We discover an example where the dissociation of the $Z_2$ vortices occurs at the second-order phase transition point. We investigate the nature of phase transition in a classical Heisenberg model on a distorted triangular lattice with competing interactions. The order parameter space of the model is SO(3)$\times Z_2$. The dissociation of the $Z_2$ vortices which comes from SO(3) and a second-order phase transition with $Z_2$ symmetry breaking occur at the same temperature. We also find that the second-order phase transition belongs to the universality class of the two-dimensional Ising model.

Experiments in the antiferromagnetic phase of a quasi-two-dimensional $S=1/2$ quasi-square-lattice antiferromagnet Cu(pz)${}_2$(ClO${}_4$)${}_2$ reveal a biaxial type of anisotropy, instead of the easy-plane one considered before. The weak in-plane anisotropy, found by means of electron spin-resonance spectroscopy and magnetization measurements, is about an order of magnitude weaker than the off-plane anisotropy. The weak in-plane anisotropy results in a spin-flop phase transition for the magnetic field aligned along the easy axis, and, thereby, in a bicritical point on the phase diagram. A remarkable feature of the weak in-plane anisotropy is the abrupt change of its sign at the spin-flop point. This anisotropy switching disappears at the tilting of the magnetic field to the easy axis by the angle of 10${}^{\circ }$ within the plane. The nature of the abrupt anisotropy reversal remains unclear. The phase diagram is characterized by the increase of the ordering temperature $T_N$ in the magnetic field used, except for a dip near the bicritical point.

The critical need in novel or alternative approaches to energy production, conversion, and harvesting, combined with the ongoing pursuit of device miniaturization, have revived interest in the long-known but “doomed” impractical caloric effects in solids. Contrary to the old belief that such effects, while common, are miniscule, a large variety of giant caloric effects has been recently discovered in some ferroic materials. These discoveries have opened the door to the use of these giant effects in an efficient and environmentally friendly solid-state refrigeration technology. Here we report first-principles simulations of multiple caloric effects in a typical ferroelastic ferroelectric PbTiO${}_3$  which offer insight into a multicaloric nature of this material. We predict the existence of unusual multicaloric effect in such electrically driven ferroelastic (extrinsic multiferroic) which far exceeds either electrocaloric or elastocaloric effect alone. Moreover, given a relative abundance of extrinsic multiferroics among ferroics with multiple order parameters, such prediction may open an unusual route to solid-state refrigeration advancement.

The atomistic structure of (Fe${}_x$Ca${}_{1-x}$O)${}_y$(SiO${}_2$)${}_{1-y}$ liquids, namely, fayalite ($x$ = 1, $y$ = 0.667), ferrosilite ($x$ = 1, $y$ = 0.5), and hedenbergite ($x$ = 0.5, $y$ = 0.5), was measured by using high-energy x-ray diffraction combined with laser-heated aerodynamic levitation. Measurements were also made for hedenbergite glass, formed via supercooling the liquid. The results unequivocally prove that Fe atoms are both four and sixfold coordinated by oxygen atoms in all compositions studied from the two distinct bond distances measured at $r_{\mathrm{FeO}}=1.93$ and $2.20\AA$, respectively. The results quantitatively reveal up to $40\%$ and $55\%$ FeO${}_6$ species residing in liquid fayalite and ferrosilite, respectively. From the real-space peak positions, the results also reveal the presence of both FeO${}_4$ and FeO${}_6$ species within a basaltic liquid and glass for which the geophysical consequences are briefly discussed.

We report quasi-two-dimensional (2D) Dirac fermions and quantum magnetoresistance in LaAgBi${}_2$. The band structure shows several narrow bands with nearly linear energy dispersion and Dirac-cone-like points at the Fermi level. The quantum oscillation experiments revealed one quasi-two-dimensional Fermi pocket and another complex pocket with a small cyclotron resonant mass. The in-plane transverse magnetoresistance exhibits a crossover at a critical field $B^{*}$ from semiclassical weak-field $B^2$ dependence to the high-field unsaturated linear magnetoresistance which is attributed to the quantum limit of the Dirac fermions. Our results suggest the existence of quasi-2D Dirac fermions in rare-earth-based layered compounds with two-dimensional double-sized Bi square nets, similar to (Ca,Sr)MnBi${}_2$, irrespective of magnetic order.

Strontium titanate ${\text{SrTiO}}_3$ is an extensively studied material. Of particular interest are its electronic properties. Here we present a theoretical study of its optical spectrum on the basis of state-of-the-art calculations including many-body effects. The latter are evaluated by solving the Bethe-Salpeter equation within the $GW$ approximation for the self-energy. Excitonic effects are strong at the onset and at higher energies up to 10 eV. Agreement between theory and experiment is excellent at the onset, but only qualitative at higher energies, where excitonic effects are stronger in theory than in experiment. The origin of structures in the spectrum, as well as the remaining discrepancy between theory and experiment, are discussed. We also present benchmark results for calculations using more approximate ways to determine the spectra, including simplified time-dependent density-functional theory.

Recently, a number of experiments indicate the possible presence of spin-liquid phases in quantum magnets with spin-$\frac{1}{2}$ and spin-1 moments sitting on triangular-lattice-based structures in Ba${}_3$CuSb${}_2$O${}_9$ and Ba${}_3$NiSb${}_2$O${}_9$, respectively. In relation to these experiments, several theoretical proposals have been made for spin-liquid phases and spin-liquid-like behaviors on the stacked triangular lattice. While the crystal structures of these materials are currently under debate, it is nonetheless interesting to understand possible spin-liquid phases on such frustrated lattices. In this work, we apply Schwinger boson mean-field theory and projective symmetry group analysis to investigate spin-liquid phases on the fully three-dimensional 6$H$-B structure, in contrast to previous works that considered two-dimensional systems. We find that a nematic $Z_2$ spin-liquid phase, where the lattice-rotational symmetry is spontaneously broken, is the most promising spin-liquid phase that is consistent with spiral magnetic ordering in the classical limit. We discuss the implications of our results to future theoretical and experimental works.

We develop a microscopic theory of electron spin dephasing in (110)-grown quantum wells where the electron scattering time is comparable to or exceeds the period of spin precession in the effective magnetic field caused by spin-orbit coupling. Structures with a homogeneous and fluctuating Rashba field, which triggers the dephasing of electron spins aligned along the growth direction, are analyzed. We show that the Dresselhaus field, which is always present in zinc-blende-type quantum wells, suppresses the spin dephasing enabling a very long spin lifetime of conduction electrons. The dependence of the spin lifetime on the electron mobility is found to be nonmonotonic reaching the minimum in structures where the scattering time is comparable to the period of spin precession in the effective magnetic field.

We report determination of structural parameters on angstrom length scale of nanostructured periodic multilayers using x-ray standing wave (XSW) enhanced elastic and Compton scattering. We show that the elastic scattered x-ray intensities emitted from the thin periodic multilayer structures, under strong XSW condition, are largely sensitive to the structural parameters of high-$z$ layers, whereas the Compton scattered intensities are sensitive to the parameters of low-$z$ layers. The utility of the methodology is demonstrated by analyzing two repetitive W/B${}_4$C multilayer structures with different surface-interface properties. The results are compared with those obtained using x-ray reflectivity and conventional x-ray standing wave fluorescence techniques. It is further shown that the Bragg angle can be derived with high accuracy (∼0.002${}^{\circ }$) from the ratio of the scattering intensities, which in turn improves the accuracy of the derived multilayer periodicity. The method presents an opportunity to probe structures on angstrom length scale of any periodic multilayer structure comprising of low atomic number layers. Unlike the conventional XSW fluorescence measurements, the present method has an advantage that it permits the determination of structural parameters of both the high- and the low-$z$ layers independently.

We study the electronic structure of chiral and achiral graphene nanoribbons with symmetric edges, including curvature and spin-orbit effects. Curved ribbons show spin-split bands, whereas flat ribbons present spin-degenerate bands. We show that this effect is due to the breaking of spatial inversion symmetry in curved graphene nanoribbons, while flat ribbons with symmetric edges possess an inversion center, regardless of their having chiral or achiral edges. We find an enhanced edge-edge coupling and a substantial gap in narrow chiral nanoribbons, which is not present in zigzag ribbons of similar width. We attribute these size effects to the mixing of the sublattices imposed by the edge geometry, yielding a behavior of chiral ribbons that is distinct from those with pure zigzag edges.

We report the electronic structures of Ta-doped anatase TiO${}_2$ thin films grown by pulsed laser deposition (PLD) with varying magnetization using a combination of first-principles calculations and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The roles of Ta doping and Ti vacancies are clarified, and the observed room-temperature ferromagnetism is attributed to the localized magnetic moments at Ti vacancy sites ferromagnetically ordered by electron charge carriers. O $K$-edge spectra exhibit significant polarization dependence which is discussed and supported by first-principles calculations in relation to both the crystal symmetry and the formation of defects. In particular, anomalous spectral-weight transfer across the entire O $K$ edge for the ferromagnetic thin film is associated exclusively with the occurrence of Ti vacancies and strong correlation effects, which result in the enhancement of the direct interaction between oxygen sites and of the anisotropy of the $e_g$-$p_{\sigma }$ hybridizations in the out-of-plane component. Our results show that O $K$-edge NEXAFS spectra can provide reliable experimental probes capable of revealing cationic defects that are intimately related to the ferromagnetism in transition metal oxides.

Large linear magnetoresistance (MR) in electron-injected $p$-type silicon at very low magnetic field is observed experimentally at room temperature. The large linear MR is induced in electron-dominated space-charge transport regime, where the magnetic field modulation of electron-to-hole density ratio controls the MR, as indicated by the magnetic field dependence of Hall coefficient in the silicon device. Contrary to the space-charge-induced MR effect in unipolar silicon device, where the large linear MR is inhomogeneity-induced, our results provide a different insight into the mechanism of large linear MR in nonmagnetic semiconductors that is not based on the inhomogeneity model. This approach enables homogeneous semiconductors to exhibit large linear MR at low magnetic fields that until now has only been appearing in semiconductors with strong inhomogeneities.

Using ground-state density functional theory we study the transition from indirect to direct band gap in hydrogen-terminated silicon nanocrystals (NCs) as a function of decreasing diameter. The studied range, from 1.0 to 4.6 nm diameter of nanocrystals, with spherical and Wulff-shape NCs, covers the transition from nano- to bulk regime. A change in the symmetry of the lowest unoccupied state as a function of decreasing NC diameter is observed, gradually increasing the oscillator strength of transitions from the highest occupied to the lowest unoccupied state. Real space and Fourier space characteristics of highest occupied and lowest unoccupied states are explored in detail and linked to a smooth transition from nano- to bulk regime.

Surface energies are important for predicting the shapes of nanocrystals and describing the faceting and roughening of surfaces. Copper surfaces are of particular interest in recent years since they are the preferred surfaces for growing graphene using chemical vapor deposition. In this study we calculate the surface energies of copper for the three low-index facets (111), (100), and (110) and one high-index facet, (210), using density-functional theory with both the local-density approximation and various parametrizations of the generalized-gradient approximation to the exchange-correlation functional. To assess the accuracy of the different functionals, we obtain the average surface energies of an isotropic crystal using a broken-bond model. We use this method, which can be generalized to other crystal structures, to compare calculated surface energies to experimental surface energies for fcc crystals. We find that the recent exchange-correlation functionals AM05 and PBEsol are the most accurate functionals for calculating the surface energies of copper. To determine how solvents affect the surface energies of copper, we perform calculations using a continuum solvation model. We find that aqueous solvation changes the overall magnitude of the surface energies only slightly but leads to more isotropic surface energies.

We identify an angle-dependent momentum scale as the fundamental property of a bilayer composed of mutually rotated graphene layers. The interlayer scattering processes at these characteristic momentum values define an effective Brillouin zone (a Jones zone) which, in general, differs from the Brillouin zone generated by the real-space lattice, which is physically irrelevant. From this we develop a numerical method that increases, for the twist bilayer, the efficiency of the standard tight-binding method by a factor of $\approx {10}^3$ at no loss of accuracy. The efficiency of the method is based on (i) the fact that the twist Hamiltonian is exceptionally sparse in a basis of single-layer graphene (SLG) states, (ii) a solution of trivial Diophantine problem (Bézout's identity) allows one to know in advance which matrix elements take nonzero values, and (iii) to access the electronic structure in a few electron volts about the Dirac point a truncated SLG basis consisting only of states in a somewhat larger energy window are required, leading to a much reduced size of the Hamiltonian. This allows a complete survey of the system which reveals (i) an angle-dependent series of van Hove singularities, (ii) an increasing mixing of SLG states as the twist angle is reduced leading to the appearance of localization of the twist bilayer wave functions at all energies in the small-angle limit, and (iii) a zero-energy peak in the density of states in an approximately self-similar small-angle regime.

We address local inelastic scattering from the vibrational impurity adsorbed onto graphene and the evolution of the local density of electron states near the impurity from a weak to strong coupling regime. For weak coupling the local electronic structure is distorted by inelastic scattering developing peaks or dips and steps. These features should be detectable in the inelastic electron tunneling spectroscopy $d^{2}I/d{V}^2$ using local probing techniques. Inelastic Friedel oscillations distort the spectral density at energies close to the inelastic mode. In the strong coupling limit, a local negative $U$ center forms in the atoms surrounding the impurity site. For those atoms, the Dirac cone structure is fully destroyed, that is, the linear energy dispersion as well as the V-shaped local density of electron states is completely destroyed. We further consider the effects of the negative $U$ formation and its evolution from weak to strong coupling. The negative $U$ site effectively acts as a local impurity such that sharp resonances appear in the local electronic structure. The main resonances are caused by elastic scattering off the impurity site, and the features are dressed by the presence of vibrationally activated side resonances. Going from weak to strong coupling, changes the local electronic structure from being Dirac-cone-like including midgap states, to a fully destroyed Dirac cone with only the impurity resonances remaining.

The magnetoelastic excitations of spin frustrated ZnCr${}_2$O${}_4$ are studied by the magnetic field dependence of the thermal conductivity $k$ down to 50 mK. Above the first-order magnetostructural transition at $T_{N,S}$ ≈ 12.5 K, spin fluctuations are strongly coupled to acoustic phonons, leading to a glasslike dependence of $k$($T$), up to ${\Theta }_{\mathrm{CW}}$. In the symmetry broken phase below $T_{N,S}$, $k$ shows a dominant magnetic contribution even at the lowest temperatures probed in this work. Application of a magnetic field above 2.5 T destabilizes the spin-bond structure, leading to a sudden increase and a nonconventional temperature dependence of the thermal conductivity. The possibility of the coexistence of gapped and gapless excitations in this magnetic phase is discussed.

We have investigated the magneto-optical response of Fe and CrO${}_2$ epitaxial films by pump-probe polarimetry, showing that charge and spin dynamics can be unambiguously disentangled. The time-resolved Kerr ellipticity and rotation in the metallic sample are essentially identical after the initial transient (shorter than a picosecond), but they considerably differ in the oxide film, even tens of picoseconds past the optical excitation. These differences are determined by the combined effects of photoexcited charge carriers and spins on the Kerr signal, but a detailed polarimetric analysis can explicitly unravel these contributions. In addition, the diagonal and off-diagonal terms of the dielectric tensor can be retrieved, providing the complete dynamical characterization of magnetic and optical properties in a ferromagnet, which is of utmost importance to understand spin evolution in magnetically correlated complex oxide materials.

The ordering of the three-dimensional isotropic $\mathit{XY}$ spin glass with nearest-neighbor random Gaussian coupling is studied by extensive Monte Carlo simulations. To investigate the ordering of the spin and the chirality, we compute several independent physical quantities including the glass order parameter, the Binder parameter, the correlation-length ratio, the overlap distribution, and the non-self-averageness parameter, etc., for both the spin-glass (SG) and the chiral-glass (CG) degrees of freedom. Evidence of spin-chirality decoupling, i.e., that the CG and SG order occur at two separate temperatures, $0<T_{\mathrm{SG}}<T_{\mathrm{CG}}$, is obtained from the glass order parameter, and is fully corroborated by the Binder parameter. By contrast, the CG correlation-length ratio yields a rather pathological and inconsistent result in the range of sizes we studied, which may originate from the finite-size effect associated with a significant short-length dropoff of the spatial CG correlations. Finite-size-scaling analysis yields the CG exponents ${\nu }_{\mathrm{CG}}=1.{36}_{-0.37}^{+0.15}$ and ${\eta }_{\mathrm{CG}}=0.{26}_{-0.26}^{+0.29}$, and the SG exponents ${\nu }_{\mathrm{SG}}=1.{22}_{-0.06}^{+0.26}$ and ${\eta }_{\mathrm{SG}}=-0.{54}_{-0.52}^{+0.24}$. The exponents obtained are close to those of the Heisenberg SG, but are very different from those of the Ising SG. The chiral overlap distribution and the chiral Binder parameter exhibit the feature of a continuous one-step replica-symmetry breaking (1RSB), consistently with previous reports. Such a 1RSB feature is again like that of the Heisenberg SG, but is different from the Ising SG, which may be the cause of the difference in the CG critical properties from those of the Ising SG despite the common $Z_2$ symmetry.

We performed angle-resolved photoemission and optical studies on electron-doped high-temperature superconductors (HTSCs), and compared the results with various theoretical models. Based on the fit to the experimental data, we conclude that itinerant electrons in the nonmagnetic phase predominantly couple to the phase fluctuations of the remnant antiferromagnetic (AF) order, rather than to the spin excitations derived from particle-hole pairs. Our observation naturally accounts for the pseudogap phenomenon and other experimental facts in electron-doped HTSCs in terms of the size of the remnant moment and the AF-correlation length. We propose a microscopic model based on the phase fluctuation scenario which leads to a $d$-wave pairing gap.