The electronic properties of single-layer antimony are studied by a combination of first-principles and tight-binding methods. The band structure obtained from relativistic density functional theory is used to derive an analytic tight-binding model that offers an efficient and accurate description of single-particle electronic states in a wide spectral region up to the mid-UV. The strong ($\lambda =0.34$ eV) intra-atomic spin-orbit interaction plays a fundamental role in the band structure, leading to splitting of the valence band edge and to a significant reduction of the effective mass of the hole carriers. To obtain an effective many-body model of two-dimensional Sb we calculate the screened Coulomb interaction and provide numerical values for the on-site ${\overline{V}}_{00}$ (Hubbard) and intersite ${\overline{V}}_{ij}$ interactions. We find that the screening effects originate predominantly from the $5p$ states, and are thus fully captured within the proposed tight-binding model. The leading kinetic and Coulomb energies are shown to be comparable in magnitude, $|t_{01}|/({\overline{V}}_{00}-{\overline{V}}_{01})\sim 1.6$, which suggests a strongly correlated character of $5p$ electrons in Sb. The results presented here provide an essential step toward the understanding and rational description of a variety of electronic properties of this two-dimensional material.

We present a computational method to quantitatively describe the linear-response conductance of nanoscale devices in the Kondo regime. This method relies on a projection scheme to extract an Anderson impurity model from the results of density functional theory and nonequilibrium Green's functions calculations. The Anderson impurity model is then solved by continuous-time quantum Monte Carlo. The developed formalism allows us to separate the different contributions to the transport, including coherent or noncoherent transport channels, and also the quantum interference between impurity and background transmission. We apply the method to a scanning tunneling microscope setup for the 1,3,5-triphenyl-6-oxoverdazyl (TOV) stable radical molecule adsorbed on gold. The TOV molecule has one unpaired electron, which when brought in contact with metal electrodes behaves like a prototypical single Anderson impurity. We evaluate the Kondo temperature, the finite-temperature spectral function, and transport properties, finding good agreement with published experimental results.

Motivated by the experiments on the rare-earth double perovskites, we propose a generalized Kitaev-Heisenberg model to describe the generic interaction between the spin-orbit-entangled Kramers doublets of the rare-earth moments. We carry out a systematic analysis of the mean-field phase diagram of this new model. In the phase diagram, there exist large regions with a continuous $U\left(1\right)$ or $O\left(3\right)$ degeneracy. Since no symmetry of the model protects such a continuous degeneracy, we predict that the quantum fluctuation lifts the continuous degeneracy and favors various magnetic orders in the phase diagram. From this order by quantum disorder mechanism, we further predict that the magnetic excitations of the resulting ordered phases are characterized by nearly gapless pseudo-Goldstone modes. We find that there exist Weyl magnon excitations for certain magnetic orders. We expect our prediction to inspire further study of Kitaev physics, the order by quantum disorder phenomenon, and topological spin-wave modes in the rare-earth magnets and the systems alike.

We have conducted hard x-ray photoelectron spectroscopy investigations of the electronic structure changes and electron correlation phenomena which take place upon the photoinduced reversible phase transition between β- and $\lambda \text{−}T{\mathrm{i}}_3\mathrm{O}$. From valence band spectra of β- and $\lambda \text{−}T{\mathrm{i}}_3{\mathrm{O}}_5$, we have identified the bipolaron caused by the σ-type bonding of $d_{xy}$ orbitals in $\beta \text{−}T{\mathrm{i}}_3{\mathrm{O}}_5$ and the π stacking between the $d_{xy}$ orbitals between different Ti sites in $\lambda \text{−}T{\mathrm{i}}_3{\mathrm{O}}_5$, previously predicted by ab initio calculations. This indicates that the single electron band picture is valid for the description of photoinduced phase transitions. On the other hand, the Ti $2p$ and Ti $1s$ core level spectra exhibit nonlocal screening satellite features, which are typical spectroscopic signs of strong electron correlation in the coherent $\mathrm{Ti}{t}_{2g}$ states. The most striking result we obtain is that correlation in the valence band also manifests to reduce the plasmon energy, which results in an enhancement of the valence electron mass by a factor of 2.7.

The convergence of a cluster expansion for lithium transition-metal (TM) oxides is improved by explicit treatment of TM magnetic moments. The approach is applied to layered $\mathrm{LiC}{\mathrm{o}}_y\mathrm{N}{\mathrm{i}}_{1-y}{\mathrm{O}}_2$ (NC). The ground state and low-lying excited state structures are identified, and the distribution of TM ions and magnetic moment in those structures is investigated to explain the origin of Ni-antisite ions and Jahn-Teller distortion. The developed model also reveals the mechanisms governing the atomic arrangement of NC, including in-plane Co-Co vs Co-Ni competition, magnetic frustration vs disproportionation competition, and cationic interactions spanning adjacent layers.

Heat transfer across metal-semiconductor interfaces involves multiple fundamental transport mechanisms such as elastic and inelastic phonon scattering, and electron-phonon coupling within the metal and across the interface. The relative contributions of these different transport mechanisms to the interface conductance remains unclear in the current literature. In this work, we use a combination of first-principles calculations under the density functional theory framework and heat transport simulations using the atomistic Green's function (AGF) method to quantitatively predict the contribution of the different scattering mechanisms to the thermal interface conductance of epitaxial ${\mathrm{CoSi}}_2$-Si interfaces. An important development in the present work is the direct computation of interfacial bonding from density functional perturbation theory (DFPT) and hence the avoidance of commonly used “mixing rules” to obtain the cross-interface force constants from bulk material force constants. Another important algorithmic development is the integration of the recursive Green's function (RGF) method with Büttiker probe scattering that enables computationally efficient simulations of inelastic phonon scattering and its contribution to the thermal interface conductance. First-principles calculations of electron-phonon coupling reveal that cross-interface energy transfer between metal electrons and atomic vibrations in the semiconductor is mediated by delocalized acoustic phonon modes that extend on both sides of the interface, and phonon modes that are localized inside the semiconductor region of the interface exhibit negligible coupling with electrons in the metal. We also provide a direct comparison between simulation predictions and experimental measurements of thermal interface conductance of epitaxial ${\mathrm{CoSi}}_2$-Si interfaces using the time-domain thermoreflectance technique. Importantly, the experimental results, performed across a wide temperature range, only agree well with predictions that include all transport processes: elastic and inelastic phonon scattering, electron-phonon coupling in the metal, and electron-phonon coupling across the interface.

Silicides are used extensively in nano- and microdevices due to their low electrical resistivity, low contact resistance to silicon, and their process compatibility. In this work, the thermal interface conductance of ${\mathrm{TiSi}}_2, {\mathrm{CoSi}}_2$, NiSi, and PtSi are studied using time-domain thermoreflectance. Exploiting the fact that most silicides formed on Si(111) substrates grow epitaxially, while most silicides on Si(100) do not, we study the effect of epitaxy, and show that for a wide variety of interfaces there is no dependence of interface conductance on the detailed structure of the interface. In particular, there is no difference in the thermal interface conductance between epitaxial and nonepitaxial silicide/silicon interfaces, nor between epitaxial interfaces with different interface orientations. While these silicide-based interfaces yield the highest reported interface conductances of any known interface with silicon, none of the interfaces studied are found to operate close to the phonon radiation limit, indicating that phonon transmission coefficients are nonunity in all cases and yet remain insensitive to interfacial structure. In the case of ${\mathrm{CoSi}}_2$, a comparison is made with detailed computational models using (1) full-dispersion diffuse mismatch modeling (DMM) including the effect of near-interfacial strain, and (2) an atomistic Green' function (AGF) approach that integrates near-interface changes in the interatomic force constants obtained through density functional perturbation theory. Above 100 K, the AGF approach significantly underpredicts interface conductance suggesting that energy transport does not occur purely by coherent transmission of phonons, even for epitaxial interfaces. The full-dispersion DMM closely predicts the experimentally observed interface conductances for ${\mathrm{CoSi}}_2$, NiSi, and ${\mathrm{TiSi}}_2$ interfaces, while it remains an open question whether inelastic scattering, cross-interfacial electron-phonon coupling, or other mechanisms could also account for the high-temperature behavior. The effect of degenerate semiconductor dopant concentration on metal-semiconductor thermal interface conductance was also investigated with the result that we have found no dependencies of the thermal interface conductances up to ($n$ or $p$ type) $\approx 1\times {10}^{19} {\mathrm{cm}}^{-3}$, indicating that there is no significant direct electronic transport and no transport effects that depend on long-range metal-semiconductor band alignment.

Dielectric and mechanical spectroscopies have been used to investigate ferroelectric transitions and twin wall dynamics in the lead-free ceramic $0.5\mathrm{Ba}\left(\mathrm{Z}{\mathrm{r}}_{0.2}\mathrm{T}{\mathrm{i}}_{0.8}\right){\mathrm{O}}_3-0.5\left(\mathrm{B}{\mathrm{a}}_{0.7}\mathrm{C}{\mathrm{a}}_{0.3}\right)\mathrm{Ti}{\mathrm{O}}_3$ (abbreviated as BZT-50BCT), which is known to have a high piezoelectric coefficient ($d_{33}>545\mathrm{pC}/\mathrm{N}$). Results from dynamical mechanical analysis in the frequency range 0.2–20 Hz and resonant ultrasound spectroscopy in the frequency range $\sim 0.1–1.2\mathrm{MHz}$ confirm the existence of three phase transitions with falling temperature, at $\sim 360\mathrm{K}$ (cubic-tetragonal), $\sim 304\mathrm{K}$ (tetragonal-orthorhombic), and $\sim 273\mathrm{K}$ (orthorhombic-rhombohedral). In comparison with $\mathrm{BaTi}{\mathrm{O}}_3$, however, the transitions are marked by rounded rather than sharp minima in the shear modulus. The pattern of acoustic loss is also quite different from that shown by $\mathrm{BaTi}{\mathrm{O}}_3$ in having a broad interval of high loss at low temperatures, consistent with a spectrum of relaxation times for interactions of ferroelastic twin walls. Differences in the dielectric properties also suggest more relaxor like characteristics for BZT-50BCT. It is proposed that the overall pattern of behavior is significantly influenced by strain heterogeneity at a local length scale in the perovskite structure due to the substitution of cations with different ionic radii. The existence of this strain heterogeneity and its influence on the elastic behavior near the transition points could be contributory factors to the development of adaptive nanoscale microstructures and enhanced piezoelectric properties.

Applying time-varying magnetic fields to Weyl metals, a pair of Weyl points becomes oscillating. This oscillating monopole and antimonopole pair gives rise to ac Berry magnetic fields, responsible for the emergence of Berry electric fields, which have not been discussed before at least in the context of Weyl metals. Introducing this information into Boltzmann transport theory, we find anomalous Hall effects beyond Berry magnetic fields as a fingerprint of Berry electric fields.

We investigate the high-pressure behavior of beryllium, magnesium, and calcium difluorides using ab initio random structure searching and density functional theory (DFT) calculations, over the pressure range $0–70$ GPa. Beryllium fluoride exhibits extensive polymorphism at low pressures, and we find two new phases for this compound—the silica moganite and ${\mathrm{CaCl}}_2$ structures—which are stable over the wide pressure range $12–57$ GPa. For magnesium fluoride, our searching results show that the orthorhombic “O-I” ${\mathrm{TiO}}_2$ structure $(Pbca,Z=8)$ is stable for this compound between 40 and 44 GPa. Our searches find no new phases at the static-lattice level for calcium difluoride between 0 and 70 GPa; however, a phase with $P\overline{6}2m$ symmetry is close to stability over this pressure range, and our calculations predict that this phase is stabilized at high temperature. The $P\overline{6}2m$ structure exhibits an unstable phonon mode at large volumes which may signal a transition to a superionic state at high temperatures. The group-II difluorides are isoelectronic to a number of other ${\mathrm{AB}}_2$-type compounds such as ${\mathrm{SiO}}_2$ and ${\mathrm{TiO}}_2$, and we discuss our results in light of these similarities.

We study the multipole interactions between $f^2$ ions with the ${\mathrm{\Gamma }}_3$ non-Kramers doublet ground state under a cubic crystalline electric field. We construct the ${\mathrm{\Gamma }}_3$ doublet state of the electrons with the total angular momentum $j=5/2$. By applying the second-order perturbation theory with respect to the intersite hopping, we derive the multipole interactions. We obtain a quadrupole interaction for a simple cubic lattice, an octupole interaction for a bcc lattice, and both quadrupole and octupole interactions for an fcc lattice. We also discuss general tendencies of the multipole interactions depending on the lattice structure by comparing the results with those for the ${\mathrm{\Gamma }}_8$ quartet systems of $f^1$ ions.

Using an efficient first-principles computational scheme for paramagnetic body-centered cubic (bcc) and face-centered cubic (fcc) Fe, we investigate the impact of thermal longitudinal spin fluctuations (LSFs) on the thermal lattice expansion. The equilibrium physical parameters are derived from the self-consistent Helmholtz free energy, in which the LSFs are considered within the adiabatic approximation and the anharmonic lattice vibration effect is included using the Debye-Grüneisen model taking into account the interplay between thermal, magnetic, and elastic degrees of freedom. Thermal LSFs are energetically more favorable in the fcc phase than in the bcc one giving a sizable contribution to the linear thermal expansion of $\gamma$-Fe. The present scheme leads to accurate temperature-dependent equilibrium Wigner-Seitz radius, bulk modulus, and Debye temperature within the stability fields of the two phases and demonstrates the importance of thermal spin fluctuations in paramagnetic Fe.

Transmittance spectra of $\mathrm{TbFe}{\mathrm{O}}_3$ single crystals have been studied in the far-infrared spectral range $15-120\mathrm{c}{\mathrm{m}}^{-1}$ in the external magnetic fields up to 9 T and at low temperatures down to 1.5 K. The temperature and magnetic-field dependencies of the antiferromagnetic resonance (AFMR) modes have been measured below and above the magnetic ordering temperature of Tb moments $T_N^{\mathrm{Tb}}=3.3\mathrm{K}$. Both the quasiferromagnetic and quasiantiferromagnetic modes of AFMR demonstrate hardening at $T<T_N^{\mathrm{Tb}}$. The quasiantiferromagnetic mode gains electric-dipole activity along the $c$ axis below $T_N^{\mathrm{Tb}}$ and thus behaves as a hybrid, i.e., both electric- and magnetic-dipole active, mode. In addition to AFMR modes, below $T_N^{\mathrm{Tb}}$ we observed the appearance of electromagnon excitation at about $27\mathrm{c}{\mathrm{m}}^{-1}$, which is electric-dipole active along the $c$ axis. The electromagnon is optically active only in a narrow temperature range, $2.7\mathrm{K}<T<T_N^{\mathrm{Tb}}$ at $H=0$, and in a narrow range of magnetic fields 2.4–2.7 T applied along the $b$ axis at $T=1.5\mathrm{K}$. We argue that this electromagnon appears in magnetic phases, which are compatible with a spontaneous electric polarization along the $b$ axis.

We study the effect of sputter-deposition conditions, namely, substrate temperature and chamber base pressure, upon the interface quality of epitaxial Pt/Co/Pt thin films with perpendicular magnetic anisotropy. Here we define interface quality to be the inverse of the sum in quadrature of roughness and intermixing. We find that samples with the top Co/Pt layers grown at $250{}^{\circ }\mathrm{C}$ exhibit a local maximum in roughness intermixing and that the interface quality is better for lower or higher deposition temperatures, up to $400{}^{\circ }\mathrm{C}$, above which the interface quality degrades. Imaging the expansion of magnetic domains in an in-plane field using wide-field Kerr microscopy, we determine the interfacial Dzyaloshinskii-Moriya interaction (DMI) in films in the deposition temperature range $100{}^{\circ }\mathrm{C}$ to $300{}^{\circ }\mathrm{C}$. We find that the net DMI increases as the difference between top and bottom Co interface quality increases. Furthermore, for sufficiently low base pressures, the net DMI increases linearly with the deposition temperature, indicating that fine-tuning of the DMI may be achieved via the deposition conditions.

Among hundreds of spinel oxides, $\mathrm{LiT}{\mathrm{i}}_2{\mathrm{O}}_4$ (LTO) is the only one that exhibits superconductivity ($T_{\mathrm{c}}\sim 13\mathrm{K}$). Although the general electron-phonon coupling is still the main mechanism for electron pairing in LTO, unconventional behaviors such as the anomalous magnetoresistance, anisotropic orbital/spin susceptibilities, etc. reveal that both the spin and the orbital interactions should also be considered for understanding the superconductivity. Here we investigate tunneling spectra of [111]-, [110]-, and [001]-oriented high quality LTO thin films. Several bosonic modes in tunneling spectra are observed in the [111]- and [110]-oriented films but not in [001]-oriented ones, and these modes still exist at $T=2T_{\mathrm{c}}$ and beyond the upper critical field, which are confirmed as stemming from electron-phonon interaction by DFT calculations. These modes only appear in special surface orientations, indicating that the electron-phonon coupling in LTO system is highly anisotropic and may be enhanced by an orbital-related state. The anisotropic electron-phonon coupling should be taken seriously in understanding the nature of LTO superconductivity.

A trimer resonating valence bond (tRVB) state consisting of an equal-weight superposition of trimer coverings on a square lattice is proposed. A model Hamiltonian of the Rokhsar-Kivelson type for which the tRVB becomes the exact ground state is written. The state is shown to have $9^g$ topological degeneracy on a genus $g$ surface and support ${\mathbb{Z}}_3$ vortex excitations. Correlation functions show exponential behavior with a very short correlation length consistent with the gapped spectrum. The classical problem of the degeneracy of trimer configurations is investigated by the transfer matrix method.

To understand emergent magnetic monopole dynamics in the spin ices ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ and ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, it is necessary to investigate the mechanisms by which spins flip in these materials. Presently there are thought to be two processes: quantum tunneling at low and intermediate temperatures and thermally activated at high temperatures. We identify possible couplings between crystal field and optical phonon excitations and construct a strictly constrained model of phonon-mediated spin flipping that quantitatively describes the high-temperature processes in both compounds, as measured by quasielastic neutron scattering. We support the model with direct experimental evidence of the coupling between crystal field states and optical phonons in ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$.

We report on the experimental verification of the rotational sense of homogeneous and antiferromagnetic spin spiral structures observed on mono- and double-layer Mn thin films formed on a W(110) substrate using spin-polarized scanning tunneling microscopy. By controlling the orientation of the tip magnetization with external magnetic fields, we determined the rotational sense of the cycloidal and transverse conical spin spiral structures of the Mn/W systems and found that both exhibit a unique chirality and the same polarity for the interfacial Dzyaloshinskii-Moriya interaction (iDMI). By comparing our results with those reported for other noncollinear magnetic states formed on the same substrate, we conclude that the polarity of the iDMI, which defines the rotational sense, is predominantly determined by the substrate rather than the overlayers.