Compelling efforts to improve the critical temperature $\left(T_c\right)$ of superconductors have been made through high-pressure application. Understanding the underlying mechanism behind such improvements is critically important; however, much remains unclear. Here we studied ortho-III ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.73}$ (YBCO) using x-ray scattering under hydrostatic pressure (HP) up to $\sim 6.0\mathrm{GPa}$. We found the reinforced oxygen order of YBCO under HP, revealing an oxygen rearrangement in the Cu-O layer, which evidently shows the charge-transfer phenomenon between the $\mathrm{Cu}{\mathrm{O}}_2$ plane and Cu-O layer. Concurrently, we also observed no disorder-pinned charge-density-wave signature in $\mathrm{Cu}{\mathrm{O}}_2$ plane under HP. This indicates that the oxygen rearrangement modifies the quenched disorder state in the $\mathrm{Cu}{\mathrm{O}}_2$ plane. Using these results, we appropriately explain why pressure condition can achieve higher $T_c$ compared with the optimal $T_c$ under ambient pressure in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$. As an implication of these results, finally we have discussed that the change in disorder could make it easier for ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ to undergo a transition to the nematic order under an external magnetic field.

We present the results of finite-temperature classical Monte Carlo simulations of a strongly spin-orbit-coupled nearest-neighbor triangular-lattice model for the candidate $\mathrm{U}\left(1\right)$ quantum spin liquid ${\text{YbMgGaO}}_4$ at large system sizes. We find a single continuous finite-temperature stripe-ordering transition with slowly diverging heat capacity that completely breaks the sixfold ground-state degeneracy, despite the absence of a known conformal field theory describing such a transition. We also simulate the effect of random-bond disorder in the model, and find that even weak bond disorder destroys the transition by fragmenting the system into very large domains—possibly explaining the lack of observed ordering in the real material. The Imry-Ma argument only partially explains this fragility to disorder, and we extend the argument with a physical explanation for the preservation of our system's time-reversal symmetry even under a disorder model that preserves the same symmetry.

Electron tunneling into a system with strong interactions is known to exhibit an anomaly, in which the tunneling conductance vanishes continuously at low energy due to many-body interactions. Recent measurements have probed this anomaly in a quantum Hall bilayer of the half-filled Landau level, and shown that the anomaly apparently gets stronger as the half-filled Landau level is increasingly spin polarized. Motivated by this result, we construct a semiclassical hydrodynamic theory of the tunneling anomaly in terms of the charge-spreading action associated with tunneling between two copies of the Halperin-Lee-Read state with partial spin polarization. This theory is complementary to our recent work (D. Chowdhury, B. Skinner, and P. A. Lee, arXiv:1709.06091) where the electron spectral function was computed directly using an instanton-based approach. Our results show that the experimental observation cannot be understood within conventional theories of the tunneling anomaly, in which the spreading of the injected charge is driven by the mean-field Coulomb energy. However, we identify a qualitatively new regime, in which the mean-field Coulomb energy is effectively quenched and the tunneling anomaly is dominated by the finite compressibility of the composite Fermion liquid.

Germanium is an indirect semiconductor which attracts particular interest as an electronics and photonics material due to low indirect-to-direct band separation. In this work we bend the bands of Ge by means of biaxial tensile strain in order to achieve a direct band gap. Strain is applied by growth of Ge on a lattice mismatched InGaAs buffer layer with variable In content. Band structure is studied by photoluminescence and photoreflectance, giving the indirect and direct bands of the material. Obtained experimental energy band values are compared with a $\mathbf{k}·\mathbf{p}$ simulation. Photoreflectance spectra are also simulated and compared with the experiment. The obtained results indicate direct band structure obtained for a Ge sample with $1.94\%$ strain applied, with preferable $\mathrm{\Gamma }$ valley to heavy hole transition.

We present a field-theoretic treatment of an adiabatic quantum motor. We explicitly discuss a motor called the Thouless motor which is based on a Thouless pump operating in reverse. When a sliding periodic potential is considered to be the motor degree of freedom, a bias voltage applied to the electron channel sets the motor in motion. We investigate a Thouless motor whose electron channel is modeled as a Luttinger liquid. Interactions increase the gap opened by the periodic potential. For an infinite Luttinger liquid the coupling-induced friction is enhanced by electron-electron interactions. When the Luttinger liquid is ultimately coupled to Fermi liquid reservoirs, the dissipation reduces to its value for a noninteracting electron system for a constant motor velocity. Our results can also be applied to a motor based on a nanomagnet coupled to a quantum spin Hall edge.

Photocurrent generation is unexpected in metallic 2D layered materials unless a photothermal mechanism is prevalent. Yet, typical high thermal conductivity and low absorption of the visible spectrum prevent photothermal current generation in metals. Here, we report photoresponse from two-terminal devices of mechanically exfoliated metallic 3R-$\mathrm{Nb}{\mathrm{S}}_2$ thin crystals using scanning photocurrent microscopy (SPCM) both at zero and finite bias. SPCM measurements reveal that the photocurrent predominantly emerges from metal/$\mathrm{Nb}{\mathrm{S}}_2$ junctions of the two-terminal device at zero bias. At finite biases, along with the photocurrent generated at metal/$\mathrm{Nb}{\mathrm{S}}_2$ junctions, now a negative photoresponse from all over the $\mathrm{Nb}{\mathrm{S}}_2$ crystal is evident. Among our results, we realized that the observed photocurrent can be explained by the local heating caused by the laser excitation. These findings show that $\mathrm{Nb}{\mathrm{S}}_2$ is among a few metallic materials in which photocurrent generation is possible.

The thermal conductivity of a freestanding single-crystal silicon membrane may be reduced significantly by attaching nanoscale pillars on one or both surfaces. Atomic resonances of the nanopillars form vibrons that intrinsically couple with the base membrane phonons causing mode hybridization and flattening at each coupling location in the phonon band structure. This in turn causes group velocity reductions of existing phonons, in addition to introducing new modes that get excited but are localized and do not transport energy. The nanopillars also reduce the phonon lifetimes at and around the hybridization zones. These three effects, which in principle may be tuned to take place across silicon's full spectrum, lead to a lowering of the in-plane thermal conductivity in the base membrane. Using equilibrium molecular dynamics simulations, and utilizing the concept of vibrons compensation, we report a staggering two orders of magnitude reduction in the thermal conductivity at room temperature by this mechanism. Specifically, a reduction of a factor of 130 is demonstrated for a roughly 10-nm-thick pillared membrane compared to a corresponding unpillared membrane. This amounts to a record reduction of a factor of 481 compared to bulk crystalline silicon and nearly a factor of 2 compared to bulk amorphous silicon. These results are obtained while providing a path for preserving performance with upscaling.

As a fundamental building block for quantum computation and communication protocols, the correct verification of the two-photon interference (TPI) contrast between two independent quantum light sources is of utmost importance. Here, we experimentally demonstrate how frequently present blinking dynamics and changes in emitter brightness critically affect the Hong–Ou–Mandel-type (HOM) correlation histograms of remote TPI experiments measured via the commonly utilized setup configuration. We further exploit this qualitative and quantitative explanation of the observed correlation dynamics to establish an alternative interferometer configuration, which is overcoming the discussed temporal fluctuations, giving rise to an error-free determination of the remote TPI visibility. We prove full knowledge of the obtained correlation by reproducing the measured correlation statistics via Monte Carlo simulations. As an exemplary system, we make use of two pairs of remote semiconductor quantum dots; however, the same conclusions apply for TPI experiments with flying qubits from any kind of remote solid-state quantum emitters.

Evolution of electronic inhomogeneities with back-gate voltage in graphene on ${\mathrm{SiO}}_2$ was studied using room temperature scanning tunneling microscopy and spectroscopy. Reversal of contrast in some places in the conductance maps and sharp changes in cross correlations between topographic and conductance maps, when graphene Fermi energy approaches its Dirac point, are attributed to the change in charge state of interface defects. The spatial correlations in the conductance maps, described by two length scales, and their growth during approach to Dirac point, show a qualitative agreement with the predictions of the screening theory of graphene. Thus a sharp change in the two length scales close to the Dirac point, seen in our experiments, is interpreted in terms of the change in charge state of some of the interface defects. A systematic understanding and control of the charge state of defects can help in memory applications of graphene.

We consider the $(2+1)$-dimensional $SU\left(2\right)$ quantum link model on the honeycomb lattice and show that it is equivalent to a quantum dimer model on the kagome lattice. The model has crystalline confined phases with spontaneously broken translation invariance associated with pinwheel order, which is investigated with either a Metropolis or an efficient cluster algorithm. External half-integer non-Abelian charges [which transform nontrivially under the $\mathbb{Z}\left(2\right)$ center of the $SU\left(2\right)$ gauge group] are confined to each other by fractionalized strings with a delocalized $\mathbb{Z}\left(2\right)$ flux. The strands of the fractionalized flux strings are domain walls that separate distinct pinwheel phases. A second-order phase transition in the three-dimensional Ising universality class separates two confining phases: one with correlated pinwheel orientations, and the other with uncorrelated pinwheel orientations.

Two-particle Green's functions and the vertex functions play a critical role in theoretical frameworks for describing strongly correlated electron systems. However, numerical calculations at the two-particle level often suffer from large computation time and massive memory consumption. We derive a general expansion formula for the two-particle Green's functions in terms of an overcomplete representation based on the recently proposed “intermediate representation” basis. The expansion formula is obtained by decomposing the spectral representation of the two-particle Green's function. We demonstrate that the expansion coefficients decay exponentially, while all high-frequency and long-tail structures in the Matsubara-frequency domain are retained. This representation therefore enables efficient treatment of two-particle quantities and opens a route to the application of modern many-body theories to realistic strongly correlated electron systems.

Electronic transport properties of thermoelectric materials containing resonant levels are discussed by analyzing the two best known examples: copper-nickel metallic alloy (Cu-Ni, constantan) and thallium-doped lead telluride (PbTe:Tl). As a contrasting example of a material with a nonresonant impurity, sodium-doped PbTe is considered. Theoretical calculations of the electronic structure, Bloch spectral functions, and energy-dependent electrical conductivity at $T=0$ K are done using the Korringa-Kohn-Rostoker method with the coherent potential approximation and the Kubo-Greenwood formalism. The effect of a resonance on the residual resistivity and electronic lifetimes in PbTe is analyzed. By using the full Fermi integrals, room-temperature thermopower is calculated, confirming its increase in PbTe:Tl versus PbTe:Na, due to the presence of the resonant level. In addition, our calculations support the self-compensation model, in which the experimentally observed reduction of carrier concentration in PbTe:Tl against the nominal one is explained by the presence of $n$-type Te vacancies.

The polarized excitons and optical activity of single-wall carbon nanotubes (SWNTs) are studied theoretically by $\pi$-electron Hamiltonian and helical-rotational symmetry. By taking advantage of the symmetrization, the single-particle energy and properties of a SWNT are characterized with the corresponding helical band structure. The dipole-moment matrix elements, magnetic-moment matrix elements, and the selection rules can also be derived. Based on different selection rules, the optical transitions can be assigned as the parallel-polarized, left-handed circularly-polarized, and right-handed circularly-polarized transitions, where the combination of the last two gives the cross-polarized transition. The absorption and circular dichroism (CD) spectra are simulated by exciton calculation. The calculated results are well comparable with the reported measurements. Built on the foundation, magnetic-field effects on the polarized excitons and optical activity of SWNTs are studied. Dark-bright exciton splitting and interband Faraday effect in the CD spectrum of SWNTs under an axial magnetic field are predicted. The Faraday rotation dispersion can be analyzed according to the selection rules of circular polarizations and the helical band structure.

We develop an improved variant of $U\left(1\right)$-symmetric infinite projected entangled-pair states (iPEPS) ansatz to investigate the ground-state phase diagram of the spin-$1/2$ square $J_1\text{−}{J}_2$ Heisenberg model. In order to improve the accuracy of the ansatz, we discuss a simple strategy to select automatically relevant symmetric sectors and also introduce an optimization method to treat second-neighbor interactions more efficiently. We show that variational ground-state energies of the model obtained by the $U\left(1\right)$-symmetric iPEPS ansatz (for a fixed bond dimension $D)$ set a better upper bound, improving previous tensor-network-based results. By studying the finite-$D$ scaling of the magnetically order parameter, we find a Néel phase for $J_2/J_1<0.53$. For $0.53<J_2/J_1<0.61$, a nonmagnetic columnar valence bond solid (VBS) state is established as observed by the pattern of local bond energy. The divergent behavior of correlation length $\xi \sim D^{1.2}$ and vanishing order parameters are consistent with a deconfined Néel-to-VBS transition at $J_2^{c_1}/J_1=0.530\left(5\right)$, where estimated critical anomalous exponents are ${\eta }_s\sim 0.6$ and ${\eta }_d\sim 1.9$ for spin and dimer correlations, respectively. We show that the associated VBS order parameter monotonically increases with $J_2/J_1$ and finally a first-order quantum phase transition takes place at $J_2^{c_2}/J_1=0.610\left(2\right)$ to the conventional Stripe phase. We compare our results with earlier DMRG and PEPS studies and suggest future directions for resolving remaining issues.

We construct a new mean-field theory for a quantum (spin-1/2) Heisenberg antiferromagnet in one (1D) and two (2D) dimensions using a Hartree-Fock decoupling of the four-point correlation functions. We show that the solution to the self-consistency equations based on two-point correlation functions does not produce any unphysical finite-temperature phase transition, in accord with the Mermin-Wagner theorem, unlike the common approach based on the mean-field equation for the order parameter. The next-neighbor spin-spin correlation functions, calculated within this approach, reproduce closely the strong renormalization by quantum fluctuations obtained via a Bethe ansatz in 1D and a small renormalization of the classical antiferromagnetic state in 2D. The heat capacity approximates with reasonable accuracy the full Bethe ansatz result at all temperatures in 1D. In 2D, we obtain a reduction of the peak height in the heat capacity at a finite temperature that is accessible by high-order $1/T$ expansions.

In ${\mathrm{Ho}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$, the Ho atoms form a distorted kagome lattice. We performed magnetization, magnetic-susceptibility, specific-heat, and ultrasound measurements on a single crystal. We find that the magnetic and magnetoelastic properties of ${\mathrm{Ho}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$ result from an interplay between geometric frustration and crystal-electric-field (CEF) effects. The Ho atoms order antiferromagnetically at $T_{\text{N}}=4.5\mathrm{K}$ with reduced magnetic moments. In applied field, the magnetization shows anomalies that can be explained by CEF level crossings. We propose a CEF level scheme for which the ground-state doublet and the first two excited singlets at about 2.7 K form a quasiquartet. Indirect interlevel transitions allow for a quadrupolar interaction. This interaction explains well changes in the elastic shear modulus $C_{44}$ as a function of temperature and magnetic field.

We present a scanning probe technique for measuring the dynamics of individual fluxoid transitions in multiply connected superconducting structures. In these measurements, a small magnetic particle attached to the tip of a silicon cantilever is scanned over a micron-size superconducting ring fabricated from a thin aluminum film. We find that near the superconducting transition temperature of the aluminum, the dissipation and frequency of the cantilever changes significantly at particular locations where the tip-induced magnetic flux penetrating the ring causes the two lowest-energy fluxoid states to become nearly degenerate. In this regime, we show that changes in the cantilever frequency and dissipation are well-described by a stochastic resonance (SR) process, wherein small oscillations of the cantilever in the presence of thermally activated phase slips (TAPS) in the ring give rise to a dynamical force that modifies the mechanical properties of the cantilever. Using the SR model, we calculate the average fluctuation rate of the TAPS as a function of temperature over a 32-dB range in frequency, and we compare it to the Langer-Ambegaokar-McCumber-Halperin theory for TAPS in one-dimensional superconducting structures.

We demonstrate that the synchronization of a lattice of solid-state condensates when intersite tunneling is switched on depends strongly on the weak local disorder. This finding is vital for implementation of condensate arrays as computation devices. The condensates here are nonlinear bosonic fluids of exciton-polaritons trapped in a weakly disordered Bose-Hubbard potential, where the nearest-neighboring tunneling rate (Josephson coupling) can be dynamically tuned. The system can thus be tuned from a localized to a delocalized fluid as the number density or the Josephson coupling between nearest neighbors increases. The localized fluid is observed as a lattice of unsynchronized condensates emitting at different energies set by the disorder potential. In the delocalized phase, the condensates synchronize and long-range order appears, evidenced by narrowing of momentum and energy distributions, new diffraction peaks in momentum space, and spatial coherence between condensates. Our paper identifies similarities and differences of this nonequilibrium crossover to the traditional Bose-glass to superfluid transition in atomic condensates.