Significant attention has been recently focused on the realization of high precision nanothermometry using the spin-resonance temperature shift of the negatively charged nitrogen-vacancy (${\mathrm{NV}}^{-}$) center in diamond. However, the precise physical origins of the temperature shift is yet to be understood. Here, the shifts of the center's optical and spin resonances are observed and a model is developed that identifies the origin of each shift to be a combination of thermal expansion and electron-phonon interactions. Our results provide insight into the center's vibronic properties and reveal implications for ${\mathrm{NV}}^{-}$ thermometry.

Pressure effects on superconducting properties of two ${\mathrm{YB}}_6$ samples $(T_c=5.9$ and 7.5 K) were investigated by measurements of electrical resistivity, magnetic susceptibility, and x-ray diffraction in the pressure range up to 320 kbar. Magnetoresistivity measurements down to 60 mK and up to 47 kbar have shown a negative pressure effect on $T_c$ as well as on the third critical field $H_{c3}$ with the slopes $d\ln T_c/dp=-0.59\%$/kbar and $d\ln H_{c3}/dp=-1.1\%$/kbar, respectively. The magnetic susceptibility measurements evidenced that the slope of $d\ln T_c/dp$ gradually decreases with pressure reaching a value three times smaller at 112 kbar. The lattice parameter measurements revealed the volume reduction of 14% at 320 kbar. The pressure-volume dependence is described by the Rose-Vinet equation of state. The obtained relative volume dependence $d\ln T_c/d\ln V$ analyzed by the McMillan formula for $T_c$ indicates that the reduction of the superconducting transition temperature is mainly due to hardening of the Einstein-like phonon mode responsible for the superconducting coupling. This is confirmed by the analysis of the resistivity measurements in the normal state up to $T=300$ K performed at pressures up to 28 kbar.

Graphene, a one-layer-thick hexagonal array of carbon atoms, when undoped, exhibits a curious mixture of properties pertinent to either metals or insulators. On the one hand, despite near absence of both charge carriers and impurities, it has a finite conductivity like a metal. On the other hand, the Coulomb interaction between electrons is unscreened like in a dielectric and hence is long range. The chemical potential is pinned right between the conical valence and conduction bands causing quasiparticles to move like massless relativistic particles. We demonstrate at small coupling that the electrodynamics of graphene exhibits nonlocality on a macroscopic level due to the combination of the long-range interactions and the linear dispersion relation. The frequency and wave vector $\mathbf{k}$-dependent conductivity tensor, in addition to a local pseudo-Ohmic part ${\sigma }_T{\delta }_{ij}$, possesses a nonlocal contribution ${\sigma }_{\mathrm{nl}}{k}_i{k}_j/k^2$. While the coefficient of the local part is ${\sigma }_T\approx e^2/4\hslash$, the coefficient of the nonlocal part is proportional to the Coulomb interaction strength $\alpha$, ${\sigma }_{\mathrm{nl}}={\sigma }_T\alpha$. This leads to several remarkable effects in transport and optical response. In particular, the resistance of the graphene flake depends on the location and the geometry of the source, drain, and probes. A voltage perpendicular to the current appears in a time-reversal symmetric situation and the polarization of reflected and transmitted light is modified, without either the magnetic field (like in the Faraday effect) or anisotropy.

Calculations of exact-exchange (EXX) and random phase approximation (RPA)-correlation energies within the formally exact adiabatic connection fluctuation-dissipation theorem formalism have recently been carried out for a number of isolated and condensed systems. Unfortunately, most of the applications have been done in a non-self-consistent procedure, and for several systems it has been found that RPA correlation energies may significantly depend on the choice of input single-particle wave functions. In this work, we develop an efficient approach to compute the EXX/RPA total energy self-consistently. We derive an expression for the RPA self-consistent potential based on the density functional perturbation theory and dielectric matrix approaches and implemented it within the plane-wave pseudopotential framework. The efficiency of this approach is greatly improved by exploiting an iterative procedure to compute the inverted Kohn-Sham density-density response function. We apply our implementation to study the binding energy curves and the structural properties of rare gasses such as Ar and Kr and alkaline-earth Be dimers. In addition, the EXX and RPA-correlation potentials of these systems at different dissociation distances are analyzed.

A new type of monovacancy (MV), MV-1, with a planelike $s{p}^3$ hybridization at its defect core has been found in two-dimensional (2D) low-buckled silicene by using the first-principles study, which has never been found to exist stably in the 2D single-layer nanostructures, including other previously studied graphene, $h$-BN sheets, and single-layer ${\mathrm{MoS}}_2$. In addition, other two possible monovacancies (MV-2 and MV-3) with higher energies have also been found to exist in 2D low-buckled silicene. More importantly, it is found that the new type MV-1 is the most stable ground structure among the three possible MVs. And the high-energy MV-3 is unstable, easily decaying into MV-1, but metastable MV-2 could coexist with MV-1 at low temperatures less than 10 K. The diffusion coefficient of MV-1 is calculated to be $2.3\times {10}^{-5}{\mathrm{cm}}^2/\mathrm{s}$, much higher than that of the MV in graphene. Finally, electronic structures of the defective silicene with MV-1 and MV-2 are calculated, showing both of them are metallic.

The properties of conductance in one-dimensional (1D) quantum wires are statistically investigated using an array of 256 lithographically identical split gates, fabricated on a GaAs/AlGaAs heterostructure. All the split gates are measured during a single cooldown under the same conditions. Electron many-body effects give rise to an anomalous feature in the conductance of a one-dimensional quantum wire, known as the “0.7 structure” (or “0.7 anomaly”). To handle the large data set, a method of automatically estimating the conductance value of the 0.7 structure is developed. Large differences are observed in the strength and value of the 0.7 structure [from $0.63$ to $0.84\times (2e^2/h)$], despite the constant temperature and identical device design. Variations in the 1D potential profile are quantified by estimating the curvature of the barrier in the direction of electron transport, following a saddle-point model. The 0.7 structure appears to be highly sensitive to the specific confining potential within individual devices.

The valley-dependent optical selection rules in recently discovered monolayer group-VI transition-metal dichalcogenides (TMDs) make possible optical control of valley polarization, a crucial step towards valleytronic applications. However, in the presence of Landau-level (LL) quantization such selection rules are taken over by selection rules between the LLs, which are not necessarily valley contrasting. Using ${\mathrm{MoS}}_2$ as an example we show that the spatial inversion-symmetry breaking results in unusual valley-dependent inter-LL selection rules, which is controlled by the sign of the magnetic field and directly locks polarization to valley. We find a systematic valley splitting for all LLs in the quantum Hall regime, whose magnitude is linearly proportional to the magnetic field and is comparable with the LL spacing. Consequently, unique plateau structures are found in the optical Hall conductivity, which can be measured by the magneto-optical Faraday rotations.

Tetrahedral liquids such as silicon, germanium, carbon, water, and silica are an important class of materials not only for industrial applications but also for our understanding of nature. The Stillinger-Weber potential is one of the most popular models for computer simulations of these systems with tetrahedral coordination, with the directionality of the interactions introduced via a three-body repulsive term which promotes locally tetrahedral arrangements. This approach has been extended to various tetrahedral liquids, providing valuable insight into the physics of group XIV elements and more recently water. Perhaps surprisingly, a consistent thermodynamic picture of this class of models is still lacking despite their widespread usage. Here we fill this gap by computing equilibrium phase diagrams for the silicon and water parametrizations and report a novel crystal structure which dominates the models' phase diagram at intermediate and high pressure, and thus warrants further theoretical and numerical investigation. Our results redefine the phase behavior of an important class of tetrahedrally coordinated systems, and also suggest that a more stringent test for simulation models is the ability to select the experimentally relevant crystalline phases, as opposed to just reproducing their mechanical stability.

Thermal interface (Kapitza) resistance expresses how hard it is for heat to flow across material junctions inside multilayer structures. This quantity plays a crucial role in the thermal performance of nanoscale devices but is still poorly understood. Here we show that conventional Fourier-based metrology overestimates metal/semiconductor resistances by up to threefold due to misinterpretation of ballistic heat flow modes. We achieve improved identification and a different physical insight with a truncated Lévy formalism. This approach properly distinguishes interfacial dynamics from nearby quasiballistic heat flow suppression in the semiconductor. Unlike conventionally extracted values, interface resistances obtained with our new approach are independent of laser modulation frequency, as physically appropriate, and much more closely approach theoretical predictions.

We demonstrate the existence of a coherent transport of thermal energy in superlattices by introducing a microscopic definition of the phonon coherence length. A criterion is provided to distinguish the coherent transport regime from diffuse interface scattering and discuss how these can be specifically controlled by several physical parameters. Our approach provides a convenient framework for the interpretation of previous thermal conductivity measurements and calculations; it also paves the way for the design of a new class of thermal interface materials.

Electronic structure, magnetism, x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD) spectra of $A$- and $B$-site-ordered quadruple perovskite $B$-site-ordered perovskite $\mathrm{Ca}{\mathrm{Cu}}_3{\mathrm{Fe}}_2{\mathrm{Sb}}_2{\mathrm{O}}_{12}$ (CCFSO) were studied using first-principles calculations with inclusion of spin-orbit coupling. The calculated XAS and XMCD spectra for Cu and Fe at the $L_{3,2}$ edges were consistent with a recent experiment and indicate ferrimagnetic ordering with antiparallel magnetic moments between Cu and Fe. The magnetic exchange coupling constants were also calculated to clarify the detailed mechanism of the ferrimagnetic ordering in CCFSO. The coupling constants for Cu-Cu, Fe-Fe, and Cu-Fe pairs were found to be moderate ferromagnetic, weak antiferromagnetic, and strong antiferromagnetic, respectively. From the analysis of the calculated density of states and magnetization density, the microscopic origin of the strong antiferromagnetic coupling between Cu and Fe was successfully elucidated by the superexchange mechanism via the $\mathrm{Cu}\left(d_{x^2-y^2}\right)-\mathrm{O}\left(p_x\right)-\mathrm{Fe}\left(t_{2g}\right)$ exchange path.

Motivated by recent experiments on ${\mathrm{EuS}/\mathrm{Bi}}_2{\mathrm{Se}}_3$ heterostructures, we study the temperature dependent screening effects on the surface of a three-dimensional topological insulator proximate to a ferromagnetically ordered system. In general, we find that besides the chemical potential and temperature, the screening energy scale also depends on the proximity-induced electronic gap in an essential way. In particular, at zero temperature the screening energy vanishes if the chemical potential is smaller than the proximity-induced electronic gap. We show that at finite temperature $T$ and/or chemical potential $\mu$, the Chern-Simons (topological) mass, which is generated by quantum fluctuations arising from the proximity effect, can be calculated analytically in the insulating regime. In this case the topological mass yields the Hall conductivity associated with edge states. We show that when the chemical potential is inside the gap the topological mass remains nearly quantized at finite temperature.

We theoretically study the pump-probe response of nonequilibrium BCS superconductors coupled to optical phonons. For ultrashort pump pulses a nonadiabatic regime emerges, which is characterized by oscillations of the superconducting order parameter as well as by the generation of coherent phonons. Using the density-matrix formalism, we compute the pump-probe response in the nonadiabatic regime of the coupled Bogoliubov quasiparticle-phonon system and determine the signatures of the order parameter and the phonon oscillations in the pump-probe conductivity. We find that the nonadiabatic dynamics of the BCS superconductor reflects itself in oscillations of the pump-probe response as functions of delay time $\delta t$ between pump and probe pulses. We argue that from the analysis of this oscillatory behavior both frequency and decay time of the algebraically decaying order-parameter oscillations can be inferred. Similarly, the coherent phonons are evidenced in the pump-probe conductivity by oscillations with the frequency of the phonons. Remarkably, we find that the oscillatory response in the pump-probe conductivity is resonantly enhanced when the frequency of the order-parameter oscillations is tuned to the phonon energy.

Nonlinear coupling between different normal modes of a mechanical resonator is a relevant issue in nanomechanical resonators with high aspect ratio, as well as in very low mass resonators based on graphene and nanotube resonators or in trapped nanoparticles. Here we demonstrate that nonlinear coupling between the two orthogonal flexural modes of a high aspect ratio microcantilever results in measurable effects down to the thermal noise level at liquid helium temperature. In particular, thermal amplitude fluctuations of the first mode are mapped into frequency fluctuations of the second mode. Furthermore, we point out non-Gaussian features in the frequency noise due to a single individual mode, an effect which is a direct consequence of the nonlinear coupling. Finally, we discuss possible implications of nonlinear thermal frequency noise in ultrasensitive force microscopy technologies.

We report a detailed investigation of magnetic relaxation and memory effects in ${\mathrm{La}}_{0.9}$${\mathrm{Sr}}_{0.1}$${\mathrm{CoO}}_3$ single crystal from dc magnetization measurements. The analysis of magnetic relaxation at different temperatures and magnetic fields below the freezing temperature $T_f$ reveals the characteristics of the spin-glass phase. Below $T_f$, striking memory effects have been clearly observed in different experimental protocols. The memory effect in the zero-field-cooled magnetization further establishes that the glassy magnetic state arises from the cooperative spin-spin interaction but not due to the independent relaxation of metastable phase clusters. The asymmetric response with respect to negative and positive temperature changes favors the hierarchical model of memory effects rather than the droplet model discussed in other works for different insulating and metallic Heisenberg spin glasses.

We present a detailed theoretical and numerical study discussing the application and optimization of phase-estimation algorithms (PEAs) to diamond spin magnetometry. We compare standard Ramsey magnetometry, the nonadaptive PEA (NAPEA), and quantum PEA (QPEA) incorporating error checking. Our results show that the NAPEA requires lower measurement fidelity, has better dynamic range, and greater consistency in sensitivity. We elucidate the importance of dynamic range to Ramsey magnetic imaging with diamond spins, and introduce the application of PEAs to time-dependent magnetometry.

We have investigated the spin and orbital magnetic moments of Fe in FePt nanoparticles in the $L$1${}_0$-ordered phase coated with $\mathrm{Si}{\mathrm{O}}_2$ by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Fe $L_{2,3}$ absorption edges. Using XMCD sum rules, we evaluated the ratio of the orbital magnetic moment ($M_{\mathrm{orb}}$) to the effective spin magnetic moment ($M_{\mathrm{spin}}^{\mathrm{eff}}$) of Fe to be $M_{\mathrm{orb}}/M_{\mathrm{spin}}^{\mathrm{eff}}=0.08$. This $M_{\mathrm{orb}}/M_{\mathrm{spin}}^{\mathrm{eff}}$ value is comparable to the value (0.09) obtained for FePt nanoparticles prepared by gas phase condensation, and is larger than the values ($\sim$0.05) obtained for FePt thin films, indicating a high degree of $L$1${}_0$ order. The hysteretic behavior of the FePt component of the magnetization was measured by XMCD. The magnetic coercivity ($H_c$) was found to be as large as 1.8 T at room temperature, $\sim$3 times larger than the thin film value and $\sim$50 times larger than that of the gas phase condensed nanoparticles. The hysteresis curve is well explained by the Stoner-Wohlfarth model for noninteracting single-domain nanoparticles with the $H_c$ distributed from 1 to 5 T.

The reversal condition of magnetization in a nanomagnet under the effect of rotating magnetic field generated by a microwave is theoretically studied based on the Landau-Lifshitz-Gilbert equation. In a rotating frame, the microwave produces a dc magnetic field pointing in the reversed direction, which energetically stabilizes the reversed state. We find that the microwave simultaneously produces a torque preventing the reversal. It is pointed out that this torque leads to a jump in the reversal field with respect to the frequency. We derive the equations determining the reversal fields in both the low- and high-frequency regions from the energy balance equation. The validities of the formulas are confirmed by a comparison with the numerical simulation of the Landau-Lifshitz-Gilbert equation.

The nontrivial topology of the electronic structure of iron pnictides can lead to the appearance of surface states. We study such states in various strip geometries with a focus on the superconducting phase. In the presence of unconventional superconducting pairing with $s_{\pm }$-wave gap structure, the topological states are quite robust and partly remain in the superconducting gap. Furthermore, Andreev bound states appear, which coexist with the topological states for small superconducting gaps and merge with them for larger gap values. The bulk and surface dispersions are obtained from exact diagonalization for two-orbital and five-orbital models in strip geometries.