Spin-Resolved STM for a Kondo Impurity

We investigate the bias dependence of the tunneling conductance between a spin-polarized (SP) tip of a scanning tunneling microscope (STM) and a metallic surface with a magnetic impurity. The Fano–Kondo interference between the conduction channels tip-host and tip-impurity is considered. We pay particular attention to the interplay between the lateral tip-impurity distance R and the ferromagnetism of the tip. We observe a strong dependence of the conductance with respect to both R and the tip magnetization degree. In particular, for small values of R, a conductance plateau around the Fermi energy is observed, due to the interplay between Kondo effect, quantum interference, and ferromagnetism of the tip. For full polarized tip, we find a shift of the Fano–Kondo profile toward negative bias voltages.     

Vortex Configurations in a Mesoscopic Superconducting Ring Structure: A Finite-Element Analysis

The time-dependent Ginzburg–Landau equations have been solved numerically by a finite-element analysis for a mesoscopic superconducting ring structure. For given applied magnetic fields we have simulated the dynamical behavior of the penetrating magnetic vortices into the superconductor. Moreover, we investigated the vortex configurations and found a vortex state with two stable vortex shells in the mesoscopic superconducting ring due to the enhanced surface superconductivity.     

Persistent Currents Versus Phase Breaking in Mesoscopic Metallic Samples

Persistent currents in mesoscopic normal metal rings represent, even a decade after their first experimental observation, a challenge to both, theorists and experimentalists. After giving a brief review of the existing - experimental and theoretical - results, we concentrate on the (propsed) relationship of the size of the persistent current to the phase breaking rate. In particular, we consider effects induced by noise, scattering at two-level systems, and magnetic impurities.     

Current Cross Correlators in Mesoscopic Conductors

We present an investigation of current cross correlators in mesoscopic conductors. Making an analogy to the optical Hanbury Brown Twiss experiment we discuss how quantum statistical effects and two-particle interference effects can be investigated with current cross correlations. We also discuss how current cross correlations can be used for detecting two-particle entanglement and to perform quantum state tomography, a complete reconstruction of the density matrix of the quantum state emitted from a mesoscopic conductor.     

Mesoscopic 3D Charge Transport in Solution-Processed Graphene-Based Thin Films: A Multiscale Analysis

Graphene and related 2D material (GRM) thin films consist of 3D assembly of billions of 2D nanosheets randomly distributed and interacting via van der Waals forces. Their complexity and the multiscale nature yield a wide variety of electrical characteristics ranging from doped semiconductor to glassy metals depending on the crystalline quality of the nanosheets, their specific structural organization ant the operating temperature. Here, the charge transport (CT) mechanisms are studied that are occurring in GRM thin films near the metal-insulator transition (MIT) highlighting the role of defect density and local arrangement of the nanosheets. Two prototypical nanosheet types are compared, i.e., 2D reduced graphene oxide and few-layer-thick electrochemically exfoliated graphene flakes, forming thin films with comparable composition, morphology and room temperature conductivity, but different defect density and crystallinity. By investigating their structure, morphology, and the dependence of their electrical conductivity on temperature, noise and magnetic-field, a general model is developed describing the multiscale nature of CT in GRM thin films in terms of hopping among mesoscopic bricks, i.e., grains. The results suggest a general approach to describe disordered van der Waals thin films.     

Vortex States and Magnetization Properties in Mesoscopic Superconducting Ring Structures: A Finite-Element Analysis

The time-dependent Ginzburg–Landau equations have been solved numerically by a finite-element analysis for mesoscopic superconducting ring structures with different inner radii. For given applied magnetic fields, we have studied the influences of the inner radius on the vortex states and the magnetization properties of these systems. Our results show that the multivortex states can be stabilized in the mesoscopic superconducting ring with proper inner radius. Magnetization curves show that the magnetic vortices penetrate easily into the superconductor, and the system is magnetized easily for the superconducting ring with smaller inner radius.     

Impurity Effect of d-wave High-Tc Superconductors in the Presence of the Charge Modulation

Based on a phenomenological model, we study theoretically the impurity effect in d-wave high-Tc superconductors in the presence of the charge-density-wave (CDW) order. Our results verify that the intensity of the resonance state induced by the impurity depends on the CDW scattering strength. It first increases and then decreases as the CDW scattering strength increases. Moreover, the impurity state exist only near the impurity site without the CDW modulation, while it would appear at quite far away from the impurity site in presence of the CDW modulation. The above results may be tested by later scanning tunneling microscopy experiments.     

Nuclear spin-lattice relaxation in a Kondo superconductor

The nuclear spin-lattice relaxation rate R _s in a Kondo superconductor has been calculated numerically in the framework of the Müller-Hartmann-Zittartz depairing theory. The calculations have been performed in the case of the usual Korringa mechanism for certain concentrations of magnetic impurities and some typical values of T _K/T _c0, where T _K is the Kondo temperature and T _c0 is the superconducting transition temperature in the absence of magnetic impurities. The temperature variation of R _s shows very interesting behavior for values of T _K/T _c0corresponding to the reentrance behavior of superconductivity. Results are compared with the experimental data on the La(Ce)Al₂ system obtained by MacLaughlin, Alloul, and Daugherty. Other possible relaxation mechanisms in the La(Gd, Ce)Al₂ systems are also discussed.     

不同热化学还原LiNbO_3：Ti或Mn晶体杂质电荷态和点缺陷

采用光学吸收和电子顺磁共振（ＥＳＲ）技术表征不同热化学还原ＬＩＮｂＯ３：Ｔｉ，Ｍｌｉ（ＬＮ：Ｔｉ，Ｍｎ）和纯的Ｌｉ／Ｎｂ＝０．９４５一致熔化ＬｉＮｂＯ３（ＬＮ）晶体的热力学还原习性．将ＬＮ：Ｔｉ（厚度为１ｍｍ）样品放在Ｌｉ２ＣＯ３中、６００℃、保温７ｈ，产生６９０ｕｒｎ（～１．８ｅＶ，Ｔ＝６７％）和峰值靠近７８５ｎｍ（Ｔ＝７１％）的７７０￣８１０ｎｍ光学吸收带，它们分别对应于Ｔｉ（３＋）的２Ｔ→２Ｅ跃迁以及室温稳定Ｆ＋心滞有一个电子的氧空位）．经真空１．２Ｐａ，８００℃２ｈ还原后，存在峰值为６７５ｎｍ（Ｔ＝５２％）的４８０～７８０ｎｍ平滑吸收带，它们是Ｔｉ（３＋）、Ｆ心和Ｆ＋心重叠吸收，但是，在Ａｒ气氛下、９００℃、８ｈ处理后，仅能看到峰值在６７５ｎｍ（Ｔ＝５２％）的６００～７８０ｎｍＴｉ（３＋）的弱吸收．来自未处理ＬＮ：Ｔｉ晶体的室温和Ｘ带的ＥＳＲ＄观察到ｇ＝４．３４８，共振磁场０．１５２ＴＨ（ｐ－ｐ）＝０．０１６３Ｔ微波吸收峰，以及四组精细结构Ｂ线（每一ｆｓ线是由６条超精细结构ｈｆｓ组成），ｇ值从３．４６０～１．６７９吸收，它们分别归为于晶体杂质Ｆｅ（３＋）和Ｍｎ（２＋）离子．真空还原后，Ｆｅ（３＋）的     

Critical magnetic fields of a Kondo superconductor:ZnMn

The superconducting critical fields of pure Zn and of a series ofZnMn alloys have been measured as a function of temperatureT down to 0.06 K. The critical fields have been used to calculate the entropy difference between the normal and superconducting states. For all the alloys studied, the entropy in the superconducting state is proportional toT asT 0, indicating a finite density of states at the Fermi energy. We interpret this result as evidence for the existence of localized bound states within the energy gap centered at the Mn impurities. Results obtained for the depression ofT _c , for the critical field atT=0, for the jump in specific heat atT _c , and for the law of corresponding states are also presented and compared with the predictions of the Abrikosov-Gor'kov theory. A simple expression relating the critical fields of these alloys to the Mn concentration has been found to be in very good agreement with experiment.Research supported by the National Science Foundation.     

Kondo effect in superconductors: The magnetic susceptibility

The vertex-part integral equation is adopted for the study of the Kondo problem in superconductors. The bound-states obtained are in good agreement with other results, but we have obtained two critical values for the bound states as a consequence of the two signs forJ (thes-d interaction). The temperature dependence of the magnetic susceptibility has been calculated using the result for the vertex function.     

Reversible switching of Kondo resonance in a single-molecule junction

The control of the Kondo effect is of great interest in single-molecule junction due to its potential applications in spin based electronics.Here,we demonstrate that the Kondo effect is reversibly switched on and off in an iron phthalocyanine(FePc)single-molecule junction by using a superconducting Nb tip.In a scanning tunneling microscope-based Nb-insulator-FePc-Au junction,we achieve a reversible switching between the Kondo dip and inelastic electronic tunneling spectra by simply adjusting the tip-sample distance to tune the tunnel coupling at low temperature.Further approaching the tip leads to the picking up of the molecule to the tip apex,which transfers the geometry of the single-molecule junction into a Nb-FePc-insulator-Au type.As the molecule forms an effective magnetic impurity embedded into the superconducting ground states of the Nb tip,the out-gap Kondo dip switched to an in-gap Yu-Shiba-Rusinov state.Our results open up a new route for manipulating the Kondo effect within a single-molecule junction.     

Magnetic Flux Penetration in a Mesoscopic Superconductor with a Slit

In this work, the vortex nucleation process in a mesoscopic squared superconductor with a slit is numerically investigated in terms of time dependent Ginzburg–Landau theory. We have calculated, simultaneously, the Gibbs free energy and the vortex configuration in function of time, it allows to identify the correspondence between special points in the Gibbs free energy and the configuration of the vortex system.     

Random-Matrix Theory: Distribution of Mesoscopic Supercurrents Through a Chaotic Cavity

We investigate the distribution of the supercurrent through a chaotic quantum dot which is strongly coupled to two superconductors when the Thouless energy is large compared to the superconducting energy gap. The distribution function of the critical currents (I_c) is known to be Gaussian in the limit of large channel number, N. For N=1, we present an analytical low-temperature expression for this distribution function, valid both in the presence and in the absence of time-reversal symmetry. It connects directly the distribution of transmission coefficients to the distribution of critical currents. The case of arbitrary channel number (N2) is discussed numerically, and for small critical currents analytically.     

ZrO_2-Y_2O_3∶Co或Ce晶体热处理前后的杂质电荷态和禁带迁移

空气中生长(YSZ)ZrO_2-Y_2O_3(88∶12mol%)∶0.3wt%CoO 晶体的透射光谱呈现二个部分:<500nm 短波部分强的非结构吸收归为 Co~(3+)+O~(2-)→Co~(2+)+O~-电荷转移;以及六配位 Co~(2+)离子特征吸收,~4T_1(F)→~4T_1(P)560nm 和~4T_1(F)→~4A_2(F)605nm.该晶体经木炭包裹下加热到900℃后,除了禁带从3.97ev 变为4.43eV 外,短波部分和500～800nm(特别是500～605nm)范围吸收降低,然而,整个谱形尚无变化.未处理晶体包含 Co~(3+)和 Co~(2+),特殊热处理使 Co~(3+)转化为 Co~(2+),晶体由紫色变为蓝色.原来吸收边(3.97eV)是氧的 p 价带的电子到 Co~(2+)+F~+激发态造成的,4.43eV 新的吸收边认为是直接光学还原 Zr~(4+)→Zr~(3+)以及部分未还原 Co~(2+)离子共同作用.YSZ∶0.3wt%CeO_2晶体经真空、1300℃、6h 还原后,连结 Ce~(3+)离子基态~4F_(5/2)(4f)到5d 态跃迁的380～600nm(峰值在460nm)宽带吸收增加,晶体颜色由浅黄色变为桔黄.它的禁带由4.00eV 变为4.43eV.为了证实 YSZ∶Ce 晶体缺陷结构的存在和杂质离子价态,测量了这个晶体热处理前后的电子顺磁共振(ESR).由单电荷(Y_(Zr)V_0)′自由自旋和 Fe~(3+)离子的 ESR 信号变化,反推铈杂质在 YSZ 晶体中的价态变化.     

Transport properties in a superconducting proximity sandwich. Kondo effect

Temperature-dependent properties are evaluated in a superconducting Kondo alloy induced by the proximity effect. These include (i) the order parameter, (ii) the dc Josephson current, and (iii) the thermal conductivity and ultrasonic attenuation, in the case when there is reentrance of superconductivity. The calculations are based on the theory given by Kaiser, in which the McMillan tunneling model and the Müller-Hartmann-Zittartz theory are combined. The temperature dependence of pair-breaking is reflected directly in the above properties in much the same way as in an intrinsic Kondo superconductor, indicating that proximity-effect studies may provide detailed information on the Kondo effect in superconductivity.     

Gate-tunable split Kondo effect in a carbon nanotube quantum dot

We show a detailed investigation of the split Kondo effect in a carbon nanotube quantum dot with multiple gate electrodes. Two conductance peaks, observed at finite bias in nonlinear transport measurements, are found to approach each other for increasing magnetic field, to result in a recovered zero bias Kondo resonance at finite magnetic field. Surprisingly, in the same charge state, but under different gate configurations, the splitting does not disappear for any value of the magnetic field, but we observe an avoided crossing. We think that our observations can be understood in terms of a two-impurity Kondo effect with two spins coupled antiferromagnetically. The exchange coupling between the two spins can be influenced by a local gate, and the non-recovery of the Kondo resonance for certain gate configurations is explained by the existence of a small antisymmetric contribution to the exchange interaction between the two spins.     

Many-impurity effects in a weakly interacting Kondo system: Comparison of theory with experiment

The theory of the many-impurity Kondo system developed in a previous paper is coupled with a statistical model, using a phenomenological relationship between the magnetic susceptibility and the effective Kondo temperature, to obtain the magnetic susceptibility of a very dilute Kondo system well below the single-impurity Kondo temperature. We find that the magnetic susceptibility can be represented as arising from a single-impurity effect proportional to the impurity concentration c, another term proportional to c ², as well as a much smaller c ³ contribution. The coefficient of the c ² term in has an approximate Curie law behavior at sufficiently low temperatures. The detailed temperature and concentration dependence of is in very good quantitative agreement with experiment, and is the first theoretical explanation of the anomalous magnetic susceptibility for Cu-Fe below T _K. The modification of the susceptibility arises in the theory from the inhibition of the spin-compensated state by the impurity-impurity interaction. The latter renormalizes the ln T term even in the region of temperature where the system is magnetically completely disordered.Part of this work was supported by USAFOR, under contract 73-2430 during a summer visit at Yeshiva University.     

Anderson impurities in a transition metal superconductor. Kondo effect

Using the approach of Matsuura, Ichinose, and Nagaoka to treat Anderson impurities dissolved in a transition metal superconductor, the Kondo effect in a two-band superconductor is studied. It is found that the two-particle propagators for the superconductor are coupled to each other through thes- andd-electron vertex functions, which are obtained as solutions to a set of coupled Bethe-Salpeter equations. By rearranging some of the terms in the two-particle propagators, the pair-breaking parameters for thes andd electrons are obtained. An expression for the decrease in the transition temperature due to the Kondo scattering is obtained.     

Nuclear alignment in a Kondo system as a very low temperature thermometer

Cu doped with less than 1 ppm Mn⁵⁴ appears to offer a number of advantages as a -ray anisotropy thermometer in the temperature range 1 to 20 mK. In order to use such a thermometer it is necessary to know how the hyperfine field at the Mn⁵⁴ nuclei varies with temperature and with applied field. We have studied this -ray anisotropy and find that the hyperfine field is independent of temperature in the range 4 mK <T<10 mK. Further, we have determined the hyperfine field to ±5% as a function of applied field in the range 60 Oe<H _a<1200 Oe. The latter data may be used to determine a Kondo temperature of 64±2 mK for this system.Work performed under the auspices of the U.S. Atomic Energy Commission.