We investigate the many-body-instability-driven electronic, magnetic, and thermodynamic properties of graphene nanoflakes described by an extended Hubbard model using exact diagonalization. Our exact results lead to a complete magnetic phase diagram in $n-V$ space, where $n$ is the filling and $V$ is the nonlocal Coulomb interaction. The phase diagram is found to consist of reentrant phases with net magnetic moment, separated by those with zero magnetic moment. The interplay of local and nonlocal Coulomb interaction, geometry, and filling gives rise to complex magnetic phases with coexisting antiferromagnetic and ferromagnetic correlations, of both short-range and long-range nature. A small change in temperature or in the strength of the nonlocal Coulomb interaction is found to drive the change in the magnetic state. The low-temperature thermodynamic behavior, driven by the crossover of eigenstates of contrasting magnetic characters, is signaled by a sharp peak in the specific heat.

We study nonlinear transport and nonequilibrium current noise in quasiclassical point contacts (PCs) defined in a low-density, high-quality two-dimensional electron system in GaAs. At not too high bias voltages $V$ across the PC, the noise temperature is determined by a Joule heat power and is almost independent on the PC resistance that can be associated with a self-heating of the electronic system. This commonly accepted scenario breaks down at increasing $V$, where we observe extra noise accompanied by a strong decrease of the PC's differential resistance. The spectral density of the extra noise is roughly proportional to the nonlinear current contribution in the PC, $\delta S\approx 2F^{*}\left|e\delta I\right|\sim V^2$, with the effective Fano factor $F^{*}<1$, indicating that a random scattering process is involved. A small perpendicular magnetic field is found to suppress both $\delta I$ and $\delta S$. Our observations are consistent with a concept of a draglike mechanism of the nonlinear transport mediated by electron-electron scattering in the leads of quasiclassical PCs.

Two distinct energy transfer (ET) mechanisms have been proposed for the conversion of blue to near-infrared (NIR) photons in YAG:${\mathrm{Ce}}^{3+},{\mathrm{Yb}}^{3+}$. The first mechanism involves downconversion by cooperative energy transfer, which would yield two NIR photons for each blue photon excitation. The second mechanism of single-step energy transfer yields only a single NIR photon for each blue photon excitation and has been argued to proceed via a ${\mathrm{Ce}}^{4+}-{\mathrm{Yb}}^{2+}$ charge transfer state (CTS). If the first mechanism were operative in YAG:${\mathrm{Ce}}^{3+},{\mathrm{Yb}}^{3+}$, this material would have the potential to greatly increase the response of crystalline Si solar cells to the blue/UV part of the solar spectrum. In this work, however, we demonstrate that blue-to-NIR conversion in YAG:${\mathrm{Ce}}^{3+},{\mathrm{Yb}}^{3+}$ goes via the single-step mechanism of ET via a ${\mathrm{Ce}}^{4+}-{\mathrm{Yb}}^{2+}$ CTS. The photoluminescence decay dynamics of the ${\mathrm{Ce}}^{3+}$ excited state are inconsistent with Monte Carlo simulations of the cooperative (one-to-two photon) energy transfer, while they are well reproduced by simulations of single-step (one-to-one photon) energy transfer via a charge transfer state. Based on temperature dependent measurements of energy transfer and luminescence quenching we construct a configuration coordinate model for the ${\mathrm{Ce}}^{3+}$-to-${\mathrm{Yb}}^{3+}$ energy transfer, which includes the ${\mathrm{Ce}}^{4+}-{\mathrm{Yb}}^{2+}$ charge transfer state.

Noncontact atomic force microscopy (NC-AFM) is now routinely capable of obtaining submolecular resolution, readily resolving the carbon backbone structure of planar organic molecules adsorbed on metal substrates. Here we show that the same resolution may also be obtained for molecules adsorbed on a reactive semiconducting substrate. Surprisingly, this resolution is routinely obtained without the need for deliberate tip functionalization. Intriguingly, we observe two chemically distinct apex types capable of submolecular imaging. We characterize our tip apices by “inverse imaging” of the silicon adatoms of the $\mathrm{Si}\left(111\right)-7\times 7$ surface and support our findings with detailed density functional theory (DFT) calculations. We also show that intramolecular resolution on individual molecules may be readily obtained at 78 K, rather than solely at 5 K as previously demonstrated. Our results suggest a wide range of tips may be capable of producing intramolecular contrast for molecules adsorbed on semiconductor surfaces, leading to a much broader applicability for submolecular imaging protocols.

Scanning tunneling microscopy (STM) provides real-space electronic state information at the atomic scale that is most commonly used to study materials surfaces. An intriguing extension of the method is to attempt to study the electronic structure at an insulator/conductor interface by performing low-bias imaging above the surface of an ultrathin insulating layer on the conducting substrate. We use first-principles theory to examine the physical mechanisms giving rise to the formation of low-bias STM images in the MgO/Ag system. We show that the main features of the low-bias STM contrast are completely determined by the atoms on the surface of MgO which overcomes prior ambiguities in assigning observed STM features to atomic positions of the substrate or thin film in such an epitaxial thin film system. Hence, the low-bias contrast is formed by states at the Fermi level in the Ag that propagate evanescently through the lattice and atomic orbitals of the MgO on their way to the surface. We develop a number of analysis techniques based on an ab initio tight-binding representation that allows identification of the origin of the STM contrast in cases where previous approaches have proven ambiguous.

Femtosecond degenerate and nondegenerate pump-probe spectroscopy in graphene reveals the evolution of photoexcited carrier distribution in the energy band during relaxation: the initial occupation of photoexcited carriers centered at the excitation state is anisotropic in momentum space; this anisotropic distribution rapidly relaxes through an intermediate state argued to be fully isotropic in the energy band due to phonon-involved cascade scattering. In addition, the experiment suggests that graphene optical absorbances for in-plane and out-of-plane optical fields are identical.

We present first-principles calculations of the thermal and thermal transport properties of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ that combine an ab initio molecular dynamics (AIMD) approach to calculate interatomic force constants (IFCs) along with a full iterative solution of the Peierls-Boltzmann transport equation for phonons. The newly developed AIMD approach allows determination of harmonic and anharmonic interatomic forces at each temperature, which is particularly appropriate for highly anharmonic materials such as ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. The calculated phonon dispersions, heat capacity, and thermal expansion coefficient are found to be in good agreement with measured data. The lattice thermal conductivity, ${\kappa }_l$, calculated using the AIMD approach nicely matches measured values, showing better agreement than the ${\kappa }_l$ obtained using temperature-independent IFCs. A significant contribution to ${\kappa }_l$ from optic phonon modes is found. Already at room temperature, the phonon line shapes show a notable broadening and onset of satellite peaks reflecting the underlying strong anharmonicity.

We study the effect of an externally imposed rotation or magnetic field on frustrated multiband superconductors and superfluids. The frustration originates with multiple superconducting bands crossing the Fermi surface in conjunction with interband Josephson couplings with a positive sign. These couplings tend to frustrate the phases of the various components of the superconducting order parameter. This in turn leads to an effective description in terms of a $U\left(1\right)\times Z_2$-symmetric system, where essentially only the $U\left(1\right)$ sector couples to the gauge field representing the rotation or magnetic field. By imposing a large enough net vorticity on the system at low temperatures, one may therefore reveal a resistive vortex-liquid state which will feature an unusual additional phase transition in the $Z_2$ sector. At low enough vorticity there is a corresponding vortex-lattice phase featuring a $Z_2$ phase transition. We argue that this transition phase should be readily observable in experiments.

We found that the transitions between delocalized electronic states in quantum-dot superlattices with smaller size dispersion can account for higher electron mobility. In particular, we solved for the quantum states of a two-dimensional quantum-dot (QD) superlattice using a one-electron approximation. Electron transport properties were studied by considering hopping transitions among coupled delocalized electronic states. Molecular dynamics simulations were employed to introduce disorders in superlattice configurations as a function of QD size and size dispersion for calculation of electron mobility. The interparticle spacing, size, and temperature dependence of the electron mobility can be well explained within the framework of our analysis.

The compound ${\mathrm{MnAu}}_2$ is one of the oldest known spin-spiral materials, yet the nature of the spiral state is still not clear. The spiral cannot be explained via relativistic effects due to the short pitch of the spiral and the weakness of the spin-orbit interaction in Mn, and another common mechanism, nesting, is ruled out as direct calculations show no features at the relevant wave vector. We propose that the spiral state is induced by a competition between the short-range antiferromagnetic exchange and a long-range interaction induced by the polarization of Au bands, similar to double exchange. We find that, contrary to earlier reports, the ground state in standard density functional theory is ferromagnetic, i.e., the latter interaction dominates. However, an accounting for Coulomb correlations via a Hubbard $U$ suppresses the Schrieffer-Wolff-type $s\text{−}d$ magnetic interaction between Mn and Au faster than the superexchange interaction, favoring a spin-spiral state. For realistic values of $U$, the resulting spiral wave vector is in close agreement with experiment.

Universality of the extraordinary Hall effect scaling was tested in granular three-dimensional ${\mathrm{Ni}\text{−}\mathrm{SiO}}_2$ films across the metal-insulator transition. Three types of magnetotransport behavior have been identified: metallic, weakly insulating, and strongly insulating. Scaling between both the ordinary and the extraordinary Hall effects and material's resistivity is absent in the weakly insulating range characterized by logarithmic temperature dependence of conductivity. The results provide compelling experimental confirmation for recent models of granular metals predicting transition from logarithmic to exponential conductivity temperature dependence when intergranular conductance drops below the quantum conductance value and loss of Hall effect scaling when intergranular conductance is higher than the quantum one. The effect was found at high temperatures and reflects the granular structure of the material rather than low-temperature quantum corrections.

In many three-dimensional metals with inversion symmetry and a weak spin-orbit interaction, Dirac points of the electron energy spectrum form band-contact lines in the Brillouin zones of these crystals, and electron topological transitions of $3\frac{1}{2}$ kind are due to certain points on these lines. We theoretically study these transitions in detail and point out that they can be detected with magnetic susceptibility which exhibits a giant diamagnetic anomaly at the $3\frac{1}{2}$-order transitions.

We study entanglement between the spin components of the BCS ground state by calculating the full entanglement spectrum and the corresponding von Neumann entanglement entropy. The entanglement spectrum is effectively modeled by a generalized Gibbs ensemble (GGE) of noninteracting electrons, which may be approximated by a canonical ensemble at the BCS critical temperature. We further demonstrate that the entanglement entropy is jointly proportional to the pairing energy and to the number of electrons about the Fermi surface (an area law). Furthermore, the entanglement entropy is also proportional to the number fluctuations of either spin component in the BCS state.

We determine the lowest and higher order collective modes in both spin-conserving and spin-reversed sectors by calculating the energy differences of appropriate linear combinations of different levels of the excitons of the composite fermions and the fully spin-polarized ground states at filling factors $\nu =1/3$, 2/5, 3/7, and 4/9. Apart from providing a detailed study of previously reported modes that have also been observed in the experiments, we predict additional higher energy modes at different filling factors. The lowest and the next higher spin-conserving modes have equal number of “magnetorotons” and the number is the same as the number of filled effective Landau-like levels of composite fermions. The higher energy modes at $\nu =1/3$ merge with the lowest mode at long wavelengths. The spin-conserving modes do not merge at other filling factors. Apart from showing a zero-energy spin-wave mode at zero momentum, thanks to Larmor's theorem, the lowest spin-reversed modes at the ferromagnetic ground states of $\nu =2/5$, 3/7, and 4/9 display one or more “spin rotons” at negative energies, signaling unstable fully polarized ground states at sufficiently small Zeeman energies. The high-energy spin-reversed modes also have spin rotons but at positive energies. The energies of these excitations depend on the finite width of the quantum well as the Coulomb interaction gets screened. We perform a finite thickness correction to the Coulomb interaction by the standard method of local density approximation and use it to calculate the critical energies such as rotons and long- and short-wavelength modes, which are detectable in inelastic light scattering experiments.

We present an experimental study on the ignition and decay of a polariton optical parametric oscillator (OPO) in a semiconductor microcavity pillar. The combination of a continuous-wave laser pump, under quasi-phase-matching conditions, and a nonresonant, 2-ps-long pulse probe allows us to obtain the full dynamics of the system. The arrival of the probe induces a blueshift in the polariton emission, bringing the OPO process into resonance with the pump, which triggers the OPO process. We time-resolve the polariton OPO signal emission for more than 1 ns in both real space and momentum space. We fully characterize the emission of the OPO signal with spectral tomography techniques. Our interpretations are backed up by theoretical simulations based on the two-dimensional coupled Gross-Pitaevskii equation for excitons and photons.

We calculate the multiphonon transition rate from the excited state of a single-occupied two-level quantum dot. The electron interacts with certain optical phonon modes which in turn transfer the transition energy to the bath of other phonon modes decoupled from the electron states. Our theory covers the previously unexplored range of transition energies several times larger than the optical phonon energy and systematically studies the role of quantum interference of the processes involving different virtual polaron states.

We investigated the Ce- and Ru-site substitution effects on the spin gap in ${\mathrm{CeRu}}_2{\mathrm{Al}}_{10}$ by measuring the magnetic and thermal properties of ${\mathrm{Ce}}_{1-y}{\mathrm{Ln}}_y{\mathrm{Ru}}_2{\mathrm{Al}}_{10}$ (Ln = La, Pr, Y) and ${\mathrm{Ce}(\mathrm{Ru}}_{1-x}{\mathrm{T}}_x{)}_2{\mathrm{Al}}_{10}$ (T = Re, Rh) system, respectively. The magnetic susceptibility shows that the itinerant and localized nature is enhanced by Re and Rh doping, respectively. In the latter, the orientation of the magnetic moment in the antiferromagnetic ordered phase suddenly changes from the $c$ to $a$ axis at $x=0.03$. The orientation of the magnetic moment to the $a$ axis is consistent with the large anisotropy of the magnetic susceptibility in the paramagnetic region. The specific heat shows that the exponential temperature dependence, $e^{-\Delta /k_{\mathrm{B}}T}$, and the electronic specific coefficient $\gamma$ is not changed in the Ce-site substituted systems at least up to $y=0.1$ but in the Ru-site substituted systems, the $e^{-\Delta /k_{\mathrm{B}}T}$ dependence disappears, and the $\gamma$ value increases rapidly with $x$. These indicate that although the spin gap is robust against the Ce-site substitution by Ln ion at least up to $y=0.1$, the spin gap is rapidly collapsed by the Ru-site substitution and in place, the conduction electron with heavy effective mass appears at the Fermi level. The spin gap which is formed under the subtle balance between the localized and itinerant nature is easily collapsed by a small amount of Re or Rh doping.

Copper impurities affect optical and electronic properties in a diverse range of materials. Using IR spectroscopy, we provide evidence for a shallow Cu acceptor in Si. The observation of $P_{1/2}$ excitation lines indicates a Cu acceptor with a hole ionization energy of only 27 meV, which is shallower than any of the group-III acceptors. The observed center has an IR spectrum that is characteristic of a hydrogenic acceptor. Variable-temperature Hall-effect measurements confirm this result. The observation of shallow, hydrogenic behavior is consistent with a universal level model, in which the Cu acceptor level is fixed relative to vacuum, irrespective of the host.