency shows a clear dependence on the bias voltage and tunneling current, because these parameters influence the escape and capture processes of electrons. Our physical model for these processes, taking into account the relevant tunneling barriers, matches well with the experimental data. By choosing the appropriate tunneling conditions, we show that a single dopant can be employed as a memory element. The STM tip serves both as an electrical gate to write and as a probe to read the information stored on a single Si atom.

Using the Kondo-Heisenberg model framework, we analyze the effect of charge fluctuations in the modulated spin liquid (MSL) and in the antiferromagnetic (AF) ordering which were introduced by Pépin et al. [Phys. Rev. Lett. 106, 106601 (2011)] and Thomas et al. [Phys. Rev. B 87, 014422 (2013)]. Coupling the spin liquid to the charge sector enables us to discuss the formation of the Kondo effect in this system. As a result, we are able to observe the emergence of two phases showing the coexistence of the Kondo effect and the MSL (Kondo-MSL phase) and the coexistence of the Kondo-MSL-AF phases. We discuss the critical lines separating the coexisting Kondo-MSL and Kondo-MSL-AF phases. Our model produces promising results as regards to the experimental temperature-pressure phase diagram and to the two gap openings observed in $\mathrm{U}{\mathrm{Ru}}_2{\mathrm{Si}}_2$.

We performed ${}^{13}\mathrm{C}$ NMR measurements on the quasi-one-dimensional (Q1D) charge-ordered system $(\mathrm{DI}-DCNQI{)}_2\mathrm{Ag}$ under ambient and applied pressure to clarify the pressure-temperature phase diagram. For pressures up to 15 kbar, the NMR spectra exhibit complicated splitting at low temperatures, indicating a “generalized 3D Wigner crystal” state. In this pressure region, we find that increased pressure causes a decrease in the charge disproportionation ratio, along with a decrease in the transition temperature of the generalized 3D Wigner crystal. In the high-pressure region, near 20 kbar, where a 1D confined liquid crosses over to a 3D Fermi liquid at high temperatures, the ground state is replaced by a nonmagnetic insulating state that is qualitatively different from the generalized 3D Wigner crystal.

We use angle-resolved photoemission spectroscopy to show that the near-surface electronic structure of a bulk insulating iridate ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ lying near a metal-Mott insulator transition exhibits weak metallicity signified by finite electronic spectral weight at the Fermi level. The surface electrons exhibit a spin structure resulting from an interplay of spin-orbit, Coulomb interaction and surface quantum magnetism. Our results shed light on understanding the exotic quantum entanglement and transport phenomena in iridate-based oxide devices.

We discuss proximity effects on the hydrogen uptake in 14 atomic layers of vanadium. The enthalpy and entropy of solution were measured and compared to ab initio density functional calculations. We show that there exists a large difference in the hydrogen uptake of V in Cr/V and Fe/V superlattices, in which the V is under close to identical strain states. The calculations show that neither local strain effects nor charge redistributions can be the cause for the observed effect. This leaves magnetic and long ranged elastic strain fields—neither captured by the current calculations—as possible mechanisms for the observed effects.

This work presents a multiscale approach to correctly evaluate the dislocation sink efficiencies in Zr. Starting from ab initio calculations which show that self-interstitial atoms in Zr induce a much stronger deformation in the basal plane than parallel to the $\mathbf{c}$ axis, a phase-field model is used to evaluate the dislocation sink efficiencies. The results, which show a preferential absorption of self-interstitials by prismatic loops, stem from the elastic interactions between dislocations and anisotropic point defects. Our predictions are consistent with the observation of basal loops of vacancy character in Zr alloys under irradiation.

We have used an effective model of ferroelectric ${\mathrm{PbTiO}}_3$, which displays a representative phase transition of the displacive nonreconstructive type, to investigate how different features of the potential-energy surface affect the transition temperature $T_{\mathrm{C}}$. We find that the energy difference between ${\mathrm{PbTiO}}_3$'s high-symmetry (cubic) and low-symmetry (tetragonal) phases (which we call ground-state energy $E_{\mathrm{gs}})$ is the parameter that most directly and strongly determines $T_{\mathrm{C}}$. We have also found that other simple features of the energy landscape, such as the amplitude of the distortion connecting the high-symmetry and low-symmetry structures, can be used as a predictor for $T_{\mathrm{C}}$ as long as they are correlated with the magnitude of $E_{\mathrm{gs}}$. We discuss how our results relate to the expected behaviors that can be derived from simpler theoretical approaches, as well as to phenomenological studies in the literature. Our findings support the empirical rule for estimating $T_{\mathrm{C}}$ proposed by Abrahams et al. [Phys. Rev. 172, 551 (1968)] and clarify its physical interpretation. The evidence also suggests that deviations from the expected behaviors are indicative of complex lattice-dynamical effects involving strong anharmonic interactions (and possibly competition) between the unstable modes driving the transition and other distortions of the material.

A recent experiment [H. Y. Gou et al., Phys. Rev. Lett. 111, 157002 (2013)] reported a measured Vickers hardness of over 60 GPa for ${\mathrm{FeB}}_4$, placing it as a superhard material far above all other similar ultrahard transition-metal borides, such as ${\mathrm{ReB}}_2,{\mathrm{WB}}_4$, and especially ${\mathrm{CrB}}_4$, which has the same crystalline structure as that of ${\mathrm{FeB}}_4$ but a much lower measured Vickers hardness of around 23 GPa. This result, however, is contradicted by theoretical calculations that predict a smaller shear modulus for ${\mathrm{FeB}}_4$ than that of ${\mathrm{CrB}}_4$, indicating that ${\mathrm{FeB}}_4$ is softer than ${\mathrm{CrB}}_4$. Here we report first-principles calculations that aim to understand the stress response of ${\mathrm{FeB}}_4$ under indentation loadings and examine whether there exists a strain-stiffening effect that might enhance the indentation strength. Our results show that there is no strain stiffening in ${\mathrm{FeB}}_4$; instead, the normal pressure beneath the indenter drives a lateral structural expansion which further stretches and weakens the boron-boron and boron-iron bonds in addition to that caused by the shear deformation in Vickers indentation tests. This effect leads to a considerably reduced strength of ${\mathrm{FeB}}_4$, producing an ideal (i.e., an upper bound) indentation strength of 17 GPa that is lower than that (27 GPa) predicted for ${\mathrm{CrB}}_4$. The present results suggest that ${\mathrm{FeB}}_4$ is unlikely to be superhard and further experimental investigation is needed to clarify this issue.

We report the direct observation of the development of long-range spatial intensity correlation and the growth of intensity fluctuations inside random media. We fabricated quasi-two-dimensional disordered photonic structures and probed light transport from a third dimension. Good agreement between experiment and theory is obtained. We were able to manipulate the long-range intensity correlation and intensity fluctuations inside the disordered waveguides by simply varying the waveguide geometry.

To investigate the microscopic magnetic nature of phospho-olivines, we have measured positive muon-spin rotation and relaxation (${\mu }^{+}$SR) spectra for ${\mathrm{Na}}_x\mathrm{Fe}\mathrm{P}{\mathrm{O}}_4$ and $\mathrm{Fe}\mathrm{P}{\mathrm{O}}_4$, using powder samples in the temperature range between 2 and 200 K. For ${\mathrm{Na}}_{0.7}\mathrm{Fe}\mathrm{P}{\mathrm{O}}_4$, ${\mu }^{+}$SR measurements under zero external field (ZF) showed the appearance of static magnetic order below $T_{\mathrm{N}}\sim 61$ K, at which the susceptibility $\chi \left(T\right)$ exhibits a sharp maximum due to an antiferromagnetic (AF) transition. The wide distribution found for the internal magnetic field ($H_{\mathrm{int}}$) was reasonably explained by the formation of AF order with the spread in $H_{\mathrm{int}}$ due to a random distribution of Na vacancies. On the other hand, the ZF spectra for FePO${}_4$ suggested the formation of the canted AF order below $T_{\mathrm{N}}=113$ K, as proposed by neutron diffraction (ND), but the ratio between the $y$ and $x$ components of the ordered moment ($M_y/M_x$) was determined as 0.6 by ${\mu }^{+}$SR, whereas 0.22 was measured by ND, probably indicating anisotropic spin fluctuations in $\mathrm{Fe}\mathrm{P}{\mathrm{O}}_4$.

We present a microscopic calculation of magnetization damping for a magnetic “toy model.” The magnetic system consists of itinerant carriers coupled antiferromagnetically to a dispersionless band of localized spins, and the magnetization damping is due to coupling of the itinerant carriers to a phonon bath in the presence of spin-orbit coupling. Using a mean-field approximation for the kinetic exchange model and assuming the spin-orbit coupling to be of the Rashba form, we derive Boltzmann scattering integrals for the distributions and spin conherences in the case of an antiferromagnetic exchange splitting, including a careful analysis of the connection between lifetime broadening and the magnetic gap. For incoherent scattering of itinerant carriers with the phonon bath, i.e., the Elliott-Yafet mechanism, we extract dephasing and magnetization times $T_1$ and $T_2$ from initial conditions corresponding to a tilt of the magnetization vector and draw a comparison to phenomenological equations such as the Landau-Lifshitz (LL) or the Gilbert damping. We also analyze magnetization precession and damping for this system including an anisotropy field and find a carrier mediated dephasing of the localized spin via the mean-field coupling.

We study the crystalline and electronic properties of the ${\text{Fe}}_{1-x}{\text{Co}}_x\text{Se}$ system ($x=0,0.25,0.5,0.75$, and 1.0) using x-ray diffraction, x-ray spectroscopy, and density functional theory. We show that the introduction of Co $3d$ states in FeSe relaxes the bond strengths and induces a structural transition from tetragonal to hexagonal whose crossover takes place at $x\approx 0.38$. This structural transition in turn modifies the magnetic order, which can be related to the spin state. Using resonant inelastic x-ray scattering, we estimate the spin state of the system; FeSe is found to be in a high-spin state $\left(S=2\right)$, but Fe is reduced to a low-spin state upon Co substitution of $x\le 0.25$, well below the structural transition. Finally, we show evidence that FeSe is a moderately correlated system but the introduction of Co into the host lattice weakens the correlation strength for $x\ge 0.25$. These novel findings are important to unravel the mechanisms responsible for the superconducting state in iron-chalcogenide superconductors.

We present a systematic approach to modeling the electrical and structural properties of charge-mismatched superlattices from first principles. Our strategy is based on bulk calculations of the parent compounds, which we perform as a function of in-plane strain and out-of-plane electric displacement field. The resulting two-dimensional phase diagrams allow us to accurately predict, without performing further calculations, the behavior of a layered heterostructure where the aforementioned building blocks are electrostatically and elastically coupled, with an arbitrary choice of the interface charge (originated from the polar discontinuity) and volume ratio. By using the $[\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3{]}_m/[\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3{]}_n$ system as test case, we demonstrate that interface polarity has a dramatic impact on the ferroelectric behavior of the superlattice, leading to the stabilization of otherwise inaccessible bulk phases.

Frustration refers to the inability to satisfy competing interactions simultaneously, often leading to a large number of degenerate ground states. This can suppress ordering tendencies, sometimes resulting in a spin-liquid phase. An intrinsic effect lifting this degeneracy is entropic order by disorder. We present strong evidence that a classical nearest-neighbor Heisenberg model on the swedenborgite lattice exhibits both an extended spin-liquid phase as well as entropic order-by-disorder choosing coplanar configurations after a first-order transition. We argue that this observation renders magnetic insulators such as ${R\mathrm{BaCo}}_4{\mathrm{O}}_7$, where $R$ denotes a rare-earth atom, prime candidates for displaying spin-liquid behavior and entropic order-by-disorder physics due to their large exchange constant.

We describe ferroelectric thin films with circular electrodes and develop a thermodynamic theory that explains exotic experimental results previously reported. It is found to be especially useful for restricted geometries such as microstructures for which boundary conditions are well known to play an important role in ferroelectric properties. We have explored a switching mechanism that consists of an inhomogeneous rotational motion of the polarization and leads to a vortex state. The vortex appearance exhibits characteristic properties of a first-order field-induced phase transition with three critical electric fields and the possibility of hysteresis behavior.

${\mathrm{Y}}_2$${\mathrm{Mo}}_3$${\mathrm{O}}_{12}$, a material that exhibits negative thermal expansion (NTE) from 10 to 1173 K, offers an excellent opportunity to examine relationships between NTE and other physical properties over a wide temperature range. We report experimental heat capacity, thermal conductivity, and elastic properties of ${\mathrm{Y}}_2$${\mathrm{Mo}}_3$${\mathrm{O}}_{12}$, as well as results of an ab initio study of the lattice dynamics, and show how the anomalously high heat capacity and low thermal conductivity are correlated with NTE. We also report the ab initio elastic tensor and experimental velocity of sound of ${\mathrm{Y}}_2$${\mathrm{Mo}}_3$${\mathrm{O}}_{12}$ and use it to calculate the thermal stresses in a simulated polycrystal using finite-element analysis, showing that elastic anisotropy and thermal expansion anisotropy couple to influence the properties of the bulk solid.

The complex oxide $\mathrm{Bi}\mathrm{Mn}{\mathrm{Fe}}_2{\mathrm{O}}_6$ crystallizing in a centrosymmetric orthorhombic structure ($Pbcm$) is known to exhibit a spiral antiferromagnetic ordering at $T_N\sim 212$ K and an anomaly in magnetization at low temperature ($T_P\sim 34$ K). To understand the origin of the low temperature anomaly and to verify experimentally whether or not the spiral magnetism induces ferroelectricity, we have carried out detailed magnetization and electrical measurements on a polycrystalline sample. Frequency-dependent ac susceptibility, dc magnetic memory effect, and dc magnetization relaxation studies show that the low temperature anomaly at $T_P$ is associated with a reentrant spin-glass transition. From dc magnetization, we found that there exists another distinct temperature ($T_{\text{irr}}$), which corresponds to a strong thermomagnetic irreversibility. The field dependence of these two temperatures, $T_P\left(H\right)$ and $T_{\text{irr}}\left(H\right)$, follow Gabay-Toulouse and de Almeida–Thouless lines, respectively, which are indicative of the Heisenberg spin-glass state. We also observe another magnetic anomaly at $T^{*}\sim 170$ K, where a dielectric anomaly with a significant magnetocapacitance effect is observed. However, pyroelectric current and positive-up-negative-down measurements reveal the absence of ferroelectricity in this compound.

The magnetic, transport, and electronic properties of ultrathin epitaxial ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) films at near half doping ($x=0.47$, 0.50, and 0.55), grown under different misfit strains on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$(001) and ${\mathrm{La}}_{0.18}{\mathrm{Sr}}_{0.82}{\mathrm{Al}}_{0.59}{\mathrm{Ta}}_{0.41}{\mathrm{O}}_3$(001) (LSAT) substrates, are investigated. We find that all films exhibit metallic behavior below the magnetic critical temperature, while the magnetic properties change markedly with both doping and strain. However, while increased doping favors antiferromagnetic ordering, strain is the driving mechanism for the change in the magnetic properties, where with increasing tensile strain the magnetic ground state changes from ferromagnetic to antiferromagnetic at a critical lattice misfit threshold of about −1%. The bulk magnetometry data are confirmed by x-ray magnetic circular dichroism spectroscopy, while x-ray magnetic linear dichroism measured at room temperature demonstrates a progressive change in the orbital occupancy with increasing misfit strain from out of plane to in-plane, leading to a preferred antiferromagnetic metallic state at larger tensile strains.