We investigate the spectroscopy and photoinduced electron dynamics within the conduction band of reduced rutile $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$ surface by multiphoton photoemission $\left(\mathrm{mPP}\right)$ spectroscopy with wavelength tunable ultrafast $(\sim 20\mathrm{fs})$ laser pulse excitation. Tuning the $\mathrm{mPP}$ photon excitation energy between 2.9 and 4.6 eV reveals a nearly degenerate pair of new unoccupied states located at $2.73\pm 0.05$ and $2.85\pm 0.05$ eV above the Fermi level, which can be analyzed through the polarization and sample azimuthal orientation dependence of the $\mathrm{mPP}$ spectra. Based on the calculated electronic structure and optical transition moments, as well as related spectroscopic evidence, we assign these resonances to transitions between $\mathrm{Ti}3d$ bands of nominally $t_{2g}$ and $e_g$ symmetry, which are split by crystal field. The initial states for the optical transition are the reduced $\mathrm{T}{\mathrm{i}}^{3+}$ states of $t_{2g}$ symmetry populated by formation oxygen vacancy defects, which exist within the band gap of $\mathrm{Ti}{\mathrm{O}}_2$. Furthermore, we studied the electron dynamics within the conduction band of $\mathrm{Ti}{\mathrm{O}}_2$ by three-dimensional time-resolved pump-probe interferometric $\mathrm{mPP}$ measurements. The spectroscopic and time-resolved studies reveal competition between 2PP and 3PP processes where the $t_{2g}\text{−}{e}_g$ transitions in the 2PP process saturate, and are overtaken by the 3PP process initiated by the band-gap excitation from the valence band of $\mathrm{Ti}{\mathrm{O}}_2$.

We study the biexciton formation process in a single CdTe/ZnTe quantum dot. Consistently with previous studies, we find subquadratic dependence of the biexciton photoluminescence intensity on the excitation power. We quantitatively explain this dependence in terms of the interplay between two alternative biexciton formation mechanisms, including the formation from a long-lived dark exciton residing in the quantum dot. This mechanism is demonstrated to be a dominant one for a wide range of excitation intensities. Our study is complemented by determination of a characteristic biexciton formation time, which is shown to be governed by the spin relaxation dynamics of an excited electron pair.

The hydrogenation of ${\mathrm{MgB}}_2$ is a critical step in the reversibility of several well-known hydrogen storage reactions. Of the many processes that must occur during rehydrogenation, at least two of them take place near the surface: the dissociation of ${\mathrm{H}}_2$ molecules and the subsequent diffusion of atomic hydrogen. Using first-principles calculations, we determine the energetic barriers for these processes on the ideal Mg-terminated ${\mathrm{MgB}}_2$ (0001) surface, as well as on surfaces containing transition metal dopants (Sc-Zn, Y-Cd, Pt, and Au). The calculated dissociation barrier for ${\mathrm{H}}_2$ on the clean surface is 0.89 eV, and the surface diffusion barrier is 0.17 eV. However, we find examples of dopants that significantly decrease the activation barrier for the dissociation of ${\mathrm{H}}_2$. Our calculations suggest that Ni, Cu, and Pd are good catalytic candidates for the surface processes involved in ${\mathrm{MgB}}_2$ rehydrogenation.

Simulation evidence of an incipient triple point is reported for xenon submonolayers adsorbed on the (111) surface of platinum. This is in stark contrast to the “normal” triple point found in simulations and experiments for xenon on the basal plane surface of graphite. The motions of the atoms in the surface plane are treated with standard 2D “NVE” molecular dynamics simulations using modern interactions. The simulation evidence strongly suggests an incipient triple point in the $120–150$ K range for adsorption on the Pt (111) surface while the adsorption on graphite shows a normal triple point at about 100 K.

The feasibility of realizing the overscreened Kondo effect in ultracold Fermi gas of atoms with spin $s\ge \frac{3}{2}$ in the presence of a localized magnetic impurity atom is proved realistic. Specifying (as a mere example) a system of ultracold ${}^{22}\mathrm{Na}$ Fermi gas and a trapped ${}^6\mathrm{Li}$ impurity, the mechanism of exchange interaction between the Na and Li atoms is elucidated and the exchange constant is found to be positive (antiferromagnetic). The corresponding exchange Hamiltonian is derived, and the Kondo temperature is estimated at the order of 500 nK. Within a weak-coupling renormalization group scheme, it is shown that the coupling renormalizes to the non-Fermi-liquid fixed point. An observable displaying multichannel features even in the weak-coupling regime is the impurity magnetization that is negative for $T\gg T_K$ and becomes positive with decreasing temperature.

A scheme is presented that is based on the alloy analogy model and allows one to account for thermal lattice vibrations as well as spin fluctuations when calculating response quantities in solids. Various models to deal with spin fluctuations are discussed concerning their impact on the resulting temperature-dependent magnetic moment, longitudinal conductivity, and Gilbert damping parameter. It is demonstrated that, by using the Monte Carlo (MC) spin configuration as input, the alloy analogy model is capable of reproducing the results of MC simulations on the average magnetic moment within all spin fluctuation models under discussion. On the other hand, the response quantities are much more sensitive to the spin fluctuation model. Separate calculations accounting for the thermal effect due to either lattice vibrations or spin fluctuations show that they give comparable contributions to the electrical conductivity and Gilbert damping. However, comparison to results accounting for both thermal effects demonstrates violation of Matthiessen's rule, showing the nonadditive effect of lattice vibrations and spin fluctuations. The results obtained for bcc Fe and fcc Ni are compared with the experimental data, showing rather good agreement for the temperature-dependent electrical conductivity and the Gilbert damping parameter.

We report an observation of a pressure-induced breakdown of the three-dimensional Dirac semimetal (3D-DSM) state in $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$, evidenced by a series of in situ high-pressure synchrotron x-ray diffraction and single-crystal transport measurements. We find that $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ undergoes a structural phase transition from a metallic tetragonal phase in space group $I{4}_1/acd$ to a semiconducting monoclinic phase in space group $P{2}_1/c$ at critical pressure 2.57 GPa; above this pressure, an activation energy gap appears, accompanied by distinct switches in Hall resistivity slope and electron mobility. These changes of crystal symmetry and corresponding transport properties manifest the breakdown of the 3DDSM state in pressurized $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$.

High-energy optical phonons are preferred in phonon-absorbing transitions, and regarding their production we analyze the phonon upconversion processes under nonequilibrium created by heterojunction transmission. For heterojunctions, steady phonon flux from a low-cutoff-frequency layer (e.g., Ge) is transmitted to a high cutoff layer (e.g., Si), creating a nonequilibrium population of low-energy phonons for upconversion. Using quantum spectral phonon transmission and first-principles calculations of the phonon interaction kinetics, we identify the high-conversion efficiency channels, i.e., modes and wave vectors. Junction-transmitted phonons, despite suffering from the interface reflection and from spreading interactions with equilibrium native phonons, have a high upconversion rate to Brillouin zone-boundary optical phonons, while nonequilibrium native phonons are efficiently upconverted over most of the zone. So, depending on the harvested optical phonon, one of these nonequilibrium phonons can be selected for an efficient upconversion rate.

We calculate the long-wavelength static screening properties of both neutral and doped graphene in the framework of density-functional theory. We use a plane-wave approach with periodic images in the third dimension and truncate the Coulomb interactions to eliminate spurious interlayer screening. We carefully address the issue of extracting two-dimensional dielectric properties from simulated three-dimensional potentials. We compare this method with analytical expressions derived for two-dimensional massless Dirac fermions in the random phase approximation. We evaluate the contributions of the deviation from conical bands, exchange correlation, and local fields. For momenta smaller than twice the Fermi wave vector, the static screening of graphene within the density-functional perturbative approach agrees with the results for conical bands within the random phase approximation and neglecting local fields. For larger momenta, we find that the analytical model underestimates the static dielectric function by $\approx 10\%$, mainly due to the conical band approximation.

We present results from density functional theory study of the electronic properties of silicon nanocrystals (Si NCs) embedded in a silicon carbide (SiC) matrix, considering different combinations of various NCs and host matrix sizes. We show that the NC and the host matrix form a type-II band alignment, with the states at the top of the valence band being in the Si NC and the states at the bottom of the conduction band in the host matrix. Moreover, this band alignment can be interchanged with introducing oxygen at the interface. This interchange in the band alignment has a rather weak influence on the absorption of the system. We demonstrate that the charge densities of some valence band states can overlap with the charge densities of the neighboring NCs. We also demonstrate that this leakage of states is significant when the distance between the neighboring NCs is less than $\sim 1.6\mathrm{nm}$.

Based on first-principles calculations, we find novel valley-polarized quantum anomalous Hall (VP-QAH) phases with a large gap—0.19 eV at an appropriate buckled angle and tunable topological phase transitions driven by the spontaneous magnetization within a half-hydrogenated Bi honeycomb monolayer. Depending on the magnetization orientation, four different phases can emerge, i.e., two VP-QAH phases, ferromagnetic insulating, and metallic states. When the magnetization is reversed from the $+\mathbf{z}$ to the $-\mathbf{z}$ directions, accompanied with a sign change in the Chern number (from $-1$ to +1), the chiral edge state is moved from valley $K$ to $K^{\prime }$. Our findings provide a platform for designing dissipationless electronics and valleytronics in a more robust manner through the tuning of the magnetization orientation.

We investigate the heat flow between different terminals in an interacting coherent conductor when inelastic scattering is present. We illustrate our theory with a two-terminal quantum dot setup. Two types of heat asymmetries are investigated: electric asymmetry ${\Delta }_E$, which describes deviations of the heat current in a given contact when voltages are exchanged, and contact asymmetry ${\Delta }_C$, which quantifies the difference between the power measured in two distinct electrodes. In the linear regime, both asymmetries agree and are proportional to the Seebeck coefficient, the latter following at low temperature a Mott-type formula with a dot transmission renormalized by inelasticity. Interestingly, in the nonlinear regime of transport we find ${\Delta }_E\ne {\Delta }_C$ and this asymmetry departure depends on the applied bias configuration. Our results may be important for the recent experiments by Lee et al. [Nature (London) 498, 209 (2013)], where these asymmetries were measured.

We employ first-principles calculations to study the atomic and electronic structure of various point defects such as vacancies, interstitials, and antisites in the stoichiometric as well as slightly off-stoichiometric ${\mathrm{Ti}}_{1-c}{\mathrm{C}}_c$ (including both C-poor and C-rich compositions, $0.49\le c\le 0.51$). The atomic structure analysis has revealed that both interstitial and antisite defects can exist in split conformations involving dumbbells. To characterize the electronic structure changes caused by a defect, we introduce differential density of states (dDOS) defined as a local perturbation of the density of states (DOS) on the defect site and its surrounding relative to the perfect TiC. This definition allows us to identify the DOS peaks characteristic of the studied defects in several conformations. So far, characteristic defect states have been discussed only in connection with carbon vacancies. Here, in particular, we have identified dDOS peaks of carbon interstitials and dumbbells, which can be used for experimental detection of such defects in TiC. The formation energies of point defects in TiC are derived in the framework of a grand-canonical formalism. Among the considered defects, carbon vacancies and interstitials are shown to have, respectively, the lowest and the second-lowest formation energies. Their formation energetics are consistent with the thermodynamic data on the phase stability of nonstoichiometric TiC. A cluster type of point defect is found to be next in energy, a titanium [100] dumbbell terminated by two carbon vacancies.

The real-time evolution of two pieces of quantum insulators, initially at different temperatures, is studied when they are glued together. Specifically, each subsystem is taken as a Bose-Hubbard model in a Mott insulator state. The process of temperature equilibration via heat transfer is simulated in real time using the minimally entangled typical thermal states algorithm. The analytic theory based on quasiparticle transport is also given.

We have investigated the noncentrosymmetric tetragonal heavy fermion compound $\mathrm{CeAuA}{\mathrm{l}}_3$ using muon spin rotation $\left(\mu \mathrm{SR}\right)$, neutron diffraction (ND), and inelastic neutron scattering (INS) measurements. We have also revisited the magnetic, transport, and thermal properties. The magnetic susceptibility reveals an antiferromagnetic transition at 1.1 K with, possibly, another magnetic transition near 0.18 K. The heat capacity shows a sharp $\lambda$-type anomaly at 1.1 K in zero field, which broadens and moves to a higher temperature in an applied magnetic field. Our zero-field $\mu \mathrm{SR}$ and ND measurements confirm the existence of a long-range magnetic ground state below 1.2 K. Further, the ND study reveals an incommensurate magnetic order with a magnetic propagation vector $\mathbf{k}=(0,0,0.52\left(1\right))$ and a spiral structure of Ce moments coupled ferromagnetically within the $ab$ plane. Our INS study reveals the presence of two well-defined crystal electric field (CEF) excitations at 5.1 and 24.6 meV in the paramagnetic phase of $\mathrm{CeAuA}{\mathrm{l}}_3$ that can be explained on the basis of the CEF theory and the Kramer's theorem for a Ce ion having a $4f^1$ electronic state. Furthermore, low energy quasielastic excitations show a Gaussian line shape below 30 K compared to a Lorentzian line shape above 30 K, indicating a slowdown of spin fluctuations below 30 K. We have estimated a Kondo temperature of $T_{\mathrm{K}}=3.5\mathrm{K}$ from the quasielastic linewidth, which is in good agreement with that estimated from the heat capacity. This study also indicates the absence of any CEF-phonon coupling unlike that observed in isostructural $\mathrm{CeCuA}{\mathrm{l}}_3$ The CEF parameters, energy level scheme, and their wave functions obtained from the analysis of INS data explain satisfactorily the single crystal susceptibility in the presence of two-ion anisotropic exchange interaction in $\mathrm{CeAuA}{\mathrm{l}}_3$.

We report strong coupling between an ensemble of N impurity $\left(P1\right)$ centers in diamond and microwave photons using a unique double-post reentrant cavity. The cavity is designed so that the magnetic component of the cavity field is spatially separated from the electric component and focused into the small volume in which the diamond sample is mounted. The novelty of the structure simultaneously allows the high magnetic filling factor (38.4%) and low frequencies necessary to interact, at low magnetic field, with transitions in diamond such as those in negatively charged nitrogen-vacancy and $P1$ centers. Coupling strength (or normal-mode splitting) of 51.42 MHz was achieved with $P1$ centers at 6.18 GHz and 220 mT in a centimeter-scale cavity, with a corresponding cooperativity factor of 4.7. This technique offers an alternative way, with some significant advantages, to couple 3D cavities to transitions in diamond and achieve the strong coupling necessary for applications to quantum information processing.

We report a combined experimental study of magnetic properties of a single crystal of the frustrated diamond lattice antiferromagnet ${\text{CoAl}}_2{\text{O}}_4$ with ${\text{Co}}^{2+}$ electron spin resonance, ${}^{27}\mathrm{Al}$ nuclear magnetic resonance, and muon spin rotation/relaxation techniques. With our local probes, we show that the frustration of spin interactions and the Co/Al site disorder strongly affect the spin dynamics. The experimental results evidence inhomogeneous and slow magnetic fluctuations and the occurrence of short-range electron spin correlations far above a characteristic temperature $T^{*}=8$ K at which the spin system turns into in a quasistatic state. Our data indicate that this spin order is likely short range and unconventional with spin fluctuations persistent even at $T\ll T^{*}$. The results of three spectroscopy techniques highlight a nontrivial role of structural disorder for the magnetism of a frustrated diamond spin lattice at the proximity to the critical point.

We report magnetic and thermodynamic properties of single crystal $\alpha -{\text{RuCl}}_3$, in which the ${\text{Ru}}^{3+}\left(4d^5\right)$ ion is in its low spin state and forms a honeycomb lattice. Two features are observed in both magnetic susceptibility and specific heat data; a sharp peak at 7 K and a broad hump near 10–15 K. In addition, we observe a metamagnetic transition between 5 and 10 T. Our neutron diffraction study of single crystal samples confirms that the low temperature peak in the specific heat is associated with a magnetic order with unit cell doubling along the honeycomb (100) direction, which is consistent with zigzag order, one of the types of magnetic order predicted within the framework of the Kitaev-Heisenberg model.

Comprehensive investigations of superconductivity in the charge-doped high-temperature superconducting cuprates often involve examining the evolution of physical properties within a series of samples having controlled variations of some dopant element—most frequently oxygen. Many important observations have been extracted from such experiments, however, the associated measurements are by nature discrete snapshots of the evolving material. We demonstrate here a novel approach to sample preparation of the $\text{high-}{\mathit{\text{T}}}_c$ cuprate ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_x$. By post-annealing a uniformly overdoped ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_x(x\approx 7.0)$ film in a low pressure ${\text{O}}_2$ atmosphere with a thermal gradient across the film length, we have successfully grown a charge-gradient ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{\nabla x}$ sample, i.e., a film having a varying oxygen doping level along the length of the substrate. Surprisingly, we observe three distinct regimes of oxygen distribution across the sample, as well as behavior pointing to a full alignment within the a-b plane.