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O. E. Kvitnitskaya

Publications and source records attributed to O. E. Kvitnitskaya.

At least 19 recordsLinked to original sources
arXiv2026

Point-contact enhanced superconductivity in trigonal PtBi2: quest for the origin of high-Tc

We studied enhanced superconductivity in point contacts (PCs) based on a type-I Weyl semimetal trigonal t-PtBi2 using both normal metal (Ag, Cu, Pt) and ferromagnetic (Fe, Co, Ni) tips by measuring the differential resistance dV/dI(V) curves. In most cases, the value of the superconducting critical temperature Tc ranges between 3 and 5 K, which is several times higher than the maximum bulk Tc. Notably, among the various PCs we examined, a few achieved Tc values as high as 8 K, including those with both normal and ferromagnetic tips. Additionally, the critical magnetic field is also highly enhanced in these PCs and reaches up to several Tesla. The common reason for the Tc increase may be related to pressure/strain caused during the PC's formation. It is worth noting that a greater increase in Tc is observed in PCs formed at the edge of the sample flake, compared to those formed on the plane of the platelet. The results also reveal that the enhancement of Tc in PCs based on t-PtBi2 is compatible with ferromagnetic tip, which may suggest a potentially complex nature of enhanced superconductivity. Our findings besides suggest that t-PtBi2 is a promising candidate for realizing topological superconductivity at more accessible temperatures.

cond-mat.supr-con↗
arXiv2025

Observation of resistive switching and diode effect in the conductivity of TiTe2 point contacts

We measured the I(V) and dV/dI(V) characteristics of TiTe2-based point contacts (PCs) from room to helium temperatures. Features indicating the emergence of a charge density wave (CDW) were detected. They represent symmetrical relatively V=0 maxima in dV/dI(V) around 150 mV at liquid helium temperatures, which disappear above 150 K, similar to the case of sister CDW compound TiSe2. Applying higher voltages above 200 mV, we observed resistive switching in TiTe2 PCs from a metallic-like low-resistance state to non-metallic type high-resistance state with a change of resistance by an order of magnitude. A unique diode-like effect was registered in "soft" TiTe2 PCs with hysteretic I(V) at negative voltage on TiTe2. Discovering the resistive switching and diode effect adds TiTe2 to the transition-metal dichalcogenides, which could be useful in developing non-volatile ReRAM and other upcoming nanotechnologies.

cond-mat.mes-hall↗
arXiv2024

Electron-phonon interaction, magnetic phase transition, charge density waves and resistive switching in VS2 and VSe2 revealed by Yanson point contact spectroscopy

VS2 and VSe2 have attracted particular attention among the transition metals dichalcogenides because of their promising physical properties concerning magnetic ordering, charge density wave (CDW), emergent superconductivity, etc., which are very sensitive to stoichiometry and dimensionality reduction. Yanson point contact (PC) spectroscopic study reveals metallic and nonmetallic states in VS2 PCs, as well as a magnetic phase transition was detected below 25 K. Analysis of PC spectra of VS2 testifies the realization of the thermal regime in PCs. At the same time, rare PC spectra, where the magnetic phase transition was not visible, shows a broad maximum of around 20 mV, likely connected with electron-phonon interaction (EPI). On the other hand, PC spectra of VSe2 demonstrate metallic behavior, which allowed us to detect features associated with EPI and CDW transition. The Kondo effect appeared for both compounds, apparently due to interlayer vanadium ions. Besides, the resistive switching was observed in PCs on VSe2 between a low resistive, mainly metallic-type state, and a high resistive nonmetallic-type state by applying bias voltage (about 0.4V). In contrast, reverse switching occurs by applying a voltage of opposite polarity (about 0.4V). The reason may be the alteration of stoichiometry in the PC core due to the displacement of V ions to interlayer under a high electric field. The observed resistive switching characterize VSe2 as a potential material, e.g., for non-volatile resistive RAM, neuromorphic engineering, and for other nanoelectronic applications. At the same time, VSe2 attracts attention as a rare layered van der Waals compound with magnetic transition.

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arXiv2023

Peculiarities of electron transport and resistive switching in point contacts on TiSe2, TiSeS and CuxTiSe2

TiSe2 has received much attention among the transition metals chalcogenides because of its thrilling physical properties concerning atypical resistivity behavior, emerging of charge density wave (CDW) state, induced superconductivity etc. Here, we report discovery of new feature of TiSe2, namely, observation of resistive switching in voltage biased point contacts (PCs) based on TiSe2 and its derivatives doped by S and Cu (TiSeS, CuxTiSe2). The switching is taking place between a low resistive mainly metallic-type state and a high resistive semiconducting-type state by applying bias voltage (usually below 0.5V), while reverse switching takes place by applying voltage of opposite polarity (usually below 0.5V). The difference in resistance between these two states can reach up to two orders of magnitude at the room temperature. The origin of the effect can be attributed to the variation of stoichiometry in PC core due to drift/displacement of Se/Ti vacancies under high electric field. Additionally, we demonstrated, that heating takes place in PC core, which can facilitate the electric field induced effect. At the same time, we did not found any evidence for CDW spectral features in our PC spectra for TiSe2. The observed resistive switching allows to propose TiSe2 and their derivatives as the promising materials, e.g., for non-volatile resistive random access memory (ReRAM) engineering.

cond-mat.mes-hall↗
arXiv2022

Electron-phonon interaction and point contact enhanced superconductivity in trigonal PtBi2

PtBi2 is a Weyl semimetal, which demonstrates superconductivity with low critical temperature Tc ~ 0.6 K in the bulk. Here, we report our study of electron-phonon interaction (EPI) in trigonal PtBi2 by the Yanson point contact (PC) spectroscopy and presenting the observation of PC enhanced superconductivity. We show, that the Yansons PC spectra display a broad maximum around 15 meV, indicating, apparently, EPI mechanism of Cooper pairing in PtBi2. Moreover, we discovered a substantial increase of Tc up to ~ 3.5 K in PCs. The observed Tc is sufficiently higher than the bulk value, as well as detected at hydrostatic pressure. We calculated the phonon density of states and Eliashberg EPI function in PtBi2 within the framework of the density functional theory. A comparison of experimental data with theoretical calculations showed acceptable agreement. The theoretical Tc is 3.5 K, which corresponds to the experimental value.

cond-mat.supr-con↗
arXiv2021

Switchable domains in point contacts based on transition metal tellurides

We report resistive switching in voltage biased point contacts (PCs) based on series of van der Waals transition metals tellurides (TMTs) such as MeTe2 (Me=Mo, W) and TaMeTe4 (Me= Ru, Rh, Ir). The switching occurs between a low resistive "metallic-type" state, which is the ground state, and a high resistive "semiconducting-type" state by applying certain bias voltage (<1V), while reverse switching takes place by applying voltage of opposite polarity. The origin of the effect can be formation of domain in PC core by applying a bias voltage, when a strong electric field (about 10kV/cm) modifies the crystal structure and controls its polarization. In addition to the discovery of the switching effect in PCs, we also suggest a simple method of material testing before functionalizing them, which offers a great advantage in finding suitable novel substances. The new functionality of studied TMTs arising from switchable domains in submicron hetero-structures that are promising, e.g., for non-volatile resistive random access memory (RRAM) engineering.

cond-mat.mes-hall↗
arXiv2020

Enhanced critical temperatures in the strongly overdoped iron-based superconductors AFe$_2$As$_2$ (A = K, Cs, Rb) observed by point contacts

A remarkable several times increase (up to 10 K) of the superconducting critical temperature Tc has been observed in point contacts created on the base of single crystals AFe$_2$As$_2$ (A = K, Cs, Rb). Possible reasons for such a Tc increase in point contacts are briefly discussed on a qualitative level. Among them, it is most likely attributed to interfacial carrier doping and/or uniaxial non-homogeneous pressure arising when the contact is created.

cond-mat.supr-con↗
arXiv2019

Yanson point-contact spectroscopy of Weyl semimetal WTe2

We carried out point contact (PC) investigation of WTe2 single crystals. We measured Yanson d2V/dI2 PC spectra of the electron-phonon interaction (EPI) in WTe2. The spectra demonstrate a main phonon peak around 8 meV and a shallow second maximum near 16 meV. Their position is in line with the calculation of the EPI spectra of WTe2 in the literature, albeit phonons with higher energy are not resolved in our PC spectra. An additional contribution to the spectra is present above the phonon energy, what may be connected with the peculiar electronic band structure and need to be clarified. We detected tiny superconducting features in d2V/dI2 close to zero bias, which broadens by increasing temperature and blurs above 6K. Thus, (surface) superconductivity may exist in WTe2 with a topologically nontrivial state. We found a broad maximum in dV/dI at large voltages (>200 mV) indicating change of conductivity from metallic to semiconducting type. The latter might be induced by the high current density (~10^8 A/cm^2) in the PC and/or local heating, thus enabling the manipulation of the quantum electronic states at the interface in the PC core.

cond-mat.mes-hall↗
arXiv2018

Surface superconductivity in the Weyl semimetal MoTe2 detected by point contact spectroscopy

MoTe2 is a Weyl semimetal, which exhibits unique non-saturating magnetoresistance and strongly reinforced superconductivity under pressure. Here, we demonstrate that a novel mesoscopic superconductivity at ambient pressure arises on the surface of MoTe2 with a critical temperature up to 5 K significantly exceeding the bulk Tc=0.1K. We measured the derivatives of I-V curves for metallic hetero-contacts of MoTe2 with Ag or Cu, homo-contacts of MoTe2 as well as "soft" point contacts (PCs). Large number of these hetero-contacts exhibit a dV/dI dependence, which is characteristic for Andreev reflection. It allows to determine the superconducting gap Δ. The average gap values are 2Δ=1.30+/-0.15 meV with a 2Δ/kBTc ratio of 3.7+/-0.4, which slightly exceeds the standard BCS value of 3.52. Furthermore, the temperature dependence of the gap follows a BCS-like behavior, which points to a nodeless superconducting order parameter with some strong-coupling renormalization. Remarkably, the observation of a "gapless-like" single minimum in the dV/dI of "soft" PCs may indicate a topological superconducting state of the MoTe2 surface as these contacts probe mainly the interface and avoid additional pressure effect. Therefore, MoTe2 might be a suitable material to study new forms of topological superconductivity.

cond-mat.supr-con↗
arXiv2017

Electron-Phonon Interaction in Ternary Rare-Earth Copper Antimonides LaCuSb2 and La(Cu0.8Ag0.2)Sb2 probed by Yanson Point-Contact Spectroscopy

Investigation of the electron-phonon interaction (EPI) in LaCuSb2 and La(Cu0.8Ag0.2)Sb2 compounds by Yanson point-contact spectroscopy (PCS) has been carried out. Point-contact spectra display a pronounced broad maximum in the range of 10÷20 mV caused by EPI. Variation of the position of this maximum is likely connected with anisotropic phonon spectrum in these layered compounds. The absence of phonon features after the main maximum allows the assessment of the Debye energy of about 40 meV. The EPI constant for the LaCuSb2 compound was estimated to be λ=0.2+/-0.03. A zero-bias minimum in differential resistance for the latter compound is observed for some point contacts, which vanishes at about 6 K, pointing to the formation of superconducting phase under point contact, while superconducting critical temperature of the bulk sample is only 1K.

cond-mat.supr-con↗
arXiv2017

The superconducting gaps in FeSe studied by soft point-contact Andreev reflection spectroscopy

FeSe single crystals have been studied by soft point-contact Andreev-reflection spectroscopy. Superconducting gap features in the differential resistance dV/dI(V) of point contacts such as a characteristic Andreev-reflection double-minimum structure have been measured versus temperature and magnetic field. Analyzing dV/dI within the extended two-gap Blonder-Tinkham-Klapwijk model allows to extract both the temperature and magnetic field dependence of the superconducting gaps. The temperature dependence of both gaps is close to the standard BCS behavior. Remarkably, the magnitude of the double-minimum structure gradually vanishes in magnetic field, while the minima position only slightly shifts with field indicating a weak decrease of the superconducting gaps. Analyzing the dV/dI(V) spectra for 25 point contacts results in the averaged gap values = 1.8+/-0.4meV and =1.0+/-0.2 meV and reduced values 2 /kTc=4.2+/-0.9 and 2 /kTc=2.3+/-0.5 for the large (L) and small (S) gap, respectively. Additionally, the small gap contribution was found to be within tens of percent decreasing with both temperature and magnetic field. No signatures in the dV/dI spectra were observed testifying a gapless superconductivity or presence of even smaller gaps.

cond-mat.supr-con↗
arXiv2015

Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state

Nonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated. Measured dV/dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the degraded surface. Superconducting (SC) feature in dV/dI like a sharp zero-bias minimum develops for relatively low ohmic PCs, where the deep areas of FeSe are involved. Analysis of dV/dI has shown that the origin of the zero-bias minimum is connected with the Maxwell part of the PC resistance, what masks energy dependent spectral peculiarities. Even so, we have found the specific features in dV/dI - the sharp side maxima, which may have connection to the SC gap, since their position follows the BCS temperature dependence. Exploring the dV/dI spectra of the rare occurrence with Andreev-like structure, the two gaps with Delta = 2.5 and 3.5 meV were identified.

cond-mat.supr-con↗
arXiv2014

Analysis of point contact spectra of kfe2as2 in the thermal regime

The point-contact spectra of the iron-based compound KFe2As2 were analyzed according to the thermal regime theory. We have obtained the values of the residual resistivity, the Lorentz number, and the electron mean free path in the contacts. It was shown that the most point-contact spectra can be described by this theory. The reasons for this behavior are discussed.

cond-mat.supr-con↗
arXiv2014

Josephson effect and Andreev reflection in Ba(1-x)NaxFe2As2 (x=0.25 and 0.35) point contacts

I(V) characteristics and their first derivatives of ScS and ScN-type (S-superconductor, c-constriction, N-normal metal) point-contacts (PCs) based on Ba{1-x}NaxFe2As2 ($x$=0.25 and 0.35) were studied. ScS-type PCs with S=Nb,Ta and Pb show Josephson-like resistively shunted I(V) curves with microwave induced Shapiro steps which satisfy relation 2eV=hw. The I_cR_N product (I_c-critical current, R_N-- normal state PC resistance) in these PCs is found to be up to 1.2mV. All this data with the observed dependence of the I_c on the microwave power of ScS PCs with Pb counterelectrode indicates the presence of the singlet s-wave type pairing in Ba(1-x)NaxFe2As2. From the dV/dI(V) curves of ScN-type PCs demonstrating Andreev-reflection like features, the superconducting gap ratio 2Δ/k_BT_c = 3.6 \pm 1 for the compound with x=0.35 was evaluated. Analysis of these dV/dI(V) at high biases V, that is well above Δ, testifies transition to the thermal regime in PCs with a voltage increase.

cond-mat.supr-con↗
arXiv2014

Single 20meV boson mode in KFe2As2 detected by point-contact spectroscopy

We report an experimental and theoretical investigation of the electron-boson interaction in KFe2As2 by point-contact (PC) spectroscopy, model, and ab-initio LDA-based calculations for the standard electron-phonon Eliashberg function. The PC spectrum viz. the second derivative of the I - V characteristic of representative PC exhibits a pronounced maximum at about 20meV and surprisingly a featureless behavior at lower and higher energies. We discuss phonon and non-phonon (excitonic) mechanisms for the origin of this peak. Analysis of the underlying source of this peak may be important for the understanding of serious puzzles of superconductivity in this type of compounds.

cond-mat.supr-con↗
arXiv2014

Exploring of point-contact spectra of Ba1-xNaxFe2As2 in the normal and superconducting state

We present study of derivatives of current-voltage I(V) characteristics of point-contacts (PCs) based on Ba{1-x}Na{x}Fe2As2 (x=0.25) in the normal and superconducting state. The detailed analysis of dV/dI(V) data (also given in Appendix A) shows that the thermal regime, when temperature increases with a voltage at a rate of about 1.8 K/mV, is realized in the investigated PCs at least at high biases V above the superconducting (SC) gap Δ. In this case, specific resistivity ρ(T) in PC core is responsible for a peculiar dV/dI(V) behavior, while a pronounced asymmetry of dV/dI(V) is caused by large value of thermopower in this material. A reproducible zero-bias minima detected on dV/dI(V) at low biases in the range \pm(6--9)mV well below the SC critical temperature T_c could be connected with the manifestation of the SC gap Δ. Evaluation of these Andreev-reflection-like structures on dV/dI(V) points out to the preferred value of 2Δ/kT_c \approx 6. The expected second gap features on dV/dI(V) are hard to resolve unambiguously, likely due to impurity scattering, spatial inhomogeneity and transition to the mentioned thermal regime as the bias further increases. Suggestions are made how to separate spectroscopic features in dV/dI(V) from those caused by the thermal regime.

cond-mat.supr-con↗
arXiv2011

Peculiarities of the superconducting gaps and the electron-boson interaction in TmNi2B2C as seen by point-contact spectroscopy

Point-contact (PC) investigations on the title compound in the normal and superconducting (SC) state (Tc=10,6 K) are presented. The temperature dependence of the SC gap of TmNi2B2C determined from Andreev-reflection (AR) spectra using the standard single-gap approximation (SGA) deviates from the BCS behavior in displaying a maximum at about Tc/2. A refined analysis within the two-gap approximation provides evidence for the presence of a second gap twice as large as the main gap (the first one), while the latter is close to that within the SGA. This way, TmNi2B2C expands the number of nickel borocarbide superconductors which exhibit a clear multiband character. Additionally, for the first time "reentrant" features were found in the AR spectra for some PCs measured in a magnetic field. The PC spectroscopy of the electron-boson interaction in TmNi2B2C in the normal state reveals a pronounced phonon maximum at 9.5meV and a more smeared one around 15 meV, while at higher energies the PC spectra are almost featureless. Additionally, the most intense peak slightly above 3meV observed in the PC spectra of TmNi2B2C is presumably caused by crystalline-electric-field (CEF) excitations. The peak near 1meV detected for some PC spectra is connected with a modification of the CEF probably due to boron or carbon vacancies, allowing to probe the local stoichiometry by PC spectroscopy.

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