The Andreev conductance in superconductor–insulator–normal metal structures

The Andreev subgap conductance at 0.08–0.2 K in thin-film superconductor (aluminum)–insulator–normal metal (copper, hafnium, or aluminum with iron-sublayer-suppressed superconductivity) structures is studied. The measurements are performed in a magnetic field oriented either along the normal or in the plane of the structure. The dc current–voltage (I–U) characteristics of samples are described using a sum of the Andreev subgap current dominating in the absence of the field at bias voltages U < (0.2–0.4)Δ_c/e (where Δ_c is the energy gap of the superconductor) and the single-carrier tunneling current that predominates at large voltages. To within the measurement accuracy of 1–2%, the Andreev current corresponds to the formula \({I_n} + {I_s} = {K_n}\tanh \left( {{{eU} \mathord{\left/ {\vphantom {{eU} {2k{T_{eff}}}}} \right. \kern-\nulldelimiterspace} {2k{T_{eff}}}}} \right) + {K_s}{{\left( {{{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} \right)} \mathord{\left/ {\vphantom {{\left( {{{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} \right)} {\sqrt {1 - {{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} }}} \right. \kern-\nulldelimiterspace} {\sqrt {1 - {{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} }}\) following from a theory that takes into account mesoscopic phenomena with properly selected effective temperature T _eff and the temperature- and fieldindependent parameters K _n and K _s (characterizing the diffusion of electrons in the normal metal and superconductor, respectively). The experimental value of K _n agrees in order of magnitude with the theoretical prediction, while K _s is several dozen times larger than the theoretical value. The values of T _eff in the absence of the field for the structures with copper and hafnium are close to the sample temperature, while the value for aluminum with an iron sublayer is several times greater than this temperature. For the structure with copper at T = 0.08–0.1 K in the magnetic field B_|| = 200–300 G oriented in the plane of the sample, the effective temperature T _eff increases to 0.4 K, while that in the perpendicular (normal) field B _⊥ ≈ 30 G increases to 0.17 K. In large fields, the Andreev conductance cannot be reliably recognized against the background of single- carrier tunneling current. In the structures with hafnium and in those with aluminum on an iron sublayer, the influence of the magnetic field is not observed.     

Negative differential conductance in nano-scale normal metal/superconductor/normal metal junctions featuring Fe1 + y Te1 − x Se x

Iron-based superconductors have been the subject of intensive study due to their high transition temperature and intriguing physical mechanisms. We describe a unique experimental approach to fabricate nano-scale normal metal/superconductor/normal metal junctions involving microcrystals of Fe₁ ?₊?_y Te₁ ?_??_x Se _x , for which we have observed a distinct phenomenon of negative differential conductance (NDC) dips along with multiple plateau features in differential conductance spectra. The evolution of the NDC dips and the plateau features is further explored as a function of both temperature and magnetic field, and their physical origin is discussed.     

Tunneling conductance oscillations in spin–orbit coupled metal–insulator–superconductor junctions

The tunneling conductance for a device consisting of a metal–insulator–superconductor (MIS) junction is studied in presence of Rashba spin–orbit coupling (RSOC) via an extended Blonder–Tinkham–Klapwijk formalism. We find that the tunneling conductance as a function of an effective barrier potential that defines the insulating layer and lies intermediate to the metallic and superconducting electrodes, displays an oscillatory behavior. The tunneling conductance shows high sensitivity to the RSOC for certain ranges of this potential, while it is insensitive to the RSOC for others. Additionally, when the period of oscillations is an odd multiple of a certain value of the effective potential, the conductance spectrum as a function of the biasing energy demonstrates a contrasting trend with RSOC, compared to when it is not an odd multiple. The explanations for the observation can be found in terms of a competition between the normal and Andreev reflections. Similar oscillatory behavior of the conductance spectrum is also seen for other superconducting pairing symmetries, thereby emphasizing that the insulating layer plays a decisive role in the conductance oscillations of a MIS junction. For a tunable Rashba coupling, the current flowing through the junction can be controlled with precision.     

Elementary Andreev processes in a driven superconductor–normal metal contact

We investigate the full counting statistics of a voltage-driven normal metal(N)–superconductor(S) contact. In the low-bias regime below the superconducting gap, the NS contact can be mapped onto a purely normal contact, albeit with doubled voltage and counting fields. Hence in this regime the transport characteristics can be obtained by the corresponding substitution of the normal metal results. The elementary processes are single Andreev transfers and electron- and hole-like Andreev transfers. Considering Lorentzian voltage pulses we find an optimal quantization for half-integer Levitons.     

Influence of the in-plane magnetic field on the Andreev conductance of a superconductor–insulator–normal metal structure

Pronounced conductance due to electrons experiencing Andreev reflection from a superconducting condensate has been observed in superconductor (aluminum)–insulator (aluminum oxide)–normal metal (copper) tunnel junctions at low voltages, along with single-electron tunneling. It has been discovered experimentally that the collective current is suppressed in the magnetic field parallel to the tunnel junction plane and the Andreev conductance decreases nearly twofold in a field of ～20–30 mT.     

正常金属/dx2-y2+idxy混合波超导隧道结中的微分电导

运用Bogoliubov-de Gennes(BdG)方程和Blonder-Tinkham-Klapwijk(BTK)理论,计算了正常金属/dx2-y2 idxy混合波超导隧道结中的准粒子输运系数和微分电导.研究表明:(1)影响电导谱中零偏压电导峰滑移的因素有杂质散射、dxy波分量、混合波两分量的强度比、界面的势垒散射强度、超导晶轴方位等,其中d-xy波分量的存在和超导晶轴方位是关键因素;(2)在θ=π/4的情况下,零偏压电导峰出现的条件为Δ2=0或α=nπ/4;(3)粒子的入射角对电导峰的高低有显著影响.     

LDA’+DMFT investigation of electronic structure of K1 − x Fe2 − y Se2 superconductor

We investigate electronic structure of the new iron chalcogenide high temperature superconductor K_1?x Fe_2?y Se₂ (hole doped case with x = 0.24, y = 0.28) in the normal phase using the novel LDA’+DMFT computational approach. We show that this iron chalcogenide is more correlated in a sense of bandwidth renormalization (energy scale compression by factor about 5 in the interval ±1.5 eV), than typical iron pnictides (compression factor about 2), though the Coulomb interaction strength is almost the same in both families. Our results for spectral densities are in general agreement with recent ARPES data on this system. It is found that all Fe-3d(t _2g ) bands crossing the Fermi level have equal renormalization, in contrast to some previous interpretations. Electronic states at the Fermi level are of predominantly xy symmetry. Also we show that LDA’+DMFT results are in better agreement with experimental spectral function maps, than the results of conventional LDA+DMFT. Finally we make predictions for photoemission spectra lineshape for K_0.76Fe_1.72Se₂.     

硅烯/dx2-y2+idxy混合波超导隧道结隧道输运性质研究

运用Blonder-Tinkham-Klapwijk(BTK)理论研究了硅烯/dx2-y2 +idxy 混合波超导隧道结的隧穿性质.研究发现:垂直施加的电场、超导配对势的方向角和两种混合波配对能隙的比值Δ1/Δ0 强烈地影响正常反射、Andreev反射和隧穿电导的值;当两种混合波的序参量比值较大时,隧道谱线在外加偏压E =Δ1 处出现谐振峰;系统的隧穿电导、正常反射幅和Andreev反射幅随超导方向角成周期性变化,变化周期为π/2;由于dxy-波的存在,通过改变外加电场可以对隧穿电流加以调控。     

Electron paramagnetic resonance of Gd3+ ions in Ca1 − x − y Y x Gd y F2 + x + y crystals

Electron paramagnetic resonance of Ca_{1 ? x ? y} Y _x Gd _y F_{2 + x + y} single crystals has revealed spectra that are not typical of gadolinium-doped CaF₂ crystals. These spectra have a nearly tetragonal symmetry and are most probably caused by Gd³⁺ ions localized in yttrium clusters. Weak spectra of tetragonal Gd³⁺ centers, whose parameters are close to those of a cubic gadolinium center caused by an isolated Gd³⁺ ion, have been also detected. These centers are attributed to isolated Gd³⁺ ions localized near octahedral rare-earth clusters or their associations.     

Non-isovalent alkali metal “substitution” in YBa2Cu3O7−y granular ceramics

The aim of this paper is to study the influence of non-isovalent doping in YBa₂Cu₃O_7–y in particular on its synthesis conditions and on the resistive properties both with and without a magnetic field. We concentrate on the study of possible alkali ions (Na, K, Cs) substitution at the barium sites. A low temperature sintering process is used in order to induce a reactive liquid phase. The final chemical composition is discussed as a function of the amount of the liquid phase. No alkali ion is substituted. Carbonate layers are present. However, this (lack of) substitution leads to induced vacancies and improved electrical transport properties which are as good as in highly pure materials. For conciseness the case of Na substitution only is illustrated. The use of such data in order to probe the microstructure is emphasized.     

Electric field induced metal–insulator transition in VO2 thin film based on FTO/VO2/FTO structure

A VO₂ thin film has been prepared using a DC magnetron sputtering method and annealing on an F-doped SnO₂ (FTO) conductive glass substrate. The FTO/VO₂/FTO structure was fabricated using photolithography and a chemical etching process. The temperature dependence of the I–V hysteresis loop for the FTO/VO₂/FTO structure has been analyzed. The threshold voltage decreases with increasing temperature, with a value of 9.2 V at 20 °C. The maximum transmission modulation value of the FTO/VO₂/FTO structure is 31.4% under various temperatures and voltages. Optical modulation can be realized in the structure by applying an electric field.     

Above-gap and subgap differential conductance anomaly in concentrated magnetic semiconductor Zn0.32Co0.68O1 − v/Pb superconductor hybrid junctions

The Zn_0.32Co_0.68O_1 − v/Pb hybrid junctions were prepared, where the concentrated magnetic semiconductor Zn_0.32Co_0.68O_1 − v is in the region of variable range hopping transport instead of the ballistic or diffusive transport. The high differential conductance peak at gap voltage and two above-gap peaks were observed below the superconducting critical temperature. Moreover, both the zero bias conductance peak and the finite bias conductance peak were observed below the gap voltage. All these differential conductance peaks systematically evolve and finally disappear as the temperature or the magnetic field increases. These transport phenomena were explained by phase coherent Andreev reflection in the presence of strong disorder, magnetic impurity scattering, and spin polarization.     

Unconventional magnetic phase diagram of cuprate superconductor La2−xSrxCuO4 at quantum critical point x=1/9

We propose a new magnetic phase diagram of La_2?xSr_xCuO₄ around a quantum critical point x=1/9 based on field-cooled magnetization measurements and critical fittings. A new phase boundary T_m2(H) is discovered which buries deeply below the first order vortex melting line in the vortex solid phase. The coupling between superconductivity and antiferromagnetism is found to be attractive below T_m2(H) while repulsive above. The attractive coupling between superconducting order and static antiferromagnetic order provides compelling experimental evidence that the antiferromagnetism microscopically coexists and collaborates with the high temperature superconductivity in cuprates.     

Parity violation in the 17/2−–17/2+ doublet in93Tc

The parity-violating mixing of the 17/2^– and 17/2⁺ levels in⁹³Tc nuclei, polarized by the tilted multifoil interaction, was measured by the observation of the forward-backward-ray asymmetry. The nuclear polarization, induced by the tilted multifoils, was measured directly for the neighboring^88,90Zr isomers. The forward to backward asymmetry was determined to be A=(2.5±2.1) 10^–3 which implies a parity violating matrix element ¦H _PV )¦=(4.0±3.7)meV.     

Femtosecond mid-IR pump-probe spectroscopy of photoinduced insulator to metal transition in dimer Mott insulator κ-(BEDT-TTF)2X

Ultrafast dynamics of the photoinduced insulator (I) to metal (M) transition were investigated using femtosecond mid-infrared pump-probe spectroscopy in two-dimensional organic Mott insulators [bis (ethylenedithio)]-tetrathiafulvalene (BEDT-TTF) salts κ-(BEDT-TTF)₂X (κ-(ET)₂X, where X denotes anion). In κ-(d-ET)₂Cu[N(CN)₂]Br, a metallic state was photogenerated using a phonon-mediated mechanism: the effective bandwidth increases through the photoinduced molecular rearrangement. The mechanism differs fundamentally from the previously reported photoinduced filling control in one-dimensional Mott insulators.     

Theoretical explanations to the EPR g factors for Er3+ ion in La2−xSrxCuO4 superconductor

The electron paramagnetic resonance (EPR) g factors g_∥ and g_⊥ for Er³⁺ ion in La_2−xSr_xCuO₄ superconductor are theoretically explained by using the perturbation formulas of g factors for a 4f¹¹ ion in tetragonal symmetry. In these formulas, the contributions to the g factors arising from the second-order perturbation terms and the admixture of different states are included. The related crystal field parameters are calculated from the superposition model and the local structural parameters of the impurity Er³⁺ occupying the host La³⁺ site, and the superposition model parameters used in this work are comparable with those for similar tetragonal Er³⁺ centers in some zircon compounds in the previous work. The theoretical studies on g factors of Er³⁺ ion in this work would be of some use to experimentalists doing EPR on La_2−xSr_xCuO₄ (or other superconductors) with Er dopants.     

Incomplete magnetic ordering in Fe2(1−y)Mg1+y Ti y O4 spinels with intermediate compositiony

Static magnetization measurements on the ferrimagnetic spinels Fe_2(1?y)Mg_1+y Ti _y O₄ withy=0.3, 0.4, 0.5, and 0.6 show that these compounds have no well-defined orderdisorder transition temperature and that their ferrimagnetism may not be described in terms of the Néel theory. From the Mössbauer spectra we conclude that a temperature dependent number of the ferric ions does not participate in the ferrimagnetism of those compounds with compositiony≧0.4. The explanation of the observed magnetic and Mössbauer properties is based on the assumption that each ferric ion must have at least two magnetic linkages of the type Fe _A ³⁺ ?O^2??Fe _B ³⁺ in order to couple its magnetic moment to the neighbouring ones over the entire temperature interval between 0 K and the respective Néel temperature.