Point-contact Spectroscopy Research Articles

To the 85th birthday of Prof. Igor K. Yanson — Founder of the method of point-contact spectroscopy

To the 85th birthday of Prof. Igor K. Yanson — Founder of the method of point-contact spectroscopy

A new methodology for studying vortex dynamics based on point-contact spectroscopy

Vortex dynamics has attracted tremendous attention for both fundamental physics and applications of type-II superconductors. However, methods to detect local vortex motion or vortex jump with high sensitivity are still scarce. Here, we fabricated soft point contacts on the clean layered superconductor 2H–NbSe2, which are demonstrated to contain multiple parallel micro-constrictions by scanning electronic microscopy. Andreev reflection spectroscopy was then studied in detail for the contacts. Differential conductance taken at fixed bias voltages was discovered to vary spontaneously over time in various magnetic fields perpendicular to the sample surface. The conductance variations become invisible when the field is zero or large enough, or parallel to the sample surface, which can be identified as the immediate consequence of vortex motion across a finite number of micro-constrictions. These results demonstrate point contact Andreev reflection spectroscopy to be a new potential way with a high time resolution to study the vortex dynamics in type-II superconductors.

Quantum dephasing of kagome superconductivity

Recent observations for a pressurized kagome superconductor through transport, muon spin relaxation, nuclear magnetic resonance, and point contact spectroscopy demonstrate striking fluctuating superconductivity at a magic pressure. This discovery may hint at a new quantum dephasing mechanism, which can be discussed from the perspectives of correlation and topology. Three outstanding questions should be considered in decoding this mechanism: one is the intrinsic phase fragile nature of kagome superconductivity; second is the role of Coulomb interactions within different charge-ordered phases; third is the geometrical phase compatibility between superconductivity and different charge orders.

Interfacial Superconductivity and Zero Bias Peak in Quasi‐One‐Dimensional Bi2Te3/Fe1+yTe Heterostructure Nanostructures

AbstractBismuth telluride/iron telluride (Bi2Te3/Fe1+yTe) heterostructures are known to exhibit interfacial superconductivity between two non‐superconducting materials: Fe1+yTe as the parent compound of Fe‐based superconducting materials and the topological insulator Bi2Te3. Here, a top‐down approach is presented starting from 2D heterostructures to fabricate 1D Bi2Te3/Fe1+yTe nanowires or narrow nanoribbons. It is demonstrated that the Bi2Te3/Fe1+yTe heterostructure remains intact in nanostructures of widths on the order of 100 nm and the interfacial superconductivity is preserved, as evidenced by electrical transport and Andreev reflection point contact spectroscopy experiments measured at the end of the nanowire. The differential conductance shows a similar superconducting twin‐gap structure as in 2D heterostructures, but with enhanced fluctuation effects due to the lower dimensionality. A zero‐bias conductance peak indicates the presence of an Andreev bound state and, given the involvement of the topological Bi2Te3 surface state, a possible topological nature of superconductivity is discussed with strong interplay with an emerging ferromagnetism due to the interstitial excess iron (Fe) in the Fe1+yTe layer, developing in parallel with superconductivity at low temperatures.

Spectroscopic evidence for the superconductivity of elemental metal Y under pressure

Very high applied pressure induces superconductivity with the transition temperature (Tc) exceeding 19 K in elemental yttrium, but relatively little is known about the nature of that superconductivity. From point-contact spectroscopy (PCS) measurements in a diamond anvil cell (DAC), a strong enhancement in the differential conductance is revealed near the zero-biased voltage owing to Andreev reflection, a hallmark of the superconducting (SC) phase. Analysis of the PCS spectra based on the extended Blonder-Tinkham-Klapwijk (BTK) model indicates two SC gaps at 48.6 GPa, where the large gap ΔL is 3.63 meV and the small gap ΔS is 0.46 meV. When scaled against a reduced temperature, both small and large SC gaps collapse on a single curve that follows the prediction from BCS theory. The SC gap-to-Tc ratio is 8.2 for the larger gap, and the initial slope of the upper critical field is −1.9 T/K, indicating that Y belongs to a family of strongly coupled BCS superconductors. The successful application of PCS to Y in DAC environments demonstrates its utility for future research on other pressure-induced high-Tc superconductors.

Spectroscopy of electron-phonon interaction in β phase of mo-Re alloy

The mechanism of electron-phonon interaction in the Mo0.73Re0.27 alloy in the Ag-I-Mo0.73Re0.27 tunnel contact, where the dielectric I is formed on natural MoReOx oxide, was investigated for the first time. This approach helped to avoid the proximity effect, which is inherent in a wide range of contacts with the oxidized Al layer. Due to the lack of a negative proximity effect, which leads to a zero anomaly in the tunneling conductivity, the scale of which exceeds the fine structure of the electron-phonon interaction, it was possible to obtain the characteristics of the phonon spectrum of the studied molybdenum-rhenium alloy. The obtained results are in good agreement with the data from thermal conductivity measurements, point-contact and neutron spectroscopy.

Emergent superconducting fluctuations in compressed kagome superconductor CsV3Sb5

The recent discovery of superconductivity (SC) and charge density wave (CDW) in kagome metals AV3Sb5 (A = K, Rb, Cs) provides an ideal playground for the study of emergent electronic orders. Application of moderate pressure leads to a two-dome-shaped SC phase regime in CsV3Sb5 accompanied by the destabilizing of CDW phase. Nonetheless, the nature of this pressure-tuned SC state and its interplay with the CDW are yet to be explored. Here, we perform soft point-contact spectroscopy (SPCS) measurements in CsV3Sb5 to investigate the evolution of superconducting order parameter with pressure. Surprisingly, we find that the superconducting gap is significantly enhanced between the two SC domes, at which the zero-resistance temperature is suppressed and the transition is remarkably broadened. Moreover, the temperature-dependence of the SC gap in this pressure range severely deviates from the conventional Bardeen-Cooper-Schrieffer (BCS) behavior, evidencing for strong Cooper pair phase fluctuations. These findings reveal the complex intertwining of the CDW with SC in the compressed CsV3Sb5, suggesting striking parallel to the cuprate superconductor La2−xBaxCuO4. Our results point to the essential role of charge degree of freedom in the development of intertwining electronic orders, and thus provide new constraints for theories.

Role of interface and valley-mixing scattering in a ferromagnetic silicene/superconductor junction

We theoretically investigate the subgap transport in a ferromagnetic silicene/superconductor junction and highlight the role of the interface. It is demonstrated that the subgap conductance spectra for the continuous and discontinuous interface models are significantly different. In particular, the intravalley Andreev reflection (AR) may occur at the discontinuous interface, and then the subgap conductance will be remarkably enhanced when the silicene sheet is highly valley polarized. This novel AR resulted from the valley-mixing scattering, which is absent in the continuous interface model. The influence of the interface coupling strength on the subgap conductance is also computed. Our results can be directly tested experimentally using scanning tunnel microscope measurements and/or point-contact spectroscopy.

Tip-induced superconductivity enhancement in single-crystalline PdSb by point-contact spectroscopy

We report our mechanical point-contact spectroscopy (MPCS) study on single-crystalline superconductor PdSb with ${T}_{c}\ensuremath{\sim}$ 1.32 K and ${\ensuremath{\mu}}_{0}{H}_{c2}(0)\ensuremath{\sim}$ 900 Oe, whose ${T}_{c}$ changes very slightly against hydrostatic pressure below 2.5 GPa. For MPCS with a soft Au tip, the Andreev reflection signals suggest the same superconducting temperature ${T}_{c}$ as in the pristine PdSb crystal and the conductance curves show a fully gapped behavior, yielding a superconducting gap value of ${\mathrm{\ensuremath{\Delta}}}_{0}=$ 0.23 meV with $2{\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{\mathrm{B}}{T}_{c}\ensuremath{\sim}$ 4.25 in the strong-coupling regime. However, for MPCS with a hard W tip, an enhanced superconducting ${T}_{c}$ can be observed with a concomitant increase in the superconducting gap $\mathrm{\ensuremath{\Delta}}$ in a linear relation of 0.109 meV/kelvin, whereas $2{\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{\mathrm{B}}{T}_{c}$ decreases from 4.25 to 3.5 in a negative correlation with ${T}_{c}$. From first-principles calculations, we observe a Lifshitz transition related to Pd-$4{d}_{{z}^{2}}$ orbitals under uniaxial pressure, which is absent under hydrostatic pressure. Such Lifshitz transition results in an enhanced density of states near the Fermi level $n({E}_{F})$, which is in line with the enhanced ${T}_{c}$. In addition, a soft phonon mode at $H$ is hardened under uniaxial pressure, suggesting a reduced coupling strength in this case. Such local strain effect provides a natural explanation for our MPCS results on PdSb.

Fully-gapped superconductivity in single crystalline NbC and TaC probed by point-contact spectroscopy

We report our point-contact spectroscopy (PCS) study on the single-crystalline transition metal carbide NbC and TaC with a superconducting transition temperature = 12.0 and 10.2 K, respectively. The differential conductance curves from both soft-PCS and mechanical-PCS (MPCS) on NbC at 1.9 K can be well fitted by a single-gap s-wave Blonder–Tinkham–Klapwijk model, and the temperature dependent gap follows a standard Bardeen–Cooper–Schrieffer (BCS) behavior, yielding = 2.0 ± 0.3 meV and 2/ = 3.9 ± 0.3 in the intermediate coupling regime. Similarly, our MPCS data on TaC crystals also show its superconducting gap as a function of temperature follows the BCS behavior, yielding = 1.7 ± 0.1 meV and 2/ = 3.9 ± 0.1. Our results thus support that both NbC and TaC are fully gapped s-wave superconductors.

Andreev reflection in the enhanced superconducting phase of Cu0.04PdTe2

A unique superconducting phase where type I and type II superconductivity coexist was seen below 1.7 K in the Dirac semimetal PdTe2. The mixed superconducting phase was attributed to the local variation of the Ginzburg–Landau parameter(κ) in PdTe2. In such a scenario, it is imperative that introduction of disorder would make the system homogeneously type II. Here, we report our study of Point contact Andreev Reflection Spectroscopy on the 4% Cu intercalated PdTe2 single crystals. Cu-intercalates help enhance the critical temperature by donating carriers to the system, and at the same time act as scattering centres thereby reducing the effective κ. Our detailed magnetic field dependent point contact spectroscopy studies show a typical type-II behaviour in Cu-PdTe2. A conventional BTK analysis of the spectra revealed a BCS-like gap Δ0 = 330μeV. With 2Δ0/kBTc=4.25, Cu-PdTe2 falls in the strong coupling regime.

Strong-coupling superconductivity in the kagome metal CsV3Sb5 revealed by soft point-contact spectroscopy

The nature of the superconducting state in kagome metals $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ is a key issue in need of experimental clarification. Here, we report on a study of the superconducting order parameter in the kagome superconductor ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ through simultaneous ``soft'' point-contact spectroscopy and resistivity measurements under both ambient and a hydrostatic pressure. Signatures of two-gap superconductivity are resolved in the soft point-contact spectra, accompanied by an asymmetric excess conductance above ${T}_{c}$. Quantitative analysis based on the two-dimensional Blonder-Tinkham-Klapwijk model reveals an ($s+s$)-wave superconducting gap with $2{\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{B}{T}_{c}\ensuremath{\simeq}7.2$, placing ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ in the strong-coupling regime. The strong-coupling two-gap feature indicates a high electronic density of states (DOS) and possible existence of flat-band-driven multiple van Hove singularities (VHSs) at the Fermi level. The presence of asymmetric excess spectral conductance above ${T}_{c}$ hints at a modest electronic correlation in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$. Under a hydrostatic pressure of 2.1 kbar, the nodeless multigap nature of the superconducting state remains, whereas both the larger gap and the excess spectral conductance are greatly suppressed, accompanied by an enhanced ${T}_{c}$. An estimate of the spectral-weighted gap ratio reveals a weakened coupling strength, indicative of a reduced total superconducting DOS upon pressure. Our results point to key roles of both flat-band-associated VHSs and electronic correlation in the onset of kagome superconductivity and shed some light on the interplay between charge-density-wave order and superconductivity in vanadium-based kagome superconductors.

Anisotropic superconductivity in ZrB12 near the critical Bogomolnyi point

The superconductors with the Ginzburg-Landau (G-L) parameter ($\kappa$) $\sim1/\sqrt{2}$ exist near a critical Bogolomonyi (B) point where they show inter-type domains between type-I and type-II superconductivity. While such physics is well understood for isotropic superconductors, the experimental investigation of the physics of anisotropic superconductors near a critical B-point remains an unattained goal mainly due to the unavailability of model material systems. Theoretically, such superconductors are expected to show type-I or type-II behaviour for definite directions of an applied magnetic field. Here, from directional point-contact Andreev reflection spectroscopy and field-angle dependent ac magnetic susceptibility measurements, we show that ZrB$_{12}$ is an anisotropic superconductor, and it exhibits field-direction dependent type-I and type-II behaviour. These observations match remarkably well with the theoretical expectations for an anisotropic superconductor near a critical B-point. Therefore, our results project ZrB$_{12}$ as a model material system where the physics of inter-type anisotropic superconductivity can be explored experimentally.

Fermi surface gapping in the hidden order state of PrFe4P12 by point-contact spectroscopy

Fermi surface gapping in the hidden order state of PrFe4P12 by point-contact spectroscopy

Nodal multigap superconductivity in the anisotropic iron-based compound RbCa2Fe4As4F2

The 12442 compounds are a recently discovered family of iron-based superconductors, that share several features with the cuprates due to their strongly anisotropic structure, but are so far poorly understood. Here, we report on the gap structure and anisotropy of RbCa2(Fe1−xNix)4As4F2 single crystals, investigated by a combination of directional point-contact Andreev-reflection spectroscopy and coplanar waveguide resonator measurements. Two gaps were identified, with clear signatures of d-wave-like nodal structures which persist upon Ni doping, well described by a two-band d − d state with symmetry-imposed nodes. A large London penetration depth anisotropy was revealed, weakly dependent on temperature and fully compatible with the d − d model.

Strain-sensitive superconductivity in the kagome metals KV3Sb5 and CsV3Sb5 probed by point-contact spectroscopy

The kagome lattice is host to flat bands, topological electronic structures, Van Hove singularities, and diverse electronic instabilities, providing an ideal platform for realizing highly tunable electronic states. Here, we report soft and mechanical point-contact spectroscopy (SPCS and MPCS) studies of the kagome superconductors ${\mathrm{KV}}_{3}{\mathrm{Sb}}_{5}$ and $\mathrm{Cs}{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$. Compared to the superconducting transition temperature ${T}_{\mathrm{c}}$ from specific heat and electrical resistance measurements, significantly enhanced values of ${T}_{\mathrm{c}}$ are observed via the zero-bias conductance of SPCS, which become further enhanced in MPCS measurements. While the differential conductance curves from SPCS can be described by a two-gap $s$-wave model, a single $s$-wave gap reasonably captures the MPCS data, likely due to a diminishing spectral weight of the other gap. The enhanced superconductivity probably arises from local strain caused by the point contact, which also leads to two-gap or single-gap behaviors observed in different point contacts. Our results demonstrate highly strain-sensitive superconductivity in kagome metals $\mathrm{Cs}{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ and ${\mathrm{KV}}_{3}{\mathrm{Sb}}_{5}$, which may be harnessed in the manipulation of possible Majorana zero modes.

Full superconducting gap and type-I to type-II superconductivity transition in single crystalline NbGe2

We report a mechanical point-contact spectroscopy study on the single crystalline NbGe$_2$ with a superconducting transition temperature $T\rm_c$ = 2.0 - 2.1 K. The differential conductance curves at 0.3 K can be well fitted by a single gap s-wave Blonder-Tinkham-Klapwijk model and the temperature dependent gap follows a standard Bardeen-Cooper-Schrieffer behavior, yielding $\Delta_0 \sim$ 0.32 meV and 2$\Delta_0$/$k\rm_{B}$$T\rm_{c}$ = 3.62 in the weak coupling limit. In magnetic field, the superconducting gap at 0.3 K keeps constant up to $H_{c1}\sim$150 Oe and gradually decreases until $H_{c2}\sim$350 Oe, indicating NbGe$_2$ going through a transition from type-I to type-II (possible type-II/1) superconductor at low temperature.

Point-contact spectroscopy of heavy fermion superconductors Ce2PdIn8 and Ce3PdIn11 in comparison with CeCoIn5

We report point-contact spectroscopy measurements on heavy fermion cousins CeCoIn5, Ce2PdIn8 and Ce3PdIn11 to systematically study the hybridization between f and conduction electrons. Below a temperature T*, the spectrum of each compound exhibits an evolving Fano-like conductance shape, superimposed on a sloping background, that suggests the development of hybridization between local f and itinerant conduction electrons in the coherent heavy fermion state below T*. We present a quantitative analysis of the conductance curves with a two-channel model to compare the tunneling process between normal metallic silver particles in our soft point-contact and heavy-fermion single crystals CeCoIn5, Ce2PdIn8 and Ce3PdIn11.

Applications of point-contact spectroscopy in the study of topological materials

The point-contact spectroscopy was first used to study the elementary excitation in solid materials. Then it is widely used to probe the superconducting order parameters when Andreev reflection occurs at the superconductor/normal metal interfaces. With the development of topological materials and topological superconducting materials, it has also played an important role in the detection of the possible topological superconducting order parameters. Especially in recent years, the so-called tip-induced superconductivity has been detected on the surface of various topological semimetals by point-contact, which adds a new application scope to this classical experimental method. In this review, we try to introduce its basic principles and the development, as well as the applications in some fields. Then some respective studies of the topological superconductor candidates and the recently discovered tip-induced superconductivity by point-contact in various topological materials are also briefly reviewed.

Low temperature and high magnetic field performance of a commercial piezo-actuator probed via laser interferometry.

The advances in the fields of scanning probe microscopy, scanning tunneling spectroscopy, point contact spectroscopy, and point contact Andreev reflection spectroscopy to study the properties of conventional and quantum materials under cryogenic conditions have prompted the development of nanopositioners and nanoscanners with enhanced spatial resolution. Piezoelectric-actuator stacks as nanopositioners with working strokes of 10μm and positioning resolution ∼(1-10) nm are desirable for both basic research and industrial applications. However, information on the performance of most commercial piezoelectric actuators in cryogenic environment and in the presence of magnetic fields in excess of 5T is generally not available. In particular, the magnitude, the rate, and the associated hysteresis of the piezo-displacement at cryogenic temperatures are the most relevant parameters that determine whether a particular piezoelectric actuator can be used as a nanopositioner. Here, the design and realization of an experimental setup based on interferometric techniques to characterize a commercial piezoelectric actuator over a temperature range of 2K ≤ T ≤ 260K and magnetic fields up to 6T are presented. The studied piezoelectric actuator has a maximum displacement of 30μm at room temperature for a maximum driving voltage of 75V, which reduces to 1.2μm with an absolute hysteresis of 9.1±3.3nm at T = 2K. The magnetic field is shown to have no substantial effect on the piezo-properties of the studied piezoelectric-actuator stack.