arXiv subjects

Alexei Kalaboukhov

Publications and source records attributed to Alexei Kalaboukhov.

18 recordsLinked to original sources

arXiv2026

Intrinsic Negative-U Centers in Freestanding LaAlO3/SrTiO3 Micro-membranes

The LaAlO3/SrTiO3 (LAO/STO) interface hosts a rich range of electronic phenomena, including unconventional electron pairing that in quantum dots gives rise to a negative effective charging energy U. Here, we show freestanding LAO/STO micro-membranes naturally hosting negative-U centers, where lateral confinement arises intrinsically, rather than from engineered nanostructures. These centers coexist with gate-tunable superconductivity and can remain stable upon thermal cycling from millikelvin temperatures to room temperature. Transport is in excellent agreement with calculations based on a negative-U Anderson model, and electrostatic simulations indicate characteristic center sizes of 20-80 nm. Our findings suggest that negative-U centers may arise from the intrinsic interfacial inhomogeneity typical of LAO/STO, and should therefore be considered a general feature of the LAO/STO interface. This could have important consequences for the microwave response of interfacial superconducting devices.

cond-mat.mtrl-sci↗

arXiv2025

THz mixing of high-order harmonics using $YBa_2Cu_3O_{7-{\delta}}$ nanobridges

Superconducting materials are a key for technologies enabling a large number of devices including THz wave mixers and single photon detectors, though limited at very low temperatures for conventional superconductors. High temperature operation could in principle be offered using cuprate superconductors. However, the complexity of the material in thin film form, the extremely short coherence length and material stability, have hindered the realization of THz devices working at liquid nitrogen temperatures. $YBa_2Cu_3O_{7-{\delta}}$ (YBCO) nanodevices have demonstrated non-linear properties typical of Josephson-like behavior, which have the potential for the mixing of AC signals in the THz range due to the large superconducting energy gap. Here, we present AC Josephson functionalities for terahertz waves utilizing Abrikosov vortex motion in nanoscale-confined fully planar YBCO thin film bridges. We observe Shapiro step-like features in the current voltage characteristics when irradiating the device with monochromatic sub-THz waves (100 GHz to 215 GHz) at 77 K. We further explore these nonlinear effects by detecting THz high-order harmonic mixing for signals from 200 GHz up to 1.4 THz using a local oscillator at 100 GHz. Our results open a path to an easy-fabricated HTS nonlinear nanodevice based on dimensional confinement suitable for terahertz applications.

cond-mat.supr-con↗

arXiv2025

Interplay between altermagnetic order and crystal symmetry probed using magnetotransport in epitaxial altermagnet MnTe

Altermagnets are a new class of magnetic materials characterized by fully compensated spins arranged in alternating local structures, allowing for spin-split bands similar to those found in ferromagnets without net magnetism. Recently, MnTe has emerged as a prototypical altermagnetic material exhibiting spin-polarized electronic bands and anomalous transport phenomena. Although recent work has explored the magnetic and structural properties of MnTe, detailed experimental investigations into the relationship between altermagnetic order and crystal symmetry are lacking. Here, we report the relationship between altermagnetic order and crystal symmetry by investigating magnetotransport properties of MnTe epitaxial altermagnetic thin films grown by molecular beam epitaxy. We observe a spontaneous anomalous Hall effect and show the control of Hall response with the altermagnetic order using the magnetic field and the crystallographic angle dependence. Detailed measurements establish that both the longitudinal and transverse electronic responses depend on the relative orientation of the applied current and N\'eel vector as well as on the crystal orientation and altermagnetic order. These results provide new insights into the interplay between crystal symmetry and altermagnetism for future device applications.

cond-mat.mtrl-sci↗

arXiv2025

Boosting superconductivity in ultrathin YBa$_2$Cu$_3$O$_{7-\delta}$ films via nanofaceted substrates

In cuprate high-temperature superconductors the doping level is fixed during synthesis, hence the charge carrier density per CuO$_2$ plane cannot be easily tuned by conventional gating, unlike in 2D materials. Strain engineering has recently emerged as a powerful tuning knob for manipulating the properties of cuprates, in particular charge and spin orders, and their delicate interplay with superconductivity. In thin films, additional tunability can be introduced by the substrate surface morphology, particularly nanofacets formed by substrate surface reconstruction. Here we show a remarkable enhancement of the superconducting onset temperature $T_{\mathrm{c}}^{\mathrm{on}}$ and the upper critical magnetic field $H_{c,2}$ in nanometer-thin YBa$_2$Cu$_3$O$_{7-\delta}$ films grown on a substrate with a nanofaceted surface. We theoretically show that the enhancement is driven by electronic nematicity and unidirectional charge density waves, where both elements are captured by an additional effective potential at the interface between the film and the uniquely textured substrate. Our findings show a new paradigm in which substrate engineering can effectively enhance the superconducting properties of cuprates. This approach opens an exciting frontier in the design and optimization of high-performance superconducting materials.

cond-mat.supr-con↗

arXiv2025

The influence of phonon symmetry and electronic structure on the electron-phonon coupling momentum dependence in cuprates

The experimental determination of the magnitude and momentum dependence of electron-phonon coupling (EPC) is an outstanding problem in condensed matter physics. The intensity of phonon peaks in Resonant Inelastic X-ray Scattering (RIXS) spectra can be related to the underlying EPC strength under significant approximations whose validity deserves careful verification. We measured the Cu L$_3$ RIXS phonon intensity as function of incident photon energy and of momentum transfer in several layered cuprates. For CaCuO$_2$, La$_{2-x}$Sr$_{x}$CuO$_{4+\delta}$, and \ch{YBa_2Cu_3O_{6}}, using a generally accepted theoretical model, we estimate quantitatively the EPC for the bond-stretching mode along the high-symmetry directions ($\zeta$,0) and ($\zeta$,$\zeta$), and as a function of the azimuthal angle $\varphi$ at fixed $q_\parallel$. We compare our results with theoretical predictions and we find that the $\mathbf{q}_\parallel$-dependence of the phonon RIXS intensity can be largely ascribed to the phonon symmetry. However, a more satisfactory prediction of the experimental results requires an accurate description of the electronic structure close to the Fermi level. Our extensive investigation indicates that Cu L$_3$ RIXS can be reliably used to determine the momentum dependence of EPC for the bond-stretching modes of cuprates. Moreover, the large experimental basis provided in this article can serve as stringent test for advanced theoretical predictions on the EPC.

cond-mat.str-el↗

arXiv2024

Gate-tunable negative differential resistance in multifunctional van der Waals heterostructure

Two-dimensional (2D) semiconductors have emerged as exciting candidates for the development of low-power and multifunctional computing applications, thanks to their qualities such as layer-dependent band gap tunability, high carrier mobility, and excellent electrostatic control. Here, we explore a pair of 2D semiconductors with nearly broken-gap (Type-III-like) band alignment and demonstrate a highly gate-tunable p-MoTe$_2$/n-SnS$_2$ heterojunction with multifunctional behavior. Employing a dual-gated asymmetric device geometry, we unveil its functionality as both a forward and backward rectifying device. Moreover, we observe a highly gate-tunable negative differential resistance (NDR), with a gate-coupling efficiency of $\eta \simeq 0.5$ and a peak-to-valley ratio of $\sim$ 3 down to 150K. By employing density functional theory, we determine that the observed NDR is dominated by valence band-to-valence band tunneling, while additional interband tunneling contributions arise at higher bias. The combination of tunneling driven transport and gate controllability of NDR opens the pathway for realizing gate-tunable 2D material-based neuromorphic and energy-efficient electronics.

cond-mat.mes-hall↗

arXiv2024

Engineering underdoped CuO$_2$ nanoribbons in nm-thick $a$-axis YBa$_2$Cu$_3$O$_{7-δ}$ films

In underdoped cuprate high $T_{\mathrm{c}}$ superconductors, various local orders and symmetry breaking states, in addition to superconductivity, reside in the CuO$_2$ planes. The confinement of the CuO$_2$ planes can therefore play a fundamental role in modifying the hierarchy between the various orders and their intertwining with superconductivity. Here we present the growth of $a$-axis oriented YBa$_2$Cu$_3$O$_{7-δ}$ films, spanning the whole underdoped side of the phase diagram. In these samples, the CuO$_2$ planes are confined by the film thickness, effectively forming unit-cell-thick nanoribbons. The unidirectional confinement at the nanoscale enhances the in-plane anisotropy of the films. By X-ray diffraction and resistance vs temperature measurements, we have discovered the suppression of the orthorhombic-to-tetragonal transition at low dopings, and a very high anisotropy of the normal state resistance in the $b$-$c$ plane, the latter being connected to a weak coupling between adjacent CuO$_2$ nanoribbons. These findings show that the samples we have grown represent a novel system, different from the bulk, where future experiments can possibly shed light on the rich and mysterious physics occurring within the CuO$_2$ planes.

cond-mat.supr-con↗

arXiv2024

Stoichiometric control of electron mobility and 2D superconductivity at LaAlO$_3$-SrTiO$_3$ interfaces

SrTiO$_3$-based conducting interfaces, which exhibit coexistence of gate-tunable 2D superconductivity and strong Rashba spin-orbit coupling (RSOC), are candidates to host topological superconductive phases. Yet, superconductivity is usually in the dirty limit, which tends to suppress nonconventional pairing and therefore challenges these expectations. Here we report on LaAlO$_3$/SrTiO$_3$ (LAO/STO) interfaces with remarkably large mobility and mean free paths comparable to the superconducting coherence length, approaching the clean limit for superconductivity. We further show that the carrier density, mobility, and formation of the superconducting condensate are controlled by the fine-tuning of La/Al chemical ratio in the LAO film. Interestingly, we find a region in the superconducting phase diagram where the critical temperature is not suppressed below the Lifshitz transition, at odds with previous experimental investigations. These findings point out the relevance of achieving a clean-limit regime to enhance the observation of unconventional pairing mechanisms in these systems

cond-mat.supr-con↗

arXiv2024

Integration of High-Tc Superconductors with High Q Factor Oxide Mechanical Resonators

Micro-mechanical resonators are building blocks of a variety of applications in basic science and applied electronics. This device technology is mainly based on well-established and reproducible silicon-based fabrication processes with outstanding performances in term of mechanical Q factor and sensitivity to external perturbations. Broadening the functionalities of MEMS by the integration of functional materials is a key step for both applied and fundamental science. However, combining functional materials and silicon-based compounds is challenging. An alternative approach is fabricating MEMS based on complex heterostructures made of materials inherently showing a variety of physical properties such as transition metal oxides. Here, we report on the integration of a high-Tc superconductor YBa2Cu3O7 (YBCO) with high Q factor micro-bridge resonator made of a single-crystal LaAlO3 (LAO) thin film. LAO resonators are tensile strained, with a stress of 345 MPa, show Q factor in the range of tens of thousands, and have low roughness. The topmost YBCO layer deposited by Pulse Laser Deposition shows a superconducting transition starting at 90 K with zero resistance below 78 K. This result opens new possibilities towards the development of advanced transducers, such as bolometers or magnetic field detectors, as well as basic science experiments in solid state physics, material science, and quantum opto-mechanics.

physics.app-ph↗

arXiv2023

Strong in-plane magnetic anisotropy (Co0.15Fe0.85)5GeTe2/graphene van der Waals heterostructure spin-valve at room temperature

Van der Waals (vdW) magnets are promising owing to their tunable magnetic properties with doping or alloy composition, where the strength of magnetic interactions, their symmetry, and magnetic anisotropy can be tuned according to the desired application. However, most of the vdW magnet based spintronic devices are so far limited to cryogenic temperatures with magnetic anisotropies favouring out-of-plane or canted orientation of the magnetization. Here, we report room-temperature lateral spin-valve devices with strong in-plane magnetic anisotropy of the vdW ferromagnet (Co0.15Fe0.85)5GeTe2 (CFGT) in heterostructures with graphene. Magnetization measurements reveal above room-temperature ferromagnetism in CFGT with a strong in-plane magnetic anisotropy. Density functional theory calculations show that the magnitude of the anisotropy depends on the Co concentration and is caused by the substitution of Co in the outermost Fe layer. Heterostructures consisting of CFGT nanolayers and graphene were used to experimentally realize basic building blocks for spin valve devices such as efficient spin injection and detection. The spin transport and Hanle spin precession measurements prove a strong in-plane and negative spin polarization at the interface with graphene, which is supported by the calculated spin-polarized density of states of CFGT. The in-plane magnetization of CFGT at room temperature proves its usefulness in graphene lateral spin-valve devices, thus opening further opportunities for spintronic technologies.

cond-mat.mes-hall↗

arXiv2023

Strain, Young's modulus, and structural transition of EuTiO3 thin films probed by micro-mechanical methods

EuTiO3 (ETO) is a well-known complex oxide mainly investigated for its magnetic properties and its incipient ferro-electricity. In this work, we demonstrate the realization of suspended micro-mechanical structures, such as cantilevers and micro-bridges, from 100 nm-thick single-crystal epitaxial ETO films deposited on top of SrTiO3(100) substrates. By combining profile analysis and resonance frequency measurements of these devices, we obtain the Young's modulus, strain, and strain gradients of the ETO thin films. Moreover, we investigate the ETO anti-ferro-distorsive transition by temperature-dependent characterizations, which show a non-monotonic and hysteretic mechanical response. Comparison between experimental and literature data allows us to weight the contribution from thermal expansion and softening to the tuning slope, while a full understanding of the origin of such a wide hysteresis is still missing. We also discuss the influence of oxygen vacancies on the reported mechanical properties by comparing stoichiometric and oxygen-deficient samples.

cond-mat.mtrl-sci↗

arXiv2022

Mapping the phase diagram of a YBa$_2$Cu$_3$O$_{7-δ}$ nanowire through electromigration

We use electromigration (EM) to tune the oxygen content of YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) nanowi\-res. During EM, the dopant oxygen atoms in the nanowire are moved under the combined effect of electrostatic force and Joule heating. The EM current can be tuned to either deplete or replenish nanowires with oxygen, allowing fine tuning of its hole doping level. Electrical transport measurements and Kelvin probe microscopy corroborate good homogeneity of the doping level along the electromigrated nanowires. Thus, EM provides an effective method to study transport properties of YBCO in a wide doping range at the nanoscale in one and the same device.

cond-mat.supr-con↗

arXiv2021

Gate-tunable hallmarks of unconventional superconductivity in non-centrosymmetric nanowires

Two dimensional SrTiO3-based interfaces stand out among non-centrosymmetric superconductors due to their intricate interplay of gate tunable Rashba spin-orbit coupling and multi-orbital electronic occupations, whose combination theoretically prefigures various forms of non-standard superconductivity. However, a convincing demonstration by phase sensitive measurements has been elusive so far. Here, by employing superconducting transport measurements in nano-devices we present clear-cut experimental evidences of unconventional superconductivity in the LaAlO3/SrTiO3 interface. The central observations are the substantial anomalous enhancement of the critical current by small magnetic fields applied perpendicularly to the plane of electron motion, and the asymmetric response with respect to the magnetic field direction. These features have a unique trend in intensity and sign upon electrostatic gating that, together with their dependence on temperature and nanowire dimensions, cannot be accommodated within a scenario of canonical spin-singlet superconductivity. We theoretically demonstrate that the hall-marks of the experimental observations unambiguously indicate a coexistence of Josephson channels with sign difference and intrinsic phase shift. The character of these findings establishes the occurrence of independent components of unconventional pairing in the superconducting state due to inversion symmetry breaking. The outcomes open new venues for the investigation of multi-orbital non-centrosymmetric superconductivity and Josephson-based devices for quantum technologies.

cond-mat.supr-con↗

arXiv2019

Untwinned YBa$_2$Cu$_3$O$_{7-δ}$ thin films on MgO substrates: a platform to study strain effects on the local orders in cuprates

We have grown untwinned YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) films on (110) MgO substrates that were pre-annealed at high temperature in oxygen atmosphere. The annealing results in surface reconstruction with shallow facets, which induce the suppression of the YBCO twinning domains, and the preferential alignment of the CuO chains along one of the in-plane directions of the substrate. Because of the large mismatch between the in-plane lattice parameters of film and substrate, the strain induced by the MgO into the YBCO layer is strong and very peculiar. The YBCO film is compressed, with respect to the bulk, and presents a unidirectional buckling of the atomic planes, along the chains' direction, due to a deformation of the copper-oxygen octahedra. The YBCO films, which can be grown with thicknesses down to few unit cells and oxygen doping levels spanning most of the superconducting dome, are patterned into nanowires with dimensions down to 50 nm. The anisotropies due to the untwinning state are preserved in these structures; moreover, additional anisotropies appear, in ultrathin structures where strain effects become more pronounced. Such untwinned and compressively strained films can therefore be used as a platform to study the interplay between strain and the various local orders in the normal state of YBCO.

cond-mat.supr-con↗

arXiv2019

SQUID magnetometer based on Grooved Dayem nanobridges and a flux transformer

We report noise measurements performed on a SQUID magnetometer implementing Grooved Dayem nanobridge of YBCO as weak-links. The SQUID shows magnetic flux noise as low as 10 $μΦ_0$/Hz$^{0.5}$. The magnetometer is realized by coupling the SQUID to a flux transformer with a two-level coupling scheme using a flip-chip approach. This improves the effective area of the SQUID and result in a magnetic field noise of 50 fT/Hz$^{0.5}$ at T=77 K.

cond-mat.supr-con↗

arXiv2019

Grooved Dayem nanobridges as building blocks of high-performance YBa$_2$Cu$_3$O$_{7-δ}$ SQUID magnetometers

We present noise measurements performed on a YBa$_2$Cu$_3$O$_{7-δ}$ nanoscale weak-link-based magnetometer consisting of a Superconducting QUantum Interference Device (SQUID) galvanically coupled to a $3.5 \times 3.5~$mm$^2$ pick-up loop, reaching white flux noise levels and magnetic noise levels as low as $6~μΦ_0 / \sqrt{\mathrm{Hz}}$ and $100$~fT/$\sqrt{\mathrm{Hz}}$ at $T=77$~K, respectively. The low noise is achieved by introducing Grooved Dayem Bridges (GDBs), a new concept of weak-link. A fabrication technique has been developed for the realization of nanoscale grooved bridges, which substitutes standard Dayem bridge weak links. The introduction of these novel key blocks reduces the parasitic inductance of the weak links and increases the differential resistance of the SQUIDs. This greatly improves the device performance, thus resulting in a reduction of the white noise.

cond-mat.supr-con↗

arXiv2018

Induced unconventional superconductivity on the surface states of Bi$_2$Te$_3$ topological insulator

Topological superconductivity is central to a variety of novel phenomena involving the interplay between topologically ordered phases and broken-symmetry states. The key ingredient is an unconventional order parameter, with an orbital component containing a chiral $p_x$ + i$p_y$ wave term. Here we present phase-sensitive measurements, based on the quantum interference in nanoscale Josephson junctions, realized by using Bi$_2$Te$_3$ topological insulator. We demonstrate that the induced superconductivity is unconventional and consistent with a sign-changing order parameter, such as a chiral $p_x$ + i$p_y$ component. The magnetic field pattern of the junctions shows a dip at zero externally applied magnetic field, which is an incontrovertible signature of the simultaneous existence of 0 and $π$ coupling within the junction, inherent to a non trivial order parameter phase. The nano-textured morphology of the Bi$_2$Te$_3$ flakes, and the dramatic role played by thermal strain are the surprising key factors for the display of an unconventional induced order parameter.

cond-mat.supr-con↗

arXiv2009

SiC Graphene Suitable For Quantum Hall Resistance Metrology

We report the first observation of the quantum Hall effect in epitaxial graphene. The result described in the submitted manuscript fills the yawning gap in the understanding of the electronic properties of this truly remarkable material and demonstrate suitability of the silicon carbide technology for manufactiring large area high quality graphene. Having found the quantum Hall effect in several devices produced on distant parts of a single large-area wafer, we can confirm that material synthesized on the Si-terminated face of SiC promises a suitable platform for the implementations of quantum resistance metrology at elevated temperatures and, in the longer term, opens bright prospects for scalable electronics based on graphene.

cond-mat.mes-hall↗