Millimeter sized single crystals of Mo${}_3$Sb${}_7$ are grown using the self-flux technique and a thorough characterization of their structural, magnetic, thermal, and transport properties is reported. The structure parameters for the high-temperature cubic phase and the low-temperature tetragonal phase were determined with neutron single crystal diffraction. Both x-ray powder diffraction and neutron single crystal diffraction at room temperature confirmed that Mo${}_3$Sb${}_7$ crystallizes in Ir${}_3$Ge${}_7$-type cubic structure with space group $\mathit{Im}$ $\overline{3}$$m$. The cubic-tetragonal structure transition at 53 K is verified by the peak splitting of (4 0 0) reflection observed by x-ray single crystal diffraction and the dramatic intensity change of the (12 0 0) peak observed by neutron single crystal diffraction. The structural transition is accompanied by a sharp drop in magnetic susceptibility, electrical resistivity, and thermopower while cooling. A weak lambda anomaly was also observed around 53 K in the temperature dependence of specific heat, and the entropy change across the transition is estimated to be 1.80 J/mol Mo K. The temperature dependence of magnetic susceptibility was measured up to 750 K, and it follows a Curie-Weiss behavior above room temperature. Analysis of the low-temperature magnetic susceptibility suggests a spin gap of 110 K around 53 K. A typical phonon thermal conductivity was observed in the low temperature tetragonal phase. A glassy phonon thermal conductivity above 53 K suggests a structural instability in a wide temperature range. Superconductivity was observed at 2.35 K in the as-grown crystals, and the dimensionless specific heat jump $▵C\left(T\right)/{\gamma }_n{T}_c$ was determined to be 1.49, which is slightly larger than the BCS value of 1.43 for the weak-coupling limit.

Optical Rabi oscillations are coherent population oscillations of a two-level system coupled by an electric dipole transition when driven by a strong nearly resonant optical field. In quantum dot structures, these measurements have typically been performed as a function of the total pulse area $\int {\Omega }_0\left(t\right)\mathrm{d}t$ where the pulse area varies as a function of Rabi frequency. Here, we report direct detection of the time-resolved coherent transient response of the resonance fluorescence to measure the time evolution of the optical Rabi oscillations in a single charged InAs quantum dot. We extract a decoherence rate consistent with the limit from the excited state lifetime.

We investigate the charge relaxation in a mesoscopic capacitor made of an interacting multilevel quantum dot with the spin exchange coupling. The charge relaxation resistance is numerically calculated and analyzed for ferromagnetic, antiferromagnetic, and two-stage Kondo regimes of the system under the singlet-triplet transition. We find that the relaxation resistance can be enhanced over the known universal value in each regime in its own distinctive way. The enhancement of the resistance occurs when the energy to break the spin binding that is formed by the Coulomb interaction and the spin exchange coupling is compensated by the external voltage source or the finite Zeeman splitting. The resultant spin fluctuations promote the generation of particle-hole pairs accompanying spin flip, which is responsible for the increase of the relaxation resistance.

We use ab initio simulations and perturbation theory to study the $\mathit{2}{D}^{\prime }$ Raman mode of graphene subject to biaxial and uniaxial strains up to $2\%$. We demonstrate that $\mathit{2}{D}^{\prime }$ Raman measurements, as a function of polarization and laser energy $E_L$, can probe the LO phonons of graphene with arbitrary radial and angular extent around $\Gamma$. The $\mathit{2}{D}^{\prime }$ profile is highly sensitive to uniaxial strain and depends on both polarization and strain orientation. The Grüneisen parameter ${\gamma }_{\mathit{2}{D}^{\prime }}\approx 1.71$ has a mild dependency on the laser energy $E_L$, and is found to be in good agreement with experiments and comparable in value to ${\gamma }_G$. The shear deformation potential ${\beta }_{\mathit{2}{D}^{\prime }}$ depends strongly on the polarization and strain orientation, becoming negative when the polarizer and analyzer are perpendicular to each other. Finally, we describe a robust method to determine the uniaxial strain by relying solely on polarized measurements of the $\mathit{2}{D}^{\prime }$ mode.

Projective symmetry groups are the mathematical tools which make it possible to list and classify mean-field spin liquids (SLs) based on a parton construction. The seminal work of Wen [Phys. Rev. B 65, 165113 (2002)] and its subsequent extension to bosons by Wang and Vishwanath [Phys. Rev. B 74, 174423 (2006)] concerned the so-called symmetric SLs; i.e., states that break neither lattice symmetries nor time reversal invariance. Here we generalize this tool to chiral (time reversal symmetry breaking) SLs described in a Schwinger boson mean-field approach and illustrate it on the triangular lattice, which can harbor nine different weakly symmetric SLs (two symmetric SLs and seven chiral SLs) with nearest neighbor bond operators only. Results for other lattices (square and kagome) are given in the Appendixes. Application of this new approach has recently led to the discovery of two chiral ground states on the kagome lattice [Messio et al., Phys. Rev. Lett. 108, 207204 (2012); Fåk et al., Phys. Rev. Lett. 109, 037208 (2012)]. The signature of a time reversal symmetry breaking SL is the presence in the ground state of nontrivial fluxes of loop operators that break some lattice point group symmetries. The physical significance of these gauge invariant quantities is discussed both in the classical limit and in the quantum SL and their expressions in terms of spin observables are given.

We investigate the metallic transport in the organic superconductor (TMTSF)${}_2$PF${}_6$ under pressure within the framework of the spin density wave theory in the proximity of a Peierls quantum critical point (QCP). We use a simple transport model of hot and cold regions around the Fermi surface, driven by the proximity to the QCP in order to provide a template to fit the experimental results. Successful agreement with the data comforts the interpretation of extended criticality around the antiferromagnetic QCP in the normal phase of quasi-one-dimensional organic superconductors.

By calculating orbitally resolved Pauli susceptibilities within maximally localized Wannier orbital basis transformed from first principles band structures, we find that magnetism in Fe${}_{1+x}$Te still has its itinerant origin even without Fermi surface nesting, provided orbital modulation of particle-hole excitations are considered. This leads to strong magnetic instabilities at wave vector (0,$\pi$)/($\pi$,0) in $d_{xz}$/$d_{yz}$ orbitals that are responsible for the bicollinear antiferromagnetic state as extra electrons donated from excess Fe are considered. Magnetic exchange coupling between excess Fe and in-plane Fe further stabilizes the bicollinear antiferromagnetic order. Our results reveal that magnetism and superconductivity in iron chalcogenides may have different orbital origin, as Pauli susceptibilities of different orbitals evolve differently as a function of concentration of excess Fe and height of the chalcogen atom measured from the iron plane.

We study theoretically edge transport of a fractional quantum Hall liquid, in the presence of a quantum dot inside the Hall bar with well-controlled electron density and Landau-level filling factor $\nu$ and show that such transport studies can help reveal the nature of the fractional quantum Hall liquid. In our first example we study the $\nu =1/3$ and $\nu =2/3$ liquids in the presence of a $\nu =1$ quantum dot. When the quantum dot becomes large, its edge states join those of the Hall bar to reconstruct the edge states configuration. Taking randomness around the edges into account, we find that in the disorder-irrelevant phase the two-terminal conductance of the original $\nu =1/3$ system vanishes at zero temperature, while that of the $\nu =2/3$ case remains finite. This distinction is rooted in the fact that the $\nu =2/3$ state is built on the $\nu =1$ state. In the disorder-dominated phase, the two-terminal conductance of $\nu =1/3$ system is $(1/5)\frac{e^2}{h}$ and that of $\nu =2/3$ system is $(1/2)\frac{e^2}{h}$. We further apply the same idea to the $\nu =5/2$ system, which realizes either the Pfaffian state or the anti-Pfaffian state. In this case, we study the edge transport in the presence of a central $\nu =3$ quantum dot. If the quantum dot is large enough for its edge states joining those of the Hall bar, in the disorder-irrelevant phase the total two-terminal conductance in the Pfaffian case is $G_{\mathrm{tot}}^{\mathrm{Pf}}\to 2\frac{e^2}{h}$ while that of anti-Pfaffian case is higher but not universal, $G_{\mathrm{tot}}^{\mathrm{aPf}}>2\frac{e^2}{h}$. This difference can be used to determine which one of these two states is realized at $\nu =5/2$. In the disorder-dominated phase, however, the total two-terminal conductances in these two systems are exactly the same, $G_{\mathrm{tot}}^{\mathrm{Pf}/\mathrm{aPf}}=(7/3)\frac{e^2}{h}$.

We present a comprehensive experimental and theoretical study of visible light diffraction from opal-based photonic structures of different dimensions. We also developed new methodological approaches to collection, processing, and interpretation of experimental diffraction data. The key idea is to analyze the light diffraction data represented in the “incident angle-registration angle” coordinates instead of the widely used “incident angle-registration intensity” coordinates. In such coordinates, a 2D diffraction pattern consists of ovals, while a 3D Bragg diffraction pattern consists of parallel stripes owing to the specular reflection law. Therefore this representation allows one to distinguish easily the patterns originating from 2D diffraction from the ones governed by 3D diffraction. In addition, different types of structural disorder including twinning of the face-centered cubic structure become apparent in the angle coordinates.

Dirac fermions in graphene can be subjected to non-Abelian gauge fields by implementing certain modulations of the carbon site potentials. Artificial graphene, engineered with a lattice of CO molecules on top of the surface of Cu, offers an ideal arena to study their effects. In this work, we show by symmetry arguments how the underlying CO lattice must be deformed to obtain these gauge fields, and estimate their strength. We also discuss the fundamental differences between Abelian and non-Abelian gauge fields from the Dirac electrons point of view, and show how a constant (non-Abelian) magnetic field gives rise to either a Landau level spectrum or a quadratic band touching, depending on the gauge field that realizes it (a known feature of non-Abelian gauge fields known as the Wu-Yang ambiguity). We finally present the characteristic signatures of these effects in the site-resolved density of states that can be directly measured in the current molecular graphene experiment, and discuss prospects to realize the interaction induced broken symmetry states of a quadratic touching in this system.

Current methods allow quantitative local structure determination of adsorbate geometries on surfaces in ultrahigh vacuum (UHV) but are incompatible with the higher pressures required for a steady-state catalytic reactions. Here we show that photoelectron diffraction can be used to determine the structure of the methoxy and formate reaction intermediates during the steady-state oxidation of methanol over Cu(110) by taking advantage of recent instrumental developments to allow near-ambient pressure x-ray photoelectron spectroscopy. The local methoxy site differs from that under static UHV conditions, attributed to the increased surface mobility and dynamic nature of the surface under reaction conditions.

We have experimentally compared the excitonic properties of hybrid multiple quantum wells, (C${}_{\text{6}}$H${}_{\text{5}}$-C${}_{\text{2}}$H${}_{\text{4}}$NH${}_{\text{3}}$)${}_{\text{2}}$PbBr${}_{\text{4}}$ and (C${}_{\text{4}}$H${}_{\text{9}}$NH${}_{\text{3}}$)${}_{\text{2}}$PbBr${}_{\text{4}}$, using photoluminescence, reflection, and photoluminescence excitation measurements. We focused on the contribution of the image charge effect (ICE) to the excitonic energy structure in these materials which have different dielectric constants of the barrier layers. We have found that the binding energies of the $2s$ and $3s$ excitons are considerably enhanced by ICE, while the contribution of ICE to the $1s$ excitons is smaller because of the small Bohr radius, which is comparable to the well width.

We present a method called SISYPHUS (stochastic iterations to strengthen yield of path hopping over upper states) for extending accessible time scales in atomistic simulations. The method proceeds by separating phase space into basins and transition regions between the basins based on a general collective variable (CV) criterion. The transition regions are treated via traditional molecular dynamics (MD) while Monte Carlo (MC) methods are used to (i) estimate the expected time spent in each basin and (ii) thermalize the system between two MD episodes. In particular, an efficient adiabatic switching based scheme is used to estimate the time spent inside the basins. The method offers various advantages over existing approaches in terms of (i) providing an accurate real time scale, (ii) avoiding reliance on harmonic transition state theory, and (iii) avoiding the need to enumerate all possible transition events. Applications of SISYPHUS to low-temperature vacancy diffusion in bcc Ta and adatom island ripening in fcc Al are presented. A CV appropriate for such condensed phases, especially for transitions involving collective motions of several atoms, is also introduced.

The contributions to the spin relaxation in copper (Cu) nanowires are quantified by carefully analyzing measurements of both charge and spin transport in lateral spin valves as a function of temperature and thickness. The temperature dependence of the spin-flip scattering solely arises from the scattering with phonons, as in bulk Cu, whereas we identify grain boundaries as the main temperature-independent contribution of the defects in the nanowires. A puzzling maximum in the spin-diffusion length of Cu at low temperatures is found, which can be explained by the presence of magnetic impurities. The results presented here suggest routes for improving spin transport in metallic nanostructures, otherwise limited by confinement effects.

Magnetic excitations in a ferromagnetic superconductor in the presence of an Abrikosov vortex lattice have been studied using the phenomenological London and Landau-Lifshitz equations. Due to the periodicity of the vortex field the magnon spectrum has a band structure, similar to the structure of the electon spectrum in a crystal lattice. The gaps between adjacent bands have been calculated using an analog of the weak-binding approximation. When the applied magnetic field is altered the band structure undergoes a qualitative transformation due to commensurability effects, connected with the nonmonotonicity of the magnon spectrum in the Meissner state. In dirty samples the energy gaps may be smeared out because of the dissipation connected with vortex motion. In sufficiently clean samples the gaps manifest themselves as maxima in the frequency dependence of the microwave reflectivity coefficient.

The interplay between structure, magnetism, and superconductivity in single crystal Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ ($x=0.047$) has been studied using high-resolution x-ray diffraction by monitoring charge Bragg reflections in each twin domain separately. The emergence of the superconducting state is correlated with the suppression of the orthorhombic distortion around $T$${}_{\mathtt{C}}$, exhibiting competition between orthorhombicity and superconductivity. Above $T$${}_{\mathtt{S}}$, the in-plane charge correlation length increases with the decrease of temperature, possibly induced by nematic fluctuations in the paramagnetic tetragonal phase. Upon cooling, anomalies in the in-plane charge correlation lengths along $a$ (${\xi }_a$) and $b$ axes (${\xi }_b$) are observed at $T$${}_{\mathtt{S}}$ and also at $T$${}_{\mathtt{N}}$ indicative of strong magnetoelastic coupling. The in-plane charge correlation lengths are found to exhibit anisotropic behavior along and perpendicular to the in-plane component of stripe-type AFM wave vector (101)${}_{\mathrm{O}}$ below around $T$${}_{\mathtt{N}}$. The temperature dependence of the out-of-plane charge correlation length shows a single anomaly at $T$${}_{\mathtt{N}}$, reflecting the connection between Fe-As distance and Fe local moment. The origin of the anisotropic in-plane charge correlation lengths ${\xi }_a$ and ${\xi }_b$ is discussed on the basis of the antiphase magnetic domains and their dynamic fluctuations.

We describe a superconducting device capable of producing laser light in the visible range at half the Josephson generation frequency, with the optical phase of the light locked to the superconducting phase difference. An earlier proposed device, the so-called “half-Josephson laser” [Phys. Rev. Lett. 107, 073901 (2011)], cannot provide long coherence times, because of spontaneous switchings between the emitter states. To circumvent this we consider $N\gg 1$ emitters driving an optical resonator mode. We derive a general model that captures essential physics of such devices while not depending on specific microscopic details. We find the conditions under which the coherence times are exponentially long, thus surpassing the fundamental limitation on the coherence times of common lasers. For this we study the noise in the device. In particular, we are interested in the rate of large fluctuations of the light field in the limit where the typical fluctuations are small. The large fluctuations are responsible for switching of the laser between stable states of radiation and therefore determine the coherence time.

The shear modulus of solid ${}^4$He below 200 mK exhibits an unusual increase, the characteristics of which show remarkable similarities to those of the period reduction in torsional oscillator experiments. We systematically studied the drive strain and temperature dependence of the shear modulus at low temperatures. The hysteretic behavior depends strongly on the drive and cooling history, which can be associated with the thermally assisted Granato-Lucke dislocation theory. The phase diagram of the shear modulus is constructed on the basis of the emerging hysteretic behavior.

We used high-energy resolution angle-resolved photoemission spectroscopy to extract the momentum dependence of the superconducting gap of Ru-substituted Ba(Fe${}_{0.75}$Ru${}_{0.25}$)${}_2$As${}_2$ ($T_c=15$ K). Despite a strong out-of-plane warping of the Fermi surface, the magnitude of the superconducting gap observed experimentally is nearly isotropic and independent of the out-of-plane momentum. More precisely, we respectively observed 5.7 and 4.5 meV superconducting gaps on the inner and outer $\Gamma$-centered hole Fermi surface pockets, whereas a 4.8 meV gap is recorded on the $M$-centered electron Fermi surface pockets. Our results are consistent with the $J_1-J_2$ model with a dominant antiferromagnetic exchange interaction between the next-nearest Fe neighbors.