We investigate the effect of electron-electron interactions on proximity-induced $s$-wave superconductivity in one-dimensional nanowires. We treat the interactions on a self-consistent mean-field level, and find an analytic expression for the effective pairing potential in the presence of interactions, valid for a weakly tunnel coupled wire. We show that for a set of two nanowires placed in parallel on a superconducting substrate, the interaction-induced reduction of the pairing energy could result in the effective interwire pairing potential exceeding the intrawire potential, which is one of the requirements for creating a time-reversal symmetric topological superconducting state in such a two-wire system.

The conditions of formation of the Au-induced $\mathrm{Si}\text{−}(6\times 6)$ reconstruction, its stability, as well as its atomic structure are studied experimentally using grazing incidence x-ray diffraction techniques. This reconstruction is found to form at 680 K when cooling down eutectic droplets previously obtained by the dewetting of thin gold films deposited at room temperature on a $\mathrm{Si}(111)$ substrate. The quality of the reconstruction is found to depend on the annealing temperature. The formation of the $(6\times 6)$ reconstruction at low temperature after it has been destroyed by ion bombardment, together with the recovery of the Si surface, give evidence of its high stability and highlight the surfactant properties of gold atoms. The determination of the $\mathrm{Si}\left(111\right)\text{−}(6\times 6)\mathrm{Au}$ atomic structure is performed using quantitative surface x-ray diffraction with an existing complex model proposed in the literature as a starting structure. The resulting gold structure is found to be 1 ML thick and consists in two domains related by a mirror. They are composed of trimer and pentagonal units with special sites presenting a partial occupancy of $0.5$. Our experimental data set does not provide enough accuracy to determine the positions of the Si atoms of the substrate, but ab initio calculations tend to confirm that they are only slightly displaced from the bulk position.

Mixtures of light elements with heavy elements are important in inertial confinement fusion. We explore the physics of molecular scale mixing through a validation study of equation of state (EOS) properties. Density functional theory molecular dynamics (DFT-MD) at elevated temperature and pressure is used to obtain the thermodynamic state properties of pure xenon, ethane, and various compressed mixture compositions along their principal Hugoniots. To validate these simulations, we have performed shock compression experiments using the Sandia Z-Machine. A bond tracking analysis correlates the sharp rise in the Hugoniot curve with the completion of dissociation in ethane. The DFT-based simulation results compare well with the experimental data along the principal Hugoniots and are used to provide insight into the dissociation and temperature along the Hugoniots as a function of mixture composition. Interestingly, we find that the compression ratio for complete dissociation is similar for several compositions suggesting a limiting compression for C-C bonded systems.

Electron paramagnetic resonance (EPR) is used to identify a new and unique photoactive silicon-related point defect in single crystals of rutile $\mathrm{Ti}{\mathrm{O}}_2$. The importance of this defect lies in its assignment to interstitial silicon ions and the unexpected establishment of silicon impurities as a major hole trap in $\mathrm{Ti}{\mathrm{O}}_2$. Principal $g$ values of this new $S=1/2$ center are 1.9159, 1.9377, and 1.9668 with principal axes along the $\left[\overline{1}10\right],\left[001\right]$, and $\left[110\right]$ directions, respectively. Hyperfine structure in the EPR spectrum shows the unpaired spin interacting equally with two Ti nuclei and unequally with two Si nuclei. These silicon ions are present in the $\mathrm{Ti}{\mathrm{O}}_2$ crystals as unintentional impurities. Principal values for the larger of the two Si hyperfine interactions are 91.4, 95.4, and 316.4 MHz with principal axes also along the $\left[\overline{1}10\right],\left[001\right]$, and $\left[110\right]$ directions. The model for the defect consists of two adjacent Si ions, one at a tetrahedral interstitial site and the other occupying a Ti site. Together, they form a neutral nonparamagnetic ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}-\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^0$ complex. When a crystal is illuminated below 40 K with 442-nm laser light, holes are trapped by these silicon complexes and form paramagnetic ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}-\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ defects, while electrons are trapped at oxygen vacancies. Thermal anneal results show that the ${[\mathrm{S}{\mathrm{i}}_{\mathrm{int}}-\mathrm{S}{\mathrm{i}}_{\mathrm{Ti}}]}^{+}$ EPR signal disappears in two steps, coinciding with the release of electrons from neutral oxygen vacancies and singly ionized oxygen vacancies. These released electrons recombine with the holes trapped at the silicon complexes.

Quantum simulation of many-body systems are one of the most interesting tasks of quantum technology. Among them is the preparation of a many-body system in its ground state when the vanishing energy gap makes the cooling mechanisms ineffective. Adiabatic theorem, as an alternative to cooling, can be exploited for driving the many-body system to its ground state. In this paper, we study two most common disorders in quantum dot arrays, namely exchange coupling fluctuations and hyperfine interaction, in adiabatic preparation of ground state in such systems. We show that the adiabatic ground-state preparation is highly robust against those disorder effects making it a good analog simulator. Moreover, we also study the adiabatic quantum information transfer, using singlet-triplet states, across a spin chain. In contrast to ground-state preparation the transfer mechanism is highly affected by disorder and in particular, the hyperfine interaction is very destructive for the performance. This suggests that for communication tasks across such arrays adiabatic evolution is not as effective and quantum quenches could be preferable.

We investigate the excitation spectrum of the triangular-lattice antiferromagnetic XXZ model using series expansion and mean field Schwinger boson approaches. The single-magnon spectrum computed with series expansion exhibits rotonic minima at the middle points of the edges of the Brillouin zone, for all values of the anisotropy parameter in the range $0\le J^z/J\le 1$. Based on the good agreement with series expansion for the single-magnon spectrum, we compute the full dynamical magnetic structure factor within the mean field Schwinger boson approach to investigate the relevance of the XXZ model for the description of the unusual spectrum found recently in ${\text{Ba}}_3{\text{CoSb}}_2{\text{O}}_9$. In particular, we obtain an extended continuum above the spin wave excitations, which is further enhanced and brought closer to those observed in ${\text{Ba}}_3{\text{CoSb}}_2{\text{O}}_9$ with the addition of a second neighbor exchange interaction approximately $15\%$ of the nearest-neighbor value. Our results support the idea that excitation continuum with substantial spectral-weight are generically present in two-dimensional frustrated spin systems and fractionalization in terms of bosonic spinons presents an efficient way to describe them.

${\mathrm{FeF}}_3$, with its half-filled ${\mathrm{Fe}}^{3+}3d$ orbital, hence zero orbital angular momentum and $S=5/2$, is often put forward as a prototypical highly frustrated classical Heisenberg pyrochlore antiferromagnet. By employing ab initio density functional theory, we obtain an effective spin Hamiltonian for this material. This Hamiltonian contains nearest-neighbor antiferromagnetic Heisenberg, biquadratic, and Dzyaloshinskii-Moriya interactions as dominant terms and we use Monte Carlo simulations to investigate the nonzero temperature properties of this minimal model. We find that upon decreasing temperature, the system passes through a Coulomb phase, composed of short-range correlated coplanar states, before transforming into an “all-in/all-out” (AIAO) state via a very weakly first-order transition at a critical temperature $T_c\approx 22$ K, in good agreement with the experimental value for a reasonable set of Coulomb interaction $U$ and Hund's coupling $J_{\mathrm{H}}$ describing the material. Despite the transition being first order, the AIAO order parameter evolves below $T_c$ with a power-law behavior characterized by a pseudo “critical exponent” $\beta \approx 0.18$ in accord with experiment. We comment on the origin of this unusual $\beta$ value.

Between 19 and 54 GPa, potassium has a complex composite incommensurate host-guest structure which undergoes two intraphase transitions over this pressure range. The temperature dependence of these host-guest phases is further complicated by the onset of an order-disorder transition in their guest chains. Here, we report single-crystal, quasi-single-crystal, and powder synchrotron x-ray diffraction measurements of this order-disorder phenomenon in incommensurate potassium to 47 GPa and 750 K. The so-called chain melting transition is clearly visible over a 22 GPa pressure range, and there are significant changes in the slope of the phase boundary which divides the ordered and disordered phases, one of which results from the intraphase transitions in the guest structure.

The local atomic structure of the new nonmagnetic superconducting system ${\text{LaO}}_{1-x}{\text{F}}_x{\text{BiS}}_2$ is investigated using neutron diffraction and the pair density function analysis. Evidence for local charge fluctuations linked to a charge disproportionation of the Bi ions in the distorted lattice of superconducting ${\text{LaO}}_{1-x}{\text{F}}_x{\text{BiS}}_2$ is presented. In-plane short-range distortions of sulfur atoms up to 0.3 Å in magnitude break site symmetry and create two distinct environments around Bi. Out-of-plane motion of apical sulfur brings it closer to the La-O/F doping layer with increasing $x$ that may lead to a charge transfer conduit between the doping layers and the superconducting ${\text{BiS}}_2$ planes. The mechanism for superconductivity may arise from the interplay between charge density fluctuations and an enhanced spin-orbit coupling suggested theoretically that induces spin polarization.

We present the results of muon-spin relaxation measurements of spin excitations in the one-dimensional quantum Heisenberg antiferromagnet $\mathrm{Cu}\left(\mathrm{pyz}\right){\left({\mathrm{NO}}_3\right)}_2$. Using density-functional theory we propose muon sites and assess the degree of perturbation the muon probe causes on the system. We identify a site involving the muon forming a hydroxyl-type bond with an oxygen on the nitrate group that is sensitive to the characteristic spin dynamics of the system. Our measurements of the spin dynamics show that in the temperature range $T_{\mathrm{N}}<T<J$ (between the ordering temperature $T_{\mathrm{N}}$ and the exchange energy scale $J$) the field-dependent muon spin relaxation is characteristic of diffusive transport of spin excitations over a wide range of applied fields. We also identify a possible crossover at higher applied fields in the muon probe's response to the fluctuation spectrum, to a regime where the muon detects early-time transport with a ballistic character. This behavior is contrasted with that found for $T>J$ and that in the related two-dimensional system $\mathrm{Cu}{\left(\mathrm{pyz}\right)}_2{\left({\mathrm{ClO}}_4\right)}_2$.

We analyze the transfer of a quantum state between two resonators connected by a superconducting transmission line. Nearly perfect state-transfer efficiency can be achieved by using adjustable couplers and destructive interference to cancel the back-reflection into the transmission line at the receiving coupler. We show that the transfer protocol is robust to parameter variations affecting the transmission amplitudes of the couplers. We also show that the effects of the Gaussian filtering, pulse-shape noise, and multiple reflections on the transfer efficiency are insignificant. However, the transfer protocol is very sensitive to frequency mismatch between the two resonators. Moreover, the tunable coupler we considered produces time-varying frequency detuning caused by the changing coupling. This detuning requires an active frequency compensation with an accuracy better than $90\%$ to yield the transfer efficiency above $99\%$.

A symmetry-protected topologically ordered phase is a short-range entangled state, for which some imposed symmetry prohibits the adiabatic deformation into a trivial state which lacks entanglement. In this paper we argue that magnetization plateau states of one-dimensional antiferromagnets which satisfy the conditions $S-m\in$ odd integer, where $S$ is the spin quantum number and $m$ the magnetization per site, can be identified as symmetry-protected topological states if an inversion symmetry about the link center is present. This assertion is reached by mapping the antiferromagnet into a nonlinear sigma model type effective field theory containing a novel Berry phase term (a total derivative term) with a coefficient proportional to the quantity $S-m$, and then analyzing the topological structure of the ground state wave functional which is inherited from the latter term. A boson-vortex duality transformation is employed to examine the topological stability of the ground state in the absence/presence of a perturbation violating link-center inversion symmetry. Our prediction based on field theories is verified by means of a numerical study of the entanglement spectra of actual spin chains, which we find to exhibit twofold degeneracies when the aforementioned condition is met. We complete this study with a rigorous analysis using matrix product states.

We construct an optimal set of single-particle states for few-electron quantum dots (QDs) using the method of natural orbitals (NOs). The NOs include also the effects of the Coulomb repulsion between electrons. We find that they agree well with the noniteracting orbitals for GaAs QDs of realistic parameters, while the Coulomb interactions only rescale the radius of the NOs compared to the noninteracting case. We use NOs to show that four-electron QDs are less susceptible to charge noise than their two-electron counterparts.

We present a semiclassical theory for the delayed response of a quantum dot (QD) to oscillations of a coupled nanomechanical resonator (NR). We prove that the back action of the QD changes both the resonant frequency and the quality factor of the NR. An increase or decrease in the quality factor of the NR corresponds to either an enhancement or damping of the oscillations, which can also be interpreted as Sisyphus amplification or cooling of the NR by the QD.

We realize the evaporative cooling of a polariton gas and observe the formation of polariton condensate in a potential trap based on a ZnO microwire at room temperature. Due to the three-dimensional confinement, the system is characterized by a well-defined discrete spectrum of polariton states which allows accurate measurements of the polariton population distribution. With increasing pumping power, the polariton gas system is cooled down very efficiently and relaxes closely to the thermal distribution via the polariton evaporative cooling. At an optimized excitation condition, the effective temperature of the polariton gas falls down to 22 K with the condensate fraction reaching nearly 50% in room temperature experiments.

We investigate the nanocrystallite-size and dopant-concentration dependence of the photoluminescence (PL) properties of heavily phosphorus- (P) and boron- (B) codoped Si nanocrystals (Si NCs), prepared using a combination of sputtering and ion implantation techniques. We find that the heavily doped Si NC exhibits three exotic luminescence bands, A, B, and C. The peak energy of band A redshifts with increasing dopant concentration. This band is due to the band-to-band transition at the reduced Si-NC band gap caused by the formation of impurity bands together with band-tailing effects. The PL redshift becomes large when the nanocrystallite size decreases, suggesting the occurrence of the quantum-confinement-induced carrier doping effect. The peak energies of bands B and C are independent of both the concentration and size, indicating that these bands are due to transitions between defect- and/or impurity-related localized states. Band A shows stronger thermal quenching than the PL band in pure (undoped) Si NCs, the magnitude of which depends on the dopant concentration. The stronger thermal quenching in band A is probably due to the thermally induced migration of electrons in the impurity band.

The high pressure-temperature behavior of titanium has been studied with x-ray diffraction in resistively heated and laser-heated diamond anvil cells up to 200 GPa and $\sim 3500$ K. The stability fields of $\alpha \text{−}\mathrm{Ti},\omega \text{−}\mathrm{Ti},\beta \text{−}\mathrm{Ti},\gamma \text{−}\mathrm{Ti}$, and $\delta \text{−}\mathrm{Ti}$ have been determined in this range. $\gamma \text{−}\mathrm{Ti}$ and $\delta \text{−}\mathrm{Ti}$, which had been evidenced earlier under nonhydrostatic compression, are also observed in helium pressure transmitting medium. Equation-of-state parameters are proposed for $\alpha \text{−}\mathrm{Ti}$ and $\omega \text{−}\mathrm{Ti}$ at 300 K, and $\beta \text{−}\mathrm{Ti}$ at high temperature. The stability fields of the $\alpha ,\omega ,\gamma$, and $\delta$ phases are also studied using the projector-augmented wave method based on density-functional theory. Using the relevant core radius to avoid overlapping between atomic spheres, and relaxing cells and atomic positions, we show that all those phases have a stability domain at 0 K. We explain why $\gamma \text{−}\mathrm{Ti}$ and $\delta \text{−}\mathrm{Ti}$ were calculated to be unstable in earlier works. In addition, a new phase, called ${\delta }^{\prime }$-Ti, which is a distortion of $\delta \text{−}\mathrm{Ti}$, is predicted to form between 80 and 120 GPa and below $\simeq 200$ K.

The linear-response transport properties of a T-shaped double quantum dot strongly coupled to external ferromagnetic leads are studied theoretically by using the numerical renormalization group method. It is shown that when each dot is occupied by a single electron, for antiparallel alignment of leads' magnetizations, the system exhibits the two-stage Kondo effect. For parallel alignment, however, the second stage of the Kondo effect becomes suppressed due to the presence of ferromagnetic-contact-induced exchange field. The difference between the two magnetic configurations results in highly nontrivial behavior of the tunnel magnetoresistance, which for some parameters can take giant values. In addition, the dependence of the linear conductance and tunnel magnetoresistance on external magnetic field, the double-dot levels' position, and the spin polarization of the leads is thoroughly analyzed. It is shown that the second stage of the Kondo effect can be restored by fine-tuning of the magnetic field or the dots' levels. The effect of spin-dependent tunneling on the low-temperature transition from the high to low conducting state of the system, which occurs when changing the hopping between the dots, is also discussed.

Motivated by recent experiments on isovalent-doped 1111 iron-based superconductors ${\text{LaFeAs}}_{1-x}{\text{P}}_x{\text{O}}_{1-y}{\text{F}}_y$ and the theoretical study that followed, we investigate, within the five-orbital model, the correlation between spin fluctuations and the superconducting transition temperature, which exhibits a double-dome feature upon varying the Fe-As-Fe bond angle. Around the first dome with higher $T_c$, the low-energy spin fluctuation and $T_c$ are not tightly correlated because the finite-energy spin fluctuation also contributes to superconductivity. On the other hand, the strength of the low-energy spin fluctuation originating from the $d_{xz/yz}$ orbital is correlated with $T_c$ in the second dome with lower $T_c$. These calculation results are consistent with a recent NMR study, and hence strongly suggest that the pairing in iron-based superconductors is predominantly caused by multiorbital spin fluctuations.

Recent experiments have addressed the nature of the charge order seen in underdoped cuprates. Here I show that x-ray absorption and linear dichroism are excellent probes of such order. Ab initio calculations reveal that a $d$-wave charge density wave order involving the oxygen ions is a much better description of the data than alternate models.