Electric field dependence of excess electrical conductivity below transition temperature in thin superconducting lead films

Results of measurements of the electric field dependence of the excess electrical conductivity are reported in thin superconducting lead films below the transition temperature. It is observed that the normal state sheat resistance R^□_N has some effect on the nonlinearity but the theory of Yamaji still fits well to the experimental data.     

Lower critical field and vortices configuration in 2D superconducting junction arrays

Summary In order to describe properly the magnetic status of a 2D superconducting junction array, one has to consider not only the effect of the screening currents but also that of the particular experimental protocol followed to measure the physical quantities of interest like, for example, the magnetization. We show that the value of the lower critical field,f _c1, of the junction array depends strongly on the intensity of the screening currents,i.e. on the strength of the junction coupling,E _j, and that reliable results can be obtained only by considering the full-inductance matrix. We also show that the magnetic configuration of the vortices may depend on the particular experimental approach followed (static or dynamic) and, even, in some cases, on the initial configuration of the phases. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

Magnetic field induced phase branches of the superconducting transition in two-dimensional square π-loop arrays

Based on the results of explicit forms of free energy density for each possible arrangement of magnetization fluxes in large-scale two-dimensional （2D） square π-loop arrays given by Li et al [2007 Chin. Phys. 16 1450], the field-cooled superconducting phase transition is further investigated by analysing the free energy of the arrays with a simplified symmetrical model. Our analytical result is exactly the same as that obtained in Li＇s paper by means of numerical calculations. It is shown that the phase transition splits into two branches with either ferromagnetic or anti-ferromagnetic flux ordering, which depends periodically on the strength of external magnetic flux φe through each loop and monotonically on the screen parameter β of the loops in the arrays. In principle, the diagram of the phase branches is similar to that of its one-dimensional counterpart. The influence of thermal fluctuation on the flux ordering during the transition from normal to superconducting states of the π-loop arrays is also discussed.     

The charge exchange of slow highly charged ions at surfaces unraveled with freestanding 2D materials

The property of a variable charge state makes ions unique to other types of radiation a material surface can be exposed to. As a consequence of charge exchange between ions and surfaces, energy is transferred to the surface and material damage may be triggered. Furthermore, a changing charge state of the ion alters its slowing down process in solids and has important implications when back-scattered ions are to be measured for material analysis purposes. Over the last decades extensive research was devoted to the understanding of ion charge exchange with solids. Here I review recent progress in this field with special emphasize on slow ions in high charge states. This class of ions allows a detailed analysis of charge exchange in experiments, which employ also ultra-thin solid targets and therefore give experimental access to electronic processes on the femtosecond timescale. In this review I will discuss general properties of charge exchange and present typical experimental techniques. I will also discuss current developments in the modelling and simulation of ion-surface interaction. Recent findings using freestanding 2D materials are discussed as well as results from spectroscopy of emitted secondary particles. The paper concludes with a unified picture of ion charge exchange at surfaces and presents possible applications based on the understanding of the underlying physics.     

电荷非平衡super junction结构电场分布

建立了电荷非平衡情况下super junction(SJ)耐压结构的二维电场分布理论模型. 获得了浓度和宽度非平衡、梯形n-/p- 区和横向线性缓变掺杂三种非平衡SJ结构的电场分布. 理论分析结果与二维器件数值仿真软件MEDICI的仿真结果符合良好. 虽然给出的电场分布为三角级数形式, 但仍能从中获得很多重要信息. 特别地, 由此可求出非平衡SJ结构的峰值电场和耐压. 该结果有助于对SJ结构的深入分析. 关键词： super junction 电场分布 电荷非平衡     

Indications of an electronic phase transition in two-dimensional superconducting YBa2Cu3O(7-x) thin films induced by electrostatic doping

We successfully tuned an underdoped ultrathin YBa2Cu3O(7-x) film into the overdoped regime by means of electrostatic doping using an ionic liquid as a dielectric material. This process proved to be reversible. Transport measurements showed a series of anomalous features compared to chemically doped bulk samples and a different two-step doping mechanism for electrostatic doping was revealed. The normal resistance increased with carrier concentration on the overdoped side and the high temperature (180 K) Hall number peaked at a doping level of p～0.15. These anomalous behaviors suggest that there is an electronic phase transition in the Fermi surface around the optimal doping level.     

Electric field induced phase transition in first order ferroelectrics with large zero point energy

An electric field induced phase transition in first order ferroelectrics with very large zero point energy is studied on the framework of the effective field approach. It is well known that when the zero point energy of a system is relatively large, the ferroelectric behaviour is depressed and no phase transition can be observed. The critical value Ω_cf of zero point energy for whom the phase transition disappears turns out to be dependant on the order of transition. For zero point energies larger than this critical value, a phase transition may be induced applying an external electric field. This temperature dependence of the induced polarization shows a discontinuous step when the applied electric field is weak, but becoming a continuous one at a strong applied electric field. Another critical value of zero point energy Ω_cp>Ω_cf is deduced for which no phase transition at all can be attained.     

Berry phase in superconducting charge qubits interacting with a cavity field

We propose a method for analyzing Berry phase for a multi-qubit system of superconducting charge qubits interacting with a microwave field. By suitably choosing the system parameters and precisely controlling the dynamics, novel connection found between the Berry phase and entanglement creations.     

Dependence of the superconducting transition temperature on the doping level in single-crystalline diamond films

Homoepitaxial diamond layers doped with boron in the 10(20)-10(21) cm(-3) range are shown to be type II superconductors with sharp transitions (approximately 0.2 K) at temperatures increasing from 0 to 2.1 K with boron contents. The critical concentration for the onset of superconductivity in those 001-oriented single-crystalline films is about 5-7 10(20) cm(-3). The H-T phase diagram has been obtained from transport and ac-susceptibility measurements down to 300 mK.     

Electric field gradient in pure hexagonal transition metals

Using a general calculation of the nuclear quadrupole interaction in non-cubic metals which was presented in a previous paper, this article gives an interpretation of experimental data dealing with signs and temperature dependence of the electric field gradient in 3d(Sc, Ti), 4d(Y, Zr, Tc, Ru) and 5d (Hf, Re, Os) transition hexagonal close-packed metals.     

Electric field gradient in transition metal: Scandium

Corrections to results of electric field gradient (EFG) already published [Pramana — J. Phys.41, 443 (1993)] are reported. The corrected net EFG is:q=−8.01×10¹³ esu/cm³ against the published valueq=16.06×10¹³ esu/cm³. The present result agrees reasonably well with the experimental result, |q _expt|=13×10¹³ esu/cm³. Recently, a computational error is detected, which modifies the results of EFG, we have already published [1]. The error was committed mainly in the part that evaluated thep-p contribution [1] to EFG by the conduction electrons. The corrected results are summarized in table 1 which must replace the table 1 of the published work [1]. In addition, the lattice parameters as well as the temperature were also misquoted in the previous work [1]. The right parameters are:a=6.25311 au andc=9.96509 au. The temperature at which EFG’s are calculated is 293 K instead of 11 K as reported before [1]. The discussions and conclusions made in the published work [1] remain almost unchanged except that they now refer to the corrected numbers. Although the corrected net EFG suffers a sign reversal from the one already published [1], the agreement with experiment is still considered reasonably good because the sign of experimental EFG is not determined. The computational error however does not affect the introduction and theory section of the published work [1].     

Electric field gradient in transition metal — scandium

Electric field gradient inhcp transition metal scandium has been calculated as the sum of contributions from lattice ions and conduction electrons. For the lattice contributionq _latt, a point-charge model has been assumed. The contribution from conduction electronsq _el, on the other hand, has been evaluated by carrying out an energy-band calculation using non-local transition-metal model potential. The results obtained are:q _el=−106.11×10¹³ esu/cm³ andq _latt=122.17×10¹³ esu/cm³. The net field gradient (q _el+q _latt) of 16.06×10¹³ esu/cm³ agrees quite well with the experimental result, |q _expt|=13×10¹³ esu/cm³. Directions of further improvement in the theory are discussed.     

Electric field gradient at the Hf site and the phase transition in HfV2

The electric quadrupole interaction at the Hf site in the cubic HfV₂ compound, which shows lattice instability when cooled below 120 K, has been studied using the 482 keV (5/2⁺) state in¹⁸¹Ta. The time differential perturbed angular correlation (TDPAC) studies involving (133+136)-482 keV - cascade at room temperature show that only a fraction of¹⁸¹Ta nuclei see a cubic symmetry while the remaining experience a weak, randomly oriented, electric quadrupole interaction described by a Gaussian frequency distribution with _Q=6.1±1.0 Mrad/sec and the relative width =0.5. There is a dramatic change in TDPAC pattern indicating a phase transition when the compound is cooled down to 77 K. At this temperature the quadrupole interaction frequency is found to be _Q=40.6±3.0 Mrad/sec with =0.35. The experimentally observed EFG both at the Hf and the V sites are compared to the theoretical estimates based on point charge model.     

Electric charge and field in the meniscus of a dielectric fluid

Using the conformal mapping method, we calculate the electric field at the tip of a high-voltage electrode mounted in the injector of fluid particles. The space charge of the dielectric fluid charged from the high-voltage electrode of the injector of the fluid charged particles is determined. We present the results of simulation of the charge and field in the meniscus for VM-1vacuum oil.     

Electric quadrupole interactions in the CdTiO3 perovskite2

The TDPAC technique has been used to measure the EFG at the Cd site in the perovskite CdTiO₃. The experimental results obtained at 293 K are compared with ionic lattice sum calculations of the EFG. In addition measurements at 16 and 77 K show that no considerable modification of the EFG is observed at the Cd site at temperatures below and above the previously described phase transition at 50 K.     

Magnetic field induced metal-insulator transition in semiconductors

Metal-insulator transition in doped semiconductors is investigated in the presence of intense magnetic fields. Using Thomas-Fermi screening function and a screened coulomb potential in the Hamiltonian, a two-parameter varitional calculation leads to metallization. The correlation effect among the electrons is considered through an effectivemass that depends on the spatial seperation between impurities. Results are provided for a many valley semiconductor (Si) and two single valley semiconductors (GaAs and CdTe). Our results show that in a magnetic field the critical concentration at which metallization occurs increases. The correlation effects bring out one order change in the critical concentration values, in intense magnetic fields.     

Metal to insulator transition in epitaxial graphene induced by molecular doping

The capability to control the type and amount of charge carriers in a material and, in the extreme case, the transition from metal to insulator, is one of the key challenges of modern electronics. By employing angle-resolved photoemission spectroscopy we find that a reversible metal to insulator transition and a fine-tuning of the charge carriers from electrons to holes can be achieved in epitaxial bilayer and single layer graphene by molecular doping. The effects of electron screening and disorder are also discussed. These results demonstrate that epitaxial graphene is suitable for electronics applications, as well as provide new opportunities for studying the hole doping regime of the Dirac cone in graphene.