Sum rules of the moments of the nuclear resonant inelastic x-ray scattering (NRIXS) spectrum provide means of data analysis and information on atomic dynamics. We extend existing work by calculating the third and fourth moment beyond the harmonic approximation and show that NRIXS can provide a direct measurement of the anharmonic terms in lattice potentials. Projected partial phonon density of states (ppDOS) extracted from measured spectra provide vibrational mode-specific information on lattice dynamics. Furthermore, unique contributions to thermodynamic properties can be defined and calculated as the moments and other weighted integrals of ppDOS. A summary of some of these thermodynamic quantities is given. The directional dependence of NRIXS and its effects are emphasized. We derive explicit relationships between the moments of phonon excitation probability function and those of ppDOS. The comparison between the two sets of moments provides a consistency check and insights to the lattice dynamics of the system under study.

The magnetic ordering of nanocrystalline gadolinium (crystallite sizes of 21 nm and 38 nm) has been studied using neutron diffraction and compared with that of a reference ($\sim 1\mu \mathrm{m}$) sample. In contrast with bulk gadolinium in which the moments order initially parallel to the $c$-axis and then cant by as much as ${60}^{\circ }$ away from the $c$-axis below 250 K, before rotating back towards the $c$-axis on further cooling, both nanocrystalline samples order with their moments canted by about ${50}^{\circ }$ from the $c$-axis, and this canting angle is largely temperature independent.

We use neutron scattering to study temperature-dependent spin excitations in insulating antiferromagnetic (AF) Rb${}_{0.8}$Fe${}_{1.6}$Se${}_2$. In the low-temperature AF state, spin waves can be accurately described by a local moment Heisenberg Hamiltonian. On warming to around the Néel temperature of $T_N=500$ K, low-energy ($E<30$ meV) paramagnetic spin excitations form Lorentzian-like quasielastic peaks centered at the AF wave vectors associated with spin waves, while high-energy ($E>50$ meV) spin excitations become heavily damped. Upon further warming to above the structural distortion temperature of $T_s=524$ K, the entire paramagnetic excitations become overdamped. These results suggest that AF Rb${}_{0.8}$Fe${}_{1.6}$Se${}_2$ is not a copper-oxide–like Mott insulator and has less electron correlations compared with metallic iron pnictides and iron chalcogenides.

We use neutron diffraction, magnetometry, and low-temperature heat capacity to probe giant magnetoelastic coupling in CoMnSi-based antiferromagnets and to establish the origin of the entropy change that occurs at the metamagnetic transition in such compounds. We find a large difference between the electronic density of states of the antiferromagnetic and high-magnetization states. The magnetic field-induced entropy change is composed of this contribution and a significant counteracting lattice component, deduced from the presence of negative magnetostriction. In calculating the electronic entropy change, we note the importance of using an accurate model of the electronic density of states, which here varies rapidly close to the Fermi energy.

The present work reports on the new soft ferromagnetic Heusler phases Fe${}_2$NiGe, Fe${}_2$CuGa, and Fe${}_2$CuAl, which in previous theoretical studies have been predicted to exist in a tetragonal Heusler structure. Together with the known phases Fe${}_2$CoGe and Fe${}_2$NiGa these materials have been synthesized and characterized by powder x-ray diffraction, ${}^{57}$Fe Mössbauer spectroscopy, superconducting quantum interference device, and energy-dispersive x-ray measurements. In particular Mössbauer spectroscopy was used to monitor the degree of local atomic order/disorder and to estimate magnetic moments at the Fe sites from the hyperfine fields. It is shown that in contrast to the previous predictions all the materials except Fe${}_2$NiGa basically adopt the inverse cubic Heusler ($X$) structure with differing degrees of disorder. The experimental data are compared with results from ab initio electronic structure calculations on local-density approximation level incorporating the effects of atomic disorder by using the coherent potential approximation. A good agreement between calculated and experimental magnetic moments is found for the cubic inverse Heusler phases. Model calculations on various atomic configurations demonstrate that antisite disorder tends to enhance the stability of the $X$ structure.

We have investigated the dielectric response in an antiferromagnetic dimer-Mott insulator $\beta$’-(BEDT-TTF)${}_2$ICl${}_2$ with square dimer lattice, compared to a spin liquid candidate $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$. Temperature dependence of the dielectric constant shows a peak structure obeying Curie-Weiss law with a strong frequency dependence. We found an anisotropic glassy ferroelectricity by pyrocurrent measurements, which suggests the charge disproportionation resulting in an electric dipole in a dimer. Each ferroelectric and antiferromagnetic transition temperature is closely related to the antiferromagnetic interaction energy and a freezing temperature of dipole dynamics in a dimer, respectively. These correspondences suggest the possible charge-spin coupled degrees of freedom in the system.

In this paper some specific issues related to point defects in GaAs nanowires are addressed with the help of density functional theory calculations. These issues mainly arise from the growth of nanowires under conditions different from those used for thin film or bulk GaAs, such as the coexistence of zinc blende and wurtzite polytypes, the use of gold particles as catalyst, and the arsenic-limited growth regime. Hence, we carry out density functional calculations for As vacancies, Ga${}_{\mathrm{As}}$ antisites, and Au impurities in zinc blende (ZB) and wurtzite (WZ) GaAs crystals. Our results show that As vacancies can diffuse within a ZB GaAs crystal with migration barriers of $\sim$1.9 eV. Within WZ GaAs, As vacancy diffusion is found to be anisotropic, with low barriers of 1.60 up to 1.79 eV (depending on doping conditions) in the $ab$ plane, while there are higher barriers of 2.07 to 2.44 eV to diffuse along the $c$ axis. The formation energy of Au impurities is found to be generally much lower than those of arsenic vacancies or Ga${}_{\mathrm{As}}$ antisites. Thus, Au impurities will be the dominant defects formed in Au-catalyzed nanowire growth. Moreover, we find that it is energetically more favorable by 1 to 2 eV for a Au impurity to replace a lattice Ga atom than a lattice As atom in GaAs. A Au substitutional defect for a lattice Ga atom in ZB GaAs is found to create a charge transfer level in the lower half of the band gap. While our calculations locate this level at $E_v+0.22$ eV, taking into account the inaccuracy of the density functional that ought to be corrected by a downshift of $E_v$ by about 0.2 eV, results in good agreement with the experimental result of $E_v+0.4$ eV.

We propose a method for calculating Coulomb matrix elements between exciton and biexciton states in semiconductor nanocrystals based on the envelope function formalism. We show that such a calculation requires proper treatment of the Bloch parts of the carrier wave functions, which, in the leading order, leads to spin selection rules identical to those holding for optical interband transitions. Compared to the usual (intraband) Coulomb couplings, the resulting matrix elements are additionally scaled by the ratio of the lattice constant to the nanocrystal radius. We present also statistical estimates of the distribution of the coupling magnitudes and energies of the coupled states. The number of biexciton states coupled to exciton states from a certain energy range shows a power-law scaling with the ratio of the coupling magnitude to the energy separation. We estimate also the degree of exciton-biexciton mixing. The amount of biexciton admixture to exciton states at least 1 eV above the multiple exciton generation threshold can reach 80$\%$ but varies strongly with the nanocrystal size.

Transport in H${}_2$ molecules sandwiched between Cu electrodes is investigated by first principles. The theoretical results indicate the presence of two regimes characterized by high and low conductance values. Both of them comprise the molecule within the junction but differ in the adsorption geometry. An experimentally observed low conductance state is tentatively attributed to an asymmetric adsorption site at short tip-tip distances, while at longer tip-tip distances the H${}_2$ molecule is coaxial and inside the junction, giving a conductance comparable to that of the clean contact. The threshold energy for the breakdown of the ballistic regime in the high conductance configuration is estimated as the lowest vibrational energy of the bridging H${}_2$ molecule.

We propose metamaterials for THz frequencies based on multilayer graphene structures. We calculate the dielectric permittivity tensor of the effective nonlocal medium with a periodic stack of graphene layers and demonstrate that tuning from elliptic to hyperbolic dispersion can be achieved with an external gate voltage. We reveal that such graphene structures can demonstrate a giant Purcell effect that can be used for boosting the THz emission in semiconductor devices. Tunability of these structures can be enhanced further with an external magnetic field which leads to the unconventional hybridization of the TE and TM polarized waves.

The random-phase approximation (RPA) is here applied to a two-orbital model for the ${\mathrm{BiS}}_2$-based superconductors that was recently proposed by Usui et al. [Phys. Rev. B 86, 220501(R) (2012)]. Varying the density of doped electrons per Bi site $n$ in the range $0.46\le n\le 1.0$, the spin fluctuations promote competing $A_{1g}$ and $B_{2g}$ superconducting states with similar pairing strengths, in analogy with the $A_{1g}$-$B_{1g}$ near degeneracy found also within RPA in models for pnictides. At these band fillings, two hole pockets centered at $(0,0)$ and $(\pi ,\pi )$ display nearly parallel Fermi-surface segments close to wave vector $(\pi /2,\pi /2)$, whose distance increases with $n$. After introducing electronic interactions treated in the RPA, the interpocket nesting of these segments leads to pair scattering with a rather “local” character in $k$ space. The similarity between the $A_{1g}$ and $B_{2g}$ channels observed here should manifest in experiments on ${\mathrm{BiS}}_2$-based superconductors if the pairing is caused by spin fluctuations.

We study dynamic polarization and resistive detection of nuclear spins in a semiconductor quantum dot (QD) under the Kondo effect regime. We find that the differential conductance spectra of the QD exhibit hysteresis under the Kondo effect regime in magnetic fields. Relevance of nuclear spins to the hysteresis is confirmed by the detection of nuclear magnetic resonance signals by monitoring the differential conductance. We attribute the origin of the hysteresis to the dynamic nuclear spin polarization (DNP) induced in the QD. Using the DNP, we demonstrate nuclear spin relaxation rate measurements in the QD under the Kondo effect regime.

A practical first-principles scheme for time-dependent transport through realistic systems at finite temperature is established by the combination of time-dependent density functional theory and nonequilibrium Green's-function formalism with wide-band limit approximation. This method extends the adiabatic wide-band limit approximation developed earlier [Zheng et al., Phys. Rev. B 75, 195127 (2007)]. It is implemented with both time-dependent density functional theory and time-dependent density functional tight-binding method and is applied to simulate the time-dependent transport through a carbon nanotube based electronic device to demonstrate its validity.

The elastic properties of a spinel vanadate MnV${}_2$O${}_4$ that has an orbital degree of freedom in the triply degenerate $t_{2g}$ orbital at V${}^{3+}$($d^2$) sites are investigated by ultrasonic pulse-echo measurement. Considerable softening of the shear elastic constant $(C_{11}-C_{12})/2$ is observed as the temperature approaches the first-order transition from the high-temperature orbital-disordered cubic phase to the low-temperature orbital-ordered tetragonal phase. The softening is attributed to fluctuations between $d_{yz}$ and $d_{zx}$. Moreover, the elastic anomaly is found to be sensitive to the external magnetic field, revealing a coupling between spin and orbital fluctuations. The elastic anomaly is well reproduced theoretically based on Landau theory and elucidates a characteristic precursor phenomenon in MnV${}_2$O${}_4$.

The green luminescence from the triplet states of a self-trapped exciton (STE) located on a tetrahedral (MoO${}_4$)${}^{2-}$ ion in PbMoO${}_4$ splits into three bands, I, II, and III, as a result of the symmetry lowering of the (MoO${}_4$)${}^{2-}$ ion due to a Jahn-Teller effect. Time-resolved luminescence measurements have been performed for polarizations with the electric vector parallel to the crystallographic $c$ axis ($\mathbf{E}\parallel \mathbf{c}$) and perpendicular to it (E⊥c) in a temperature range between 8 and 225 K. All three bands are confirmed to exhibit significant polarizations. The main band II at 520 nm is polarized with $\mathbf{E}\parallel \mathbf{c}$ at low temperatures ($T$ < 75 K) but changes its polarization from $\mathbf{E}\parallel \mathbf{c}$ to E⊥c at $T$ > 125 K. Band III at 600 nm is polarized with E⊥c at $T$ = 170 and 225 K. The degree of polarization of these two bands does not depend on the polarization of excitation light. On the other hand, band I at 405 nm is polarized with E⊥c ($\mathbf{E}\parallel \mathbf{c}$) under the excitation by photons polarized parallel (perpendicular) to the $c$ axis, in the range of $T$ = 150–225 K. The observation of polarized luminescence provides convincing evidence for further lowering of the symmetry of (MoO${}_4$)${}^{2-}$ ions due to a uniaxial crystal field in addition to the Jahn-Teller distortion. The sublevels of the triplet STE state responsible for the PbMoO${}_4$ luminescence are identified on the basis of group-theoretical considerations.

Graphitic carbon nitride ($g$-C${}_3$N${}_4$) has recently triggered extensive investigations due to its potential applications, such as in direct photochemical water splitting, CO${}_2$ activation, and transition-metal-free spintronics. However, electronic, and particularly the optical properties of $g$-C${}_3$N${}_4$ still have not been well established. Based on one of the state-of-the-art approaches—many-body Green's function theory (i.e., $GW$ $+$ BSE)—absorption of ultraviolet light by $g$-C${}_3$N${}_4$ is found to be determined by strong excitonic effects with a significantly large binding energy assigned to the bound excitons. Dark states have also been found in $g$-C${}_3$N${}_4$, which can affect the photoluminescence yield of $g$-C${}_3$N${}_4$. We find that the band gap of $g$-C${}_3$N${}_4$ probably can be tuned by adjusting the condensation (dimensionality) to initiate excitonic absorption in the visible light region, which might help improve the solar energy conversion efficiency.

We investigate theoretically under what conditions a stable and high-fidelity preparation of the biexciton state in a quantum dot can be realized by means of adiabatic rapid passage in the presence of acoustic phonon coupling. Our analysis is based on a numerically complete real-time path-integral approach and comprises two different schemes of optical driving using frequency-swept (chirped) pulses. We show that depending on the size of the biexciton binding energy, resonant two-photon excitations or two-color schemes can be favorable. It is demonstrated that the carrier-phonon interaction strongly affects the efficiency of both protocols and that a robust preparation of the biexciton is restricted to positive chirps and low temperatures. A considerable increase of the biexciton yield can be achieved realizing temperatures below 4 K.

Bistable behavior of single Si dopants in the (110) surface layer of GaAs was studied with a scanning tunneling microscope (STM). The Si atom acts as either a positively charged substitutional donor or a negatively charged interstitial. Its configuration can switch under the influence of a local biased STM tip. To independently manipulate the charge state, the sample was illuminated by a laser during STM operation. The Si atom can be reversibly switched between its positive and negative charge states by turning the laser on and off, respectively. This process occurs mostly with the photon energy tuned above the band gap of GaAs, indicating that photogenerated electron-hole pairs play an important role in the process. The occupation of the donor and interstitial configurations depends on the carrier dynamics, i.e., the possibility of the electrons to escape or to be captured. If the tip-induced band bending is large enough, it is possible for electrons to tunnel into the conduction band and the donor configuration is observed. Another escape path is created when the sample is illuminated and photogenerated holes can recombine with the bound electrons of the dopant.

The form and the role of the entropy flux in the thermodynamic analysis of the transport equations are essentially open questions in nonequilibrium thermodynamics. In particular, nonlocal heat-transport equations at nanoscale may exhibit some peculiar behaviors which seem to violate well-known statements of the second law of thermodynamics. Here we examine one of these behaviors in axial heat transport from the perspective of a generalized entropy flux, i.e., ${\mathbf{J}}^{\left(s\right)}=\mathbf{q}/T+\mathbf{k}$, and show that such a generalization allows it to be consistent with the second law. In contrast with previous formal analyses, this paper provides an explicit form for the nonclassical part of the entropy flux, that is, $\mathbf{k}={\ell }^2/\left(\lambda T^2\right)\nabla {\mathbf{q}}^{\mathrm{T}}·\mathbf{q}$ and links it to a concrete physical phenomenon which is accessible to current experimental possibilities for systems with sufficiently long mean-free path $\ell$, whereas for short enough $\ell$ the classical results are recovered. The derivation of the nonclassical part of the entropy flux is obtained within the frame of extended irreversible thermodynamics from two different perspectives, namely, a 13-field theory with higher-order fluxes and a 4-field theory with higher-order gradients.

We present detailed experimental and theoretical investigations of hotspots produced by trapped vortex bundles and their effect on the radio-frequency (rf) surface resistance $R_s$ of superconductors at low temperatures. Our measurements of $R_s$, combined with the temperature mapping and laser scanning of a 2.36-mm-thick Nb plate incorporated into a 3.3-GHz Nb resonator cavity cooled by the superfluid He at 2 K, revealed spatial scales and temperature distributions of hotspots and showed that they can be moved or split by thermal gradients produced by the scanning laser beam. These results, along with the observed hysteretic field dependence of $R_s$ which can be tuned by the scanning laser beam, show that the hotspots in our Nb sample are due to trapped vortex bundles which contain $\sim {10}^6$ vortices spread over regions $\sim 0.1–1$ cm. We calculated the frequency dependence of the rf power dissipated by oscillating vortex segments trapped between nanoscale pinning centers, taking into account all bending modes and the nonlocal line tension of the vortex driven by rf Meissner currents. We also calculated the temperature distributions caused by trapped vortex hotspots, and suggested a method of reconstructing the spatial distribution of vortex dissipation sources from the observed temperature maps. Vortex hotspots can dominate the residual surface resistance at low temperatures and give rise to a significant dependence of $R_s$ on the rf field amplitude $H_p$, which can have important implications for the rf resonating cavities used in particle accelerators and for thin-film structures used in quantum computing and photon detectors.