Point-contact spectroscopy in metals

Anharmonic theory of superconductivity and its applications to emerging quantum materials

The role of anharmonicity on superconductivity has often been disregarded in the past. Recently, it has been recognized that anharmonic decoherence could play a fundamental role in determining the superconducting properties (electron-phonon coupling, critical temperature, etc) of a large class of materials, including systems close to structural soft-mode instabilities, amorphous solids and metals under extreme high-pressure conditions. Here, we review recent theoretical progress on the role of anharmonic effects, and in particular certain universal properties of anharmonic damping, on superconductivity. Our focus regards the combination of microscopic-agnostic effective theories for bosonic mediators with the well-established BCS theory and Migdal-Eliashberg theory for superconductivity. We discuss in detail the theoretical frameworks, their possible implementation within first-principles methods, and the experimental probes for anharmonic decoherence. Finally, we present several concrete applications to emerging quantum materials, including hydrides, ferroelectrics and systems with charge density wave instabilities.

Thermal and electrical transport at nanosized metallic contacts: In the diffusive-ballistic region at room temperature

The Wiedemann-Franz law has been proved at the quantized metallic contacts but has never been verified at the nanosized contacts when the electrons travel in the diffusive-ballistic region. Herein, by developing a home-made inelastic tunneling spectroscope, the electrical and thermal resistances of the nanosized metallic contacts are investigated. The contact is established by pressing two wires crosswise against each other under the Lorentz force in the magnetic field. The nonmetallic surface layer is in situ removed by the resistive heating under high vacuum. The temperature dependence of the electrical contact resistance is used to separate the contributions from the diffusive and the ballistic transports. The thermal contact resistance is found to increase linearly with the electrical counterpart, indicating the validity of the Wiedemann-Franz law at the clean metallic contacts.

Anomalous Nonlinear Shot Noise at High Voltage Bias

Since the work of Walter Schottky, it is known that the shot-noise power for a completely uncorrelated set of electrons increases linearly with the time-averaged current. At zero temperature and in the absence of inelastic scattering, the linearity relation between noise power and average current is quite robust, in many cases even for correlated electrons. Through high-bias shot-noise measurements on single Au atom point contacts, we find that the noise power in the high-bias regime shows highly nonlinear behavior even leading to a decrease in shot noise with voltage. We explain this nonlinearity using a model based on quantum interference of electron waves with varying path difference due to scattering from randomly distributed defect sites in the leads, which makes the transmission probability for these electrons both energy and voltage dependent.

Point contact tunneling spectroscopy apparatus for large scale mapping of surface superconducting properties

We describe the design and testing of a point contact tunneling spectroscopy device that can measure material surface superconducting properties (i.e., the superconducting gap Δ and the critical temperature T(C)) and density of states over large surface areas with size up to mm(2). The tip lateral (X,Y) motion, mounted on a (X,Y,Z) piezo-stage, was calibrated on a patterned substrate consisting of Nb lines sputtered on a gold film using both normal (Al) and superconducting (PbSn) tips at 1.5 K. The tip vertical (Z) motion control enables some adjustment of the tip-sample junction resistance that can be measured over 7 orders of magnitudes from a quasi-ohmic regime (few hundred Ω) to the tunnel regime (from tens of kΩ up to few GΩ). The low noise electronic and LabVIEW program interface are also presented. The point contact regime and the large-scale motion capabilities are of particular interest for mapping and testing the superconducting properties of macroscopic scale superconductor-based devices.

Theory of point contact spectroscopy in correlated materials

We developed a microscopic theory for the point-contact conductance between a metallic electrode and a strongly correlated material using the nonequilibrium Schwinger-Kadanoff-Baym-Keldysh formalism. We explicitly show that, in the classical limit, contact size shorter than the scattering length of the system, the microscopic model can be reduced to an effective model with transfer matrix elements that conserve in-plane momentum. We found that the conductance dI/dV is proportional to the effective density of states, that is, the integrated single-particle spectral function A(ω = eV) over the whole Brillouin zone. From this conclusion, we are able to establish the conditions under which a non-Fermi liquid metal exhibits a zero-bias peak in the conductance. This finding is discussed in the context of recent point-contact spectroscopy on the iron pnictides and chalcogenides, which has exhibited a zero-bias conductance peak.

Spear-anvil point-contact spectroscopy in pulsed magnetic fields

We describe a new design and experimental technique for point-contact spectroscopy in non-destructive pulsed magnetic fields up to 70 T. Point-contact spectroscopy uses a quasi-dc four-point measurement of the current and voltage across a spear-anvil point-contact. The contact resistance could be adjusted over three orders of magnitude by a built-in fine pitch threaded screw. The first measurements using this set-up were performed on both single-crystalline and exfoliated graphite samples in a 150 ms, pulse length 70 T coil at 4.2 K and reproduced the well known point-contact spectrum of graphite and showed evidence for a developing high field excitation above 35 T, the onset field of the charge-density wave instability in graphite.

Ultra low power consumption for self-oscillating nanoelectromechanical systems constructed by contacting two nanowires

We report here the observation of a new self-oscillation mechanism in nanoelectromechanical systems (NEMS). A highly resistive nanowire was positioned to form a point-contact at a chosen vibration node of a silicon carbide nanowire resonator. Spontaneous and robust mechanical oscillations arise when a sufficient DC voltage is applied between the two nanowires. An original model predicting the threshold voltage is used to estimate the piezoresistivity of the point-contact in agreement with the observations. The measured input power is in the pW-range which is the lowest reported value for such systems. The simplicity of the contacting procedure and the low power consumption open a new route for integrable and low-loss self-excited NEMS devices.

Aluminum conducts better than copper at the atomic scale: a first-principles study of metallic atomic wires

Using a first-principles density functional method, we have studied the electronic structure, electron-phonon coupling, and quantum transport properties of atomic wires of Ag, Al, Au, and Cu. Non-equilibrium Green's function-based transport studies of finite atomic wires suggest that the conductivity of Al atomic wires is higher than that of Ag, Au, and Cu in contrast to the bulk where Al has the lowest conductivity among these systems. This is attributed to the higher number of eigenchannels in Al wires, which becomes the determining factor in the ballistic limit. On the basis of density functional perturbation theory, we find that the electron-phonon coupling constant of the Al atomic wire is lowest among the four metals studied, and more importantly, that the value is reduced by a factor of 50 compared to the bulk.

Scaling limits of resistive memories

This paper is intended to provide an expository, physics-based, framework for the estimation of the performance potential and physical scaling limits of resistive memory. The approach taken seeks to provide physical insights into those parameters and physical effects that define device performance and scaling properties. The mechanisms of resistive switching are based on atomic rearrangements in a material. The three model cases are: (1) formation of a continuous conductive path between two electrodes within an insulating matrix, (2) formation of a discontinuous path of conductive atoms between two electrodes within an insulating matrix and (3) rearrangement of charged defects/impurities near the interface between the semiconductor matrix and an electrode, resulting in contact resistance changes. The authors argue that these three model mechanisms or their combinations are representative of the operation of all known resistive memories. The central question addressed in this paper is: what is the smallest volume of matter needed for resistive memory? The two related tasks explored in this paper are: (i) resistance changes due to addition or removal of a few atoms and (ii) stability of a few-atom system.

What Part do Adhesion and Deformation Play in Fine-Scale Static and Sliding Contact?

We describe experiments involving the static or sliding contact of a 'single asperity' and a flat surface. Plastic deformation and possibly junction growth can occur even at zero applied load, as a consequence of surface forces. The conditions for this to occur, and the different modes of separation of an adhesive contact, are clarified with the help of 'maps': use of the macroscopic concepts involved, such as yield stress and work of adhesion, is sometimes questionable and evidence of the importance of nanometre-scale stepped topography is presented. Strong adhesion between hard non-metals can required thermal activation. Sliding contacts at the sub-micrometre level have shown behaviour ranging from 'unstable ploughing' to adhesive peeling, but the frictional mechanisms involved are often unclear.

Observation of phonons with resonant inelastic x-ray scattering

Phonons, the quantum mechanical representation of lattice vibrations, and their coupling to the electronic degrees of freedom are important for understanding thermal and electric properties of materials. For the first time, phonons have been measured using resonant inelastic x-ray scattering (RIXS) across the Cu K-edge in cupric oxide (CuO). Analyzing these spectra using an ultra-short core-hole lifetime approximation and exact diagonalization techniques, we can explain the essential inelastic features. The relative spectral intensities are related to the electron-phonon coupling strengths.

Modeling and detecting localized nonlinearity in continuum systems with a multistage transform

A general method is presented for modeling spatially extended systems that may contain a localized source of nonlinearity. It has direct applications to structural health monitoring (SHM) where physical damage may cause such nonlinearity and also communications channels which may exhibit localized nonlinearity due to bad electrical contacts or component nonlinearity. The method uses a multistage nonlinear transform in order to model the system dynamics. We discuss the application to SHM and provide a preliminary test of the method with experimental data from a randomly shaken beam with loose bolts. We discuss the application to telecommunications, provide an experimental observation of symmetric nonlinearity in a "bad" electrical contact, and provide a preliminary test of using this method to remove nonlinear echo (and thereby improve data rate) on a telephone line used for data transmission.

Passing current through touching molecules

The charge flow from a single C(60) molecule to another one has been probed. The conformation and electronic states of both molecules on the contacting electrodes have been characterized using a cryogenic scanning tunneling microscope. While the contact conductance of a single molecule between two Cu electrodes can vary up to a factor of 3 depending on electrode geometry, the conductance of the C(60)-C(60) contact is consistently lower by 2 orders of magnitude. First-principles transport calculations reproduce the experimental results, allow a determination of the actual C(60)-C(60) distances, and identify the essential role of the intermolecular link in bi- and trimolecular chains.

Andreev reflection and order parameter symmetry in heavy-fermion superconductors: the case of CeCoIn(5)

We review the current status of Andreev reflection spectroscopy on the heavy fermions, mostly focusing on the case of CeCoIn(5), a heavy-fermion superconductor with a critical temperature of 2.3 K. This is a well-established technique to investigate superconducting order parameters via measurements of the differential conductance from nanoscale metallic junctions. Andreev reflection is clearly observed in CeCoIn(5) as in other heavy-fermion superconductors. Considering the large mismatch in Fermi velocities, this observation seemingly appears to disagree with the Blonder-Tinkham-Klapwijk (BTK) theory. The measured Andreev signal is highly reduced to the order of maximum ∼13% compared to the theoretically predicted value (100%). The background conductance exhibits a systematic evolution in its asymmetry over a wide temperature range from above the heavy-fermion coherence temperature down to well below the superconducting transition temperature. Analysis of the conductance spectra using the extended BTK model provides a qualitative measure for the superconducting order parameter symmetry, which is determined to be the d(x(2)-y(2)) wave in CeCoIn(5). It is found that existing models do not quantitatively account for the data, which we attribute to the intrinsic properties of the heavy fermions. A substantial body of experimental data and extensive theoretical analysis point to the existence of two-fluid components in CeCoIn(5) and other heavy-fermion compounds. A phenomenological model is proposed employing a Fano interference effect between two conductance channels in order to explain both the conductance asymmetry and the reduced Andreev signal. This model appears plausible not only because it provides good fits to the data but also because it is highly likely that the electrical conduction occurs via two channels, one into the heavy-electron liquid and the other into the conduction electron continuum. Further experimental and theoretical investigations will shed new light on the mechanism of how the coherent heavy-electron liquid emerges out of the Kondo lattice, a prototypical strongly correlated electron system. Unresolved issues and future directions are also discussed.

Electron-plasmon and electron-electron interactions at a single atom contact

The transition from tunneling to contact is investigated by detecting light emitted from Au(111) in a scanning tunneling microscope. Optical spectra reflect single and multielectron processes and their distinct evolutions as a single-atom contact is formed. The experimental data are analyzed in terms of plasmon excitation and hot-hole processes.

Changing exchange bias in spin valves with an electric current

We show that a high-density electric current, injected from a point contact into an exchange-biased spin valve, systematically changes the exchange bias. The bias can either increase or decrease depending upon the current direction. This observation is not readily explained by the well-known spin-transfer torque effect in ferromagnetic metal circuits, but could be evidence for the recently predicted current-induced torques in antiferromagnetic metals.

Structural contributions to charge transport across Ni-octanedithiol multilayer junctions

We report the fabrication and characterization of multilayer thin films incorporating 1,8-octanedithiols and Ni atoms. Low-temperature charge transport measurements exhibit inelastic co-tunneling and resonant tunneling features that correspond energetically to vibrational excitations of the molecular multilayer. Several junctions exhibit changes in conductance features characteristic of charge defect-gating. Transport through our junctions is shown to be dominated by the intrinsic properties of the multilayer.

From ballistic transport to tunneling in electromigrated ferromagnetic breakjunctions

We fabricate ferromagnetic nanowires with constrictions whose cross section can be reduced gradually from 100 x 30 nm(2) to the atomic scale and eventually to the tunneling regime by means of electromigration. The contacts are mechanically and thermally stable. We measure low-temperature magnetoresistances (MR) < 3% for contacts < 400 Omega, reproducible MR variations that are nonmonotonic in the regime 400 Omega - 25 kOmega, and a maximum MR of 80% for atomic-scale widths. These results for devices > 400 Omega differ from previous room-temperature studies of electrodeposited devices. For samples in the tunneling regime, we observe large fluctuations in MR, between -10 and 85%.

Spin-dependent transport in molecular tunnel junctions

We present measurements of magnetic tunnel junctions made using a self-assembled-monolayer molecular barrier. Ni-octanethiol-Ni samples were fabricated in a nanopore geometry. The devices exhibit significant changes in resistance as the angle between the magnetic moments in the two electrodes is varied, demonstrating that low-energy electrons can traverse the molecular barrier while remaining spin polarized. An analysis of the voltage and temperature dependence of the data suggests that the spin-polarized transport signals can be degraded by localized states in the molecular barriers.

Current-driven excitations in symmetric magnetic nanopillars

We study experimentally the current-driven magnetic excitations in symmetric Co/Cu/Co nanopillars. In contrast with all the previous observations where the current of only one polarity is capable of exciting a multilayer system saturated by an externally applied magnetic field, we observe that both polarities of the applied current trigger excitations in a symmetric multilayer. This may indicate that in symmetric structures the current propels high-frequency magnetic oscillations in all magnetic layers. We argue, however, that only one layer is excited in our multilayers but, interestingly, currents of opposite polarities excite different layers. This hypothesis is supported by modeling the spin accumulation in symmetric magnetic multilayers.

Current-induced spin-wave excitations in a single ferromagnetic layer

Evidence for a current-induced spin-transfer torque effect has been investigated in a series of point contacts to single ferromagnetic layers. At specific current densities, abrupt resistance changes, similar to those attributed to current-induced spin-wave excitations in multilayers, have been observed for one current polarity. The critical current for these resistance changes depends linearly on the external field applied perpendicular to the layer. The observed effect is interpreted as a current-driven heterogeneous instability in an otherwise uniform ferromagnetic layer.

Measurement of the conductance of a hydrogen molecule

Recent years have shown steady progress towards molecular electronics, in which molecules form basic components such as switches, diodes and electronic mixers. Often, a scanning tunnelling microscope is used to address an individual molecule, although this arrangement does not provide long-term stability. Therefore, metal-molecule-metal links using break-junction devices have also been explored; however, it is difficult to establish unambiguously that a single molecule forms the contact. Here we show that a single hydrogen molecule can form a stable bridge between platinum electrodes. In contrast to results for organic molecules, the bridge has a nearly perfect conductance of one quantum unit, carried by a single channel. The hydrogen bridge represents a simple test system in which to understand fundamental transport properties of single-molecule devices.

Determination of the spin polarization of half-metallic CrO(2) by point contact Andreev reflection

Andreev reflection at a Pb/CrO(2) point contact has been used to determine the spin polarization of single-crystal CrO(2) films made by chemical vapor deposition. The spin polarization is found to be 0.96 +/- 0.01, which confirms that CrO(2) is a half-metallic ferromagnet, as theoretically predicted.

Generation and detection of phase-coherent current-driven magnons in magnetic multilayers

The magnetic state of a ferromagnet can affect the electrical transport properties of the material; for example, the relative orientation of the magnetic moments in magnetic multilayers underlies the phenomenon of giant magnetoresistance. The inverse effect--in which a large electrical current density can perturb the magnetic state of a multilayer--has been predicted and observed experimentally with point contacts and lithographically patterned samples. Some of these observations were taken as indirect evidence for current-induced excitation of spin waves, or 'magnons'. Here we probe directly the high-frequency behaviour and partial phase coherence of such current-induced excitations, by externally irradiating a point contact with microwaves. We determine the magnon spectrum and investigate how the magnon frequency and amplitude vary with the exciting current. Our observations support the feasibility of a spin-wave maser' or 'SWASER' (spin-wave amplification by stimulated emission of radiation).

Current-induced switching of domains in magnetic multilayer devices

Current-induced switching in the orientation of magnetic moments is observed in cobalt/copper/cobalt sandwich structures, for currents flowing perpendicularly through the layers. Magnetic domains in adjacent cobalt layers can be manipulated controllably between stable parallel and antiparallel configurations by applying current pulses of the appropriate sign. The observations are in accord with predictions that a spin-polarized current exerts a torque at the interface between a magnetic and nonmagnetic metal, due to local exchange interactions between conduction electrons and the magnetic moments.