Compressed H<Subscript>3</Subscript>S,Superfluid Density and the Quest for Room-Temperature Superconductivity

The discovery of superconductivity at 203 K in highly compressed sulphur hydride validates the ideas put forward by Ashcroft 50 years ago and galvanises the quest for room-temperature superconductivity. But at such temperatures, thermal fluctuations might be expected to break up Cooper pairs. For example, in the cuprates, fluctuations reduce T _c by 30% or more below the mean-field value. Similar effects are found in iron pnictides. Here, we ask: how does superconductivity survive in sulphur hydride at such high temperatures? To answer this, we examine the superfluid density which is the key parameter for quantifying fluctuations in both amplitude and phase. We show that dimensionality plays a key role in suppressing or enhancing thermal fluctuations to the benefit of hydrogen sulphide and the detriment of its more layered 2D competitors. We find that the temperature scale for phase fluctuations, T _φ , in superconducting H₃S exceeds 1200 K, and therefore, these are irrelevant at 200 K. But the amplitude fluctuation temperature scale, T _amp, at around 300 K is much lower and this has important implications for the ongoing quest for room temperature superconductivity. Appealing to the way in which superfluid density, T _φ , T _amp and T _c scale with each other it seems that room temperature superconductivity is nearly ruled out, but perhaps not quite. It will require 3D systems with a large Fermi velocity to achieve this goal.     

Rate constant of <Emphasis Type="Italic">VT</Emphasis>/<Emphasis Type="Italic">VV</Emphasis> energy exchange in the collision of di- and polyatomic molecules within the SFO model

The numerical-analytical investigation of the shock forced oscillator (SFO) model is complete. Approaches for calculating the probabilities of quantum transitions from the initial to some final state with VV energy exchange of diatomic molecules and VV and VT energy exchange of polyatomic molecules are considered. Formulas for calculating the probabilities of the \({W_{{i_1},{i_2} \to {f_1},{f_2}}}\) transition for VV energy exchange in collision of molecules AB and CD within the harmonic approximation are represented (SFHO model). It is shown that the probabilities of a quantum transition in VV and VT energy exchange of polyatomic molecules can be calculated in terms of the quantum transition probability for VT energy exchange of diatomic molecules on the assumption of “frozen” quantum transitions of polyatomic molecules. The problem of determining the dissociation rate constant is considered by the example of a nitrogen molecule (N₂) in the N₂–N₂ system for the “improved” Lennard-Jones potential in VV energy exchange. The calculated dissociation rate constant is compared with the experimental data obtained for a shock tube.     

Field-Controllable Spin Filter Based on Parallel Quantum Dot Systems

In this paper, we investigate the spin-polarized transport through parallel N?dot (N = 1, 2, 3) systems in the strongly correlated regime. We focus our attention on the responses of the N _{t o t} = i n t e g e r states to an increasing magnetic field B, where N _{t o t} is the total charge number on the dots. We show that when the charge level ?? is chosen at the particle-hole (p-h) symmetric case, spin filtering is difficult to occur. While if ?? is beyond the p-h symmetric point, perfect spin-polarized currents could be achieved, and the spin directions can be easily manipulated by tuning external electric and/or magnetic fields, making it easy to be realized in future experiments of the ideas. To approach these problems, the celebrated numerical renormalization group (NRG) technique is implemented, the dynamical properties and the quantum fluctuations are shown.     

Preparation and characterization of monodisperse silica nanoparticles via miniemulsion sol–gel reaction of tetraethyl orthosilicate

Monodisperse silica nanoparticles were prepared via miniemulsion sol–gel reaction of tetraethyl orthosilicate (TEOS). Hexadecane (HD) or hexadecyltrimethoxysilane was used as costabilizer to effectively retard the Ostwald ripening process involved in TEOS miniemulsion. The Ostwald ripening behavior was characterized by dynamic light scattering (DLS), and it was adequately described by the modified Kabal’nov equation. The miniemulsion sol–gel reaction of TEOS/HD with a volume fraction (φ _c) of 0.024 at 80 °C is stable in the pH range 6–10. By contrast, gelation of reacting miniemulsions occurs at 70 and 100 min at pH 4 and 5, respectively. The weight-average silica particle size (d _w) of colloidal products prepared at 80 °C and pH 7 decreases from 59 to 36 nm with low polydispersity index (PDI, in the range 1.02–1.03), determined by transmission electron microscopy, when the φ _c of HD increases from 0.024 to 0.23. At constant φ _c (0.024), the resultant silica nanoparticles show larger d _w (83 nm) and PDI (1.35) for the TEOS/HD system at pH 10 as compared to the counterpart of pH 7. Furthermore, for the TEOS/HD system at pH 7 and low φ _c (0.024), d _w increases significantly with temperature being increased from 25 to 80 °C. By contrast, the effect of temperature on silica nanoparticle size becomes insignificant when a high level of HD (φ _c = 0.23) is used. Zeta potential measurements and field emission scanning electron microscopy were used to characterize the surface charge density and morphology of resultant silica nanoparticles.     

How much is a quantum controller controlled by the controlled system?

We consider unitary transformations on a bipartite system A × B. To what extent entails the ability to transmit information from A to B the ability to transfer information in the converse direction? We prove a dimension-dependent lower bound on the classical channel capacity C(A ← B) in terms of the capacity C(A → B) for the case that the bipartite unitary operation consists of controlled local unitaries on B conditioned on basis states on A. If the local operations are given by the regular representation of a finite group G we have C(A → B) = log |G| and C(A ← B) = log N where N is the sum over the degrees of all inequivalent representations. Hence the information deficit C(A → B) ? C(A ← B) between the forward and the backward capacity depends on the “non-abelianness” of the control group. For regular representations, the ratio between backward and forward capacities cannot be smaller than 1/2. The symmetric group S _n reaches this bound asymptotically. However, for the general case (without group structure) all bounds must depend on the dimensions since it is known that the ratio can tend to zero. Our results can be interpreted as statements on the strength of the inevitable backaction of a quantum system on its controller.     

The Anomalous Unloaded Quality Factor Behavior of High T<Emphasis Type="Italic">c</Emphasis> Superconducting Microstrip Resonator in Weak dc Magnetic Field

This paper reports the anomalous behavior of unloaded quality factor (Q _u ) of high T _c superconducting (HTSC) microstrip resonator in weak dc magnetic field (B_dc). The Q _u behavior increases with the elevated applied B_dc, but beyond the certain B_dc value, decreases linearly with B_dc. We speculate that this behavior may be caused by edge effect. The behavior is modeled both by the coupled-grain model accounting for a distribution of the grain dimension and Coffey-Clem model.     

Theoretical Studies on Magnetic Structures,Hysteresis Loops and Size Effects of a Pair of Frustrated Double-Walled Nanotubes

We propose a theoretical quantum model and derive a set of analytic formulas to study the physical properties of a pair of double-walled magnetic nanotubes. The Heisenberg exchange parameters between the two walls of the nanotubes are assumed to differ only in sign. Thus, in the absence of external magnetic field, our calculated macroscopic properties of this pair of nanotubes are almost precisely identical, exhibiting fascinating duality of the nanosystems and demonstrating the correctness of our theoretical model. The two spin systems are all frustrated, so that sudden changes in the macroscopic properties are observed around T _M2 that is well below the transition temperature T _M1. However, only the inner shell consisting of smaller A-type spins has been obviously affected. In the temperature range T _M2 < T < T _M1, this shell becomes semi-antiferromagnetic and its magnetization is considerably suppressed, whereas as temperature falls below T _M2 the shell gradually restores its ferromagnetic nature. The longitudinal hysteresis behavior of such a double-waled nanotube is ferromagnetic-like below T _M2, but antiferromagnetic-like in the temperature interval T _M2 < T < T _M1. Moreover, we find that the diameter of the nanotube has seemly no effects on its physical properties, whereas its length does affects the two temperatures slightly, and also its spin configuration at very low temperatures if the tube is sufficiently long. More importantly, the theoretical results presented in this paper can be precisely reproduced with the quantum computational method we develop in recent years, justifying the validity of the numerical approach once again.     

Synthesis and photoelectric properties of intrinsic oxide-Cd<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te heterostructures

We propose a new technology for the formation of an energy barrier on crystals of Cd_{1 ? x} Mn _x Te solid solutions. Rectifying photosensitive heterostructures of the intrinsic oxide-Cd_{1 ? x} Mn _x Te (x = 0.6?1) type have been obtained for the first time by means of thermal oxidation. Stationary current-voltage characteristics and relative quantum efficiency spectra of these heterostructures have been measured. The energy spectrum and the character of interband transitions in Cd_{1 ? x} Mn _x Te are discussed. It is shown that the thermal oxidation process can be used for the fabrication of heterophotoconverters based on single crystals of dilute magnetic semiconductors of the Cd_{1 ? x} Mn _x Te system.     

Influence of graded interfaces on the exciton energy of type-I and type-II Si/Si<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript> Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> quantum wires

The exciton properties of Si/Si _1-x Ge _x cylindrical quantum wires (QWRs) are calculated using the variational method and taking into account the existence of an interface layer between the materials. We consider two possibilities for the conduction band lineup, type-I and type-II. Our numerical results show that an interfacial fluctuation of 15Å in a Si _0.85 Ge _0.15 (Si_0.70Ge_0.30) type-I (type-II) wire of 50Å wire radius leads to an exciton energy blue shift of the order of 10 (10) meV.     

Effect of Nuclear Field on Magnetotransport Quantum Oscillations in InSb<Superscript>1</Superscript>

In combining magneto-transport investigations with NMR (nuclear magnetic resonance) we measured the effect of the nuclear hyperfine field B _H F on quantum oscillations in the transverse magneto-resistivity ρ _xx and in the Hall resistivity ρ _xy of metallically doped n-InSb. Quantitative analysis of the B _H F-induced change in ρ _xx demonstrates that this experiment allows to separate spin splitting phenomena in magneto-transport from effects due to the external magnetic field B. This is used to show that an oscillatory structure in R _H =ρ _xy /B is directly related to a redistribution in the occupation of the two spin states in the lowest Landau level.     

Spatial and Quantum Confinement of Si Nanoparticles Deposited by Laser Electrodispersion onto Crystalline Si

C–V characteristics and DLTS spectra of heterostructures made up of layers of closely packed amorphous Si nanoparticles deposited by laser electrodispersion onto single-crystal p-Si substrates have been examined. The patterns observed in the behavior of the C–V characteristics and DLTS spectra measured in the dark and under illumination with white light at various bias pulse voltages U _b and filling pulse voltages U _f suggest that the spatially localized amorphous Si nanoparticles have an average size of less than 2 nm, which is comparable with the de Broglie electron wavelength, and are characterized by quantum confinement. The ground and excited states of quantum dots are formed and exhibit the Stark effect and effects of electricdipole and controllable metastable occupancy under illumination.     

Effect of Charge Reservoir Layer on the Structural and Transport Properties of CuTlBa<Subscript>2−Y</Subscript>Sr<Subscript><Emphasis Type="Italic">Y</Emphasis></Subscript>-1223 (<Emphasis Type="Italic">Y</Emphasis> = 0 − 0.25) Superconductor

The bulk superconducting composites Cu_0.5Tl_0.5 Ba_2?YSr _Y Ca₂Cu₃O₁0?δ (Y = 0, 0.15, and 0.25) have been synthesized at an ambient pressure. The techniques used to characterize the samples were X-ray scans, Fourier transform infrared spectroscopy (FTIR), and dc resistivity (ρ) measurements. In CuTl-1223 system, we replaced Sr atom at Ba site and studied the superconducting properties of squeezed charge reservoir layer (CRL). From the XRD analysis, it is confirmed that the samples have orthorhombic structure and the dimensional parameters of the unit cell suppressed with the dopant atom which is most probably due to small in size of Sr atom as compared with Ba. The normal-state resistivity and critical temperatures, i.e., T _c (R = 0) and \(T_{\mathrm {c}}^{\text {onset}}\) are observed to be suppressed. The lower values of critical temperature T _c (R = 0) and activation energy U _o (eV) might be possible due to a weak flux pinning. Accordingly, a reduction of weak links and enhanced insulating nature of inter-grain coupling were observed with the doping of Sr atom in CRL. Moreover, the doping in CRL of Sr atom is also confirmed with the FTIR technique. The intrinsic parameters, i.e., coherence length ξ _c(0), crossover temperatures (T _3D?2D), inter-layer coupling (J), etc. were calculated by fluctuation-induced conductivity (FIC) analysis.     

The surface energy of cryocrystals

An expression for the surface energy σ₀₀ of a crystal at T = 0 and P = 0 is obtained with allowance for the zero-point vibrations of atoms in the crystal. Particular calculations are performed for the cryocrystals of inert gases and hydrogen isotopes, in which the energy of zero-point vibrations is comparable with the energy of interatomic interactions. It is established that σ₀₀ exhibits highly correlated dependences of the same kind on the atomic (molecular) mass m, the melting temperature T_m, and the interatomic interaction potential D/k_B (k_B is the Boltzmann constant), whereby the function σ₀₀(m, T_m, D/k_B) exhibits nonlinear growth with each argument. The ratio σ₀₀/σ_liquid, where σ_liquid is the surface tension of the liquid phase at T = T_m, also exhibits highly correlated dependences on m, T_m, and D/k_B, which can be divided into two parts of linear growth corresponding to the quantum and classical domains. The σ₀₀/σ_liquid ratio, being smaller in the quantum case than in the classical one, grows with m, T_m, and D/k_B much faster in the quantum than in the classical domain.     

Spin Glass in a Geometrically Frustrated Magnet of ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Nanoparticles

We investigated the magnetic properties of a geometrically frustrated magnet of ZnFe₂O₄ nanoparticles by carrying out the comprehensive measurements of dc magnetization and ac susceptibility. Spin glass is confirmed to occur in this weakly Fe(Zn)-ionic-inversed sample. The common characteristics of spin glass including the aging, the memory, and the rejuvenation effects were observed. By using the critical-power law to study the spin dynamics, the spin-glass transition temperature T_g is obtained to be 18 K and the dynamic exponent zν is 8.0. The weak exchange interaction disorder introduced by the ferrimagnetic J_AB interaction, together with the weak nonmagnetic dilute disorder induced by the ionic inversion, is responsible for the formation of spin glass in the ZnFe₂O₄ nanoparticles.     

Explanation of Non-linear In-Plane Resistivity and Hall Coefficient in the Normal State of Cuprates: Polaronic Approach

We present a theoretical study of the in-plane resistivity ρ _{a b} (T) and Hall coefficient R _H (T) within the polaronic model and precursor pairing scenario by considering a two-component charge carrier picture in the normal state of high-temperature superconducting cuprates (HTSC). Here, we use a Boltzmann-equation approach and extended BCS-like model to compute ρ _{a b} (T) and R _H (T) in the τ-approximation. The opening of the pseudogap (PG) in the normal state of the cuprates should affect their transport properties. We have found that the transition to the PG regime and the effective conductivity of charge carriers in the normal state are responsible for the pronounced non-linear temperature dependence of ρ _{a b} and R _H . With the two-component model analysis, we conclude that the opening of the BCS-like PG, while the non-linear temperature dependence of ρ _{a b} and R _H could be understood as a consequence of pairing fluctuations in the PG state of cuprate superconductors. The calculated results for ρ _{a b} (T) and R _H (T) were compared with the experimental data obtained for various hole-doped cuprates. For all the considered cases, a good quantitative agreement was found between theory and experimental data. We also show that the energy scales of the binding energies of charge carriers are identified by PG crossover temperature on the cuprate phase diagram.     

Pseudogap and (An)isotropic Scattering in the Fluctuating Charge-Density Wave Phase of Cuprates

We present a general scenario for high-temperature superconducting cuprates, based on the presence of dynamical charge density waves (CDWs) and to the occurrence of a CDW quantum critical point, which occurs, e.g., at doping p ≈ 0.16 in YBa₂Cu₃O_{6 + δ} (YBCO). In this framework, the pseudogap temperature T ^? is interpreted in terms of a reduction of the density of states due to incipient CDW and, at lower temperature to the possible formation of incoherent superconducting pairs. The dynamically fluctuating character of CDW accounts for the different temperatures at which the CDW onset revealed by X-ray scattering (T _{o n s} (p)), and the static three-dimensional CDW ordering appear. We also investigate the anisotropic character of the CDW-mediated scattering. We find that this is strongly anisotropic only close to the CDW quantum critical point (QCP) at low temperature and very low energy. It rapidly becomes nearly isotropic and marginal-Fermi-liquid-like away from the CDW QCP and at finite (even rather small) energies. This may reconcile the interpretation of Hall measurements in terms of anisotropic CDW scattering (arXiv:1604.07852v1) with recent photoemission experiments Bok, J.M., et al. Sci. Adv. 2, e1501329 (2016).     

Magnetic-Memory Effects in High-Purity Beryllium

We study the magnesium-thermal beryllium condensate in the course of natural aging after several procedures of treatment by weak constant magnetic fields. The amplitude-time dependences of the effective shear modulus (G _ef ~ f ²), low-frequency internal friction, the parameter r, and the ratio of dislocation velocities V _afef /V ₀ are analyzed. It was revealed that the hysteresis in the f ²(γ) curves disappears after ~120 h of natural aging and the sign of r(γ) and V _afef /V ₀(γ) is inverted. The nature of the detected effects is connected with the appearance of microcracks in the stage of elastic deformation and their healing as a result of the viscous motion of unpinned dislocations in the field of point defects in the basal slip plane.     

Effect of Oxygen Pressure on the Surface Roughness and Intergranular Behavior of YBCO Thin Films

YBa₂Cu₃ O _7?δ (YBCO) thin films were deposited by pulsed laser deposition on single-crystal LaAlO₃ (100) substrates in O₂ partial pressure (P _O2) from 400 to 500 mTorr. The XRD data shows the presence of a-axis-oriented grains in the YBCO films deposited at P _O2 = 500 mTorr. The a-axis grains lead to increase of strain. Atomic force microscopy images show as oxygen pressure has increased, average surface roughness of the films and size of droplets were increased. The grain misorientaion could be the reason for high average roughness. However, surface analysis by statistical methods reveals that three surfaces have multi-affine structure irrespective of oxygen pressure. Analysis of the temperature dependence of the AC susceptibility near the transition temperature indicates that with increasing oxygen pressure, intergranular critical current density has decreased. It is also suggested that the nature of weak links in the samples is of superconductor-normal-superconductor (SNS) type irrespective of the oxygen pressure.     

Superconducting current in a thin film of poly(phthalidylidene biphenylene)

We report on the Josephson oscillations observed in a weak transverse magnetic field applied to a superconductor-poly(phthalidylidene biphenylene)-superconductor structure. The weak coupling is provided by point contacts at the polymer-superconductor interface. It is experimentally established that a characteristic scale of the Josephson oscillations depends on the I/I_c ratio, where I_c is the critical current.