arXiv subjects

Venkat Chandrasekhar

Publications and source records attributed to Venkat Chandrasekhar.

At least 19 recordsLinked to original sources

arXiv2026

Non-local Tunneling Spectroscopy of Inelastic Quasiparticle Relaxation in Superconducting 1-D Wires

Non-local conductance experiments using tunnel junctions can provide valuable spectroscopic information on both the transport and relaxation of quasiparticles in superconductors, as these techniques directly probe the quasiparticle charge and energy imbalance even at mK temperatures. In this work, we employ mesoscopic three terminal Cu and Al NIS devices to study non-local quasiparticle transport over length-scales on the order of the superconducting coherence length in this regime. Via a dual-bias scheme, which utilizes detector biases both above and below the superconducting gap, we are able to extract the effect of quasiparticle energy imbalance via its impact on the self consistent pair potential by symmetry considerations. We observe non-local conductance features due to pair-breaking which are anti-symmetric with respect to the polarity of the voltage bias, with a sharp onset during single electron tunneling at energies around $3Δ$. We compare these findings with quasiclassical simulations including inelastic effects to obtain estimates of the energy dependent inelastic scattering time. In addition, we demonstrate kinetic effects due to a large applied supercurrent which can also be captured in this formalism and decomposed with respect to the particle-hole symmetry and supercurrent direction, and discuss further opportunities for the advancement of this method.

cond-mat.supr-con↗

arXiv2025

Using Crossed Andreev Reflection to Split Electrons

Mesoscopic systems possess shot noise in their currents due to the quantization of the conducting quasiparticles. Measurements of this shot noise are useful to study phenomena that do not manifest themselves in standard conductance or resistance measurements, such as the statistics of the conducting quasiparticles or quantum entanglement via Bell tests. The corresponding particle statistics can be determined via two particle quantum interference experiments, such as the Hong-Ou-Mandel effect which demonstrates a bunching effect for bosons or an anti-bunching effect in fermions. In superconducting proximity junctions, electrons incident on a superconductor can induce holes via crossed Andreev reflection (CAR) in spatially separated normal metal leads, where the resulting hole currents have nontrivial partition noise due to the four terminal configuration. These nonlocally generated currents, using a superconductor as a mesoscopic beam splitter, enable fabrication of mesoscopic analogs to quantum optics interferometers using metallic and superconducting films with multiport geometries.

cond-mat.mes-hall↗

arXiv2025

Tuning the the fundamental periodicity of the current-phase relation in multiterminal diffusive Josephson junctions

Conventional superconductor/insulator/superconductor (SIS) Josephson junctions, devices where two superconductors are separated by a tunnel barrier are technologically important as elements in quantum circuits, particularly with their key role in superconducting qubits. An important characteristic of Josephson junctions is the relation between the supercurrent Is and the phase difference $ϕ$ between them. For SIS junctions, the current-phase relation is sinusioidal and 2$π$ periodic. Other types of Josephson junctions, where the material between the superconductors is a weak link or a normal metal (N) may have non-sinusoidal current-phase relations that are still 2$π$ periodic. We show here that a multi-terminal diffusive SNS Josephson junction with 4 superconducting contacts can show a current phase relation between two of the contacts that is a superposition of 2$π$ and 4$π$ periodic components whose relative strength is controlled by the phase difference between the other two contacts, becoming 2$π$ or 4$π$ periodic for certain values of this phase difference. This tunability might have applications in tailoring the Hamiltonians of superconducting quantum circuits.

cond-mat.supr-con↗

arXiv2025

Supercurrent tuning of the Josephson coupling energy

The ability to non-dissipatively tune the Josephson coupling energy of Josephson junctions is a useful tool in frequency-tunable qubits. This is typically done by threading magnetic flux through two junctions connected in a loop, a geometry that exposes the qubit to magnetic environmental noise. In this paper, we show that by biasing a junction with supercurrent from a separate pair of superconducting leads coupled to the device, the Josephson energy can be tuned without the need for a flux loop. Our multiterminal device may enable the realization of a frequency-tunable qubit with greatly reduced susceptibility to flux noise.

cond-mat.mes-hall↗

arXiv2025

Identifying Materials-Level Sources of Performance Variation in Superconducting Transmon Qubits

The Superconducting Materials and Systems (SQMS) Center, a DOE National Quantum Information Science Research Center, has conducted a comprehensive and coordinated study using superconducting transmon qubit chips with known performance metrics to identify the underlying materials-level sources of device-to-device performance variation. Following qubit coherence measurements, these qubits of varying base superconducting metals and substrates have been examined with various nondestructive and invasive material characterization techniques at Northwestern University, Ames National Laboratory, and Fermilab as part of a blind study. We find trends in variations of the depth of the etched substrate trench, the thickness of the surface oxide, and the geometry of the sidewall, which when combined, lead to correlations with the T$_1$ lifetime across different devices. In addition, we provide a list of features that varied from device to device, for which the impact on performance requires further studies. Finally, we identify two low-temperature characterization techniques that may potentially serve as proxy tools for qubit measurements. These insights provide materials-oriented solutions to not only reduce performance variations across neighboring devices, but also to engineer and fabricate devices with optimal geometries to achieve performance metrics beyond the state-of-the-art values.

quant-ph↗

arXiv2024

Transport signatures of phase fluctuations in superconducting qubits

Josephson junctions supply the nonlinear inductance element in superconducting qubits. In the widely used transmon configuration, where the junction is shunted by a large capacitor, the low charging energy minimizes the sensitivity of the qubit to charge noise while maintaining the necessary anharmonicity to qubit states. We report here low-frequency transport measurements on small standalone junctions and identically fabricated capacitively-shunted junctions that show two distinct features normally attributed to small capacitance junctions near zero bias: reduced switching currents and prominent finite resistance associated with phase diffusion in the current-voltage characteristic. Our transport data reveals the existence of phase fluctuations in transmons arising from intrinsic junction capacitance.

cond-mat.mes-hall↗

arXiv2024

Mapping the topological proximity-induced gap of multiterminal Josephson junctions

Multiterminal Josephson junctions (MTJJs), devices in which a normal metal is in contact with three or more superconducting leads, have been proposed as artificial analogs of topological crystals. The topological nature of MTJJs manifests as a modulation of the quasiparticle density of states (DOS) in the normal metal that may be probed by tunneling measurements. We show that one can reveal this modulation by measuring the resistance of diffusive MTJJs with normal contacts, which shows rich structure as a function of the phase differences $\{ϕ_i \}$. Our approach demonstrates a simple yet powerful technique for exploring topological effects in MTJJs.

cond-mat.mes-hall↗

arXiv2024

Enhanced Quasiparticle Relaxation in a Superconductor via the Proximity Effect

Quasiparticle relaxation in pure superconductors is thought to be determined by the intrinsic inelastic scattering rate in the material. In certain applications, i.e. superconducting qubits and circuits, excess quasiparticles exist at densities far beyond the thermal equilibrium level, potentially leading to dephasing and energy loss. In order to engineer superconductors with shorter overall quasiparticle lifetimes, we consider the impact of a proximity layer on the transport of quasiparticles in a superconductor. We find that a normal metal layer can be used to significantly increase the relaxation rate of quasiparticles in a superconductor, as seen by a large reduction in the quasiparticle charge imbalance in a fully proximitized Cu/Al bilayer wire. The mechanism for this effect may be useful for preventing quasiparticle poisoning of qubits using carefully chosen proximity bilayers consisting of clean superconductors and disordered normal metals.

cond-mat.supr-con↗

arXiv2024

Nonlinear Hall Effect in KTaO$_3$ Two-Dimensional Electron Gases

The observation of a Hall effect, a finite transverse voltage induced by a longitudinal current, usually requires the breaking of time-reversal symmetry, for example through the application of an external magnetic field or the presence of long range magnetic order in a sample. Recently it was suggested that under certain symmetry conditions, the presence of finite Berry curvatures in the band structure of a system with time-reversal symmetry but without inversion symmetry can give rise to a nonlinear Hall effect in the presence of a probe current. In order to observe the nonlinear Hall effect, one requires a finite component of a so-called Berry dipole along the direction of the probe current. We report here measurements of the nonlinear Hall effect in two-dimensional electron gases fabricated on the surface of KTaO$_3$ with different surface crystal orientations as a function of the probe current, a transverse electric field and back gate voltage. For all three crystal orientations, the transverse electric field modifies the nonlinear Hall effect. We discuss our results in the context of the current understanding of the nonlinear Hall effect as well as potential experimental artifacts that may give rise to the same effects.

cond-mat.mes-hall↗

arXiv2024

Current dependence of the low bias resistance of small capacitance Josephson junctions

The dc current-voltage characteristics of small Josephson junctions reveal features that are not observed in larger junctions, in particular, a switch to the finite voltage state at current values much less than the expected critical current of the junction and a finite resistance in the nominally superconducting regime. Both phenomena are due to the increased sensitivity to noise associated with the small capacitance of the Josephson junction and have been extensively studied a few decades ago. Here I focus on the current bias dependence of the differential resistance of the junction at low current bias in the nominally superconducting regime, using a quantum Langevin equation approach that enables a physically transparent incorporation of the noise environment of the junction. A similar approach might be useful in modeling the sensitivity of superconducting qubits to noise in the microwave regime.

cond-mat.supr-con↗

arXiv2024

Intrinsic magnetism in KTaO$_3$ heterostructures

There has been intense recent interest in the two-dimensional electron gases (2DEGs) that form at the surfaces and interfaces of KTaO$_3$ (KTO), with the discovery of superconductivity at temperatures significantly higher than those of similar 2DEGs based on SrTiO$_3$ (STO). Like STO heterostructures, these KTO 2DEGs are formed by depositing an overlayer on top of appropriately prepared KTO surfaces. Some of these overlayers are magnetic, and the resulting 2DEGs show signatures of this magnetism, including hysteresis in the magnetoresistance (MR). Here we show that KTO 2DEGs fabricated by depositing AlO$_x$ on top of KTO also show hysteretic MR, indicative of long range magnetic order, even though the samples nominally contain no intrinsic magnetic elements. The hysteresis appears in both the transverse and longitudinal resistance in magnetic fields both perpendicular to and in the plane of the 2DEG. The hysteretic MR has different characteristic fields and shapes for surfaces of different crystal orientations, and vanishes above a few Kelvin. Density functional theory (DFT) calculations indicate that the magnetism likely arises from Ta$^{4+}$ local moments created in the presence of oxygen vacancies.

cond-mat.mes-hall↗

arXiv2023

Colossal Spontaneous Hall Effect and Emergent Magnetism in KTaO$_3$ Two-Dimensional Electron Gases

There has been intense recent interest in the two-dimensional electron gases (2DEGs) that form at the surfaces and interfaces of KTaO$_3$ (KTO), with the discovery of superconductivity at temperatures significantly higher than those of similar 2DEGs based on SrTiO$_3$ (STO). Here we demonstrate that KTO 2DEGs fabricated under conditions that suppress the superconductivity show a large spontaneous Hall effect at low temperatures. The transverse response is asymmetric in an applied perpendicular magnetic field and becomes hysteretic at millikelvin temperatures. The hysteresis is due to long range magnetic order arising from local Ta$^{4+}$ moments. However, the most striking features of the data are the asymmetry of the transverse response and the large spontaneous transverse resistance at zero field, which can be a significant fraction of the longitudinal resistance and depends on crystal orientation. Both effects are due to the presence of a dominant contribution to the transverse response that is symmetric in perpendicular field, suggesting that its origin is topological in nature. We argue that this contribution arises from Berry curvature dipoles coupled with nonequilibrium conditions induced by the measuring current.

cond-mat.mes-hall↗

arXiv2022

Nonlocal Differential Resistance in AlO$_x$/KTaO$_3$ Heterostructures

Local and nonlocal differential resistance measurements on Hall bars defined in AlO$_x$/KTaO$_3$ heterostructures show anomalous behavior that depends on the crystal orientation and the applied back gate voltage. The local differential resistance is asymmetric in the dc bias current, with an antisymmetric component that grows with decreasing gate voltage. More surprisingly, a large nonlocal differential resistance is observed that extends between measurement probes that are separated by 100s of microns. The potential source of this anomalous behavior is discussed.

cond-mat.mes-hall↗

arXiv2022

Probing the topological band structure of diffusive multiterminal Josephson junction devices with conductance measurements

The energy of an Andreev bound state in a clean normal metal in contact with two superconductors disperses with the difference $Δϕ$ in the superconducting phase between the superconductors in much the same way as the energies of electrons in a one-dimensional crystal disperse with the crystal momentum $k$ of the electrons. A normal metal with $n$ superconductors maps on to a $n-1$ dimensional crystal, each dimension corresponding to the phase difference $ϕ_i$ between a specific pair of superconductors. The resulting band structure as a function of the phase differences $\{Δϕ_i\}$ has been proposed to have a topological nature, with gapped regions characterized by different Chern numbers separated by regions where the gap in the quasiparticle spectrum closes. A similar complex evolution of the quasiparticle spectrum with $\{Δϕ_i\}$ has also been predicted for diffusive normal metals in contact with multiple superconductors. Here we show that the variation of the density of states at the Fermi energy of such a system can be directly probed by relatively simple conductance measurements, allowing rapid characterization of the energy spectrum.

cond-mat.supr-con↗

arXiv2022

Characterization of Nb films for superconducting qubits using phase boundary measurements

Continued advances in superconducting qubit performance require more detailed understandings of the many sources of decoherence. Within these devices, two-level systems arise due to defects, interfaces, and grain boundaries, and are thought to be a major source of qubit decoherence at millikelvin temperatures. In addition to Al, Nb is a commonly used metalization layer for superconducting qubits. Consequently, a significant effort is required to develop and qualify processes that mitigate defects in Nb films. As the fabrication of complete superconducting qubits and their characterization at millikelvin temperatures is a time and resource intensive process, it is desirable to have measurement tools that can rapidly characterize the properties of films and evaluate different treatments. Here we show that measurements of the variation of the superconducting critical temperature $T_c$ with an applied external magnetic field $H$ (of the phase boundary $T_c - H$) performed with very high resolution show features that are directly correlated with the structure of the Nb films. In combination with x-ray diffraction measurements, we show that one can even distinguish variations quality and crystal orientation of the grains in a Nb film by small but reproducible changes in the measured superconducting phase boundary.

cond-mat.supr-con↗

arXiv2021

Nonlocal superconducting quantum interference device

Superconducting quantum interference devices (SQUIDs) that incorporate two superconductor/insulator/superconductor (SIS) Josephson junctions in a closed loop form the core of some of the most sensitive detectors of magnetic and electric fields currently available. SQUIDs in these applications are typically operated with a finite voltage which generates microwave radiation through the ac Josephson effect. This radiation may impact the system being measured. We describe here a SQUID in which the Josephson junctions are formed from strips of normal metal (N) in good electrical contact with the superconductor (S). Such SNS SQUIDs can be operated under a finite voltage bias with performance comparable or potentially better than conventional SIS SQUIDs. However, they also permit a novel mode of operation that is based on the unusual interplay of quasiparticle currents and supercurrents in the normal metal of the Josephson junction. The new method allows measurements of the flux dependence of the critical current of the SNS SQUID without applying a finite voltage bias across the SNS junction, enabling sensitive flux detection without generating microwave radiation.

cond-mat.mes-hall↗

arXiv2014

A real-time software simulator for scanning force microscopy

We describe software that simulates the hardware of a scanning force microscope. The essential feature of the software is its real-time response, which is critical for mimicking the behavior of real scanning probe hardware. The simulator runs on an open-source real time Linux kernel, and can be used to test scanning probe microscope control software as well as theoretical models of different types of scanning probe microscopes. We describe the implementation of a tuning-fork based atomic force microscope and a dc electrostatic force microscope, and present representative images obtained from these models.

cond-mat.mes-hall↗

arXiv2014

Evidence of Josephson junction behavior in top-gated LaAlO$_3$-SrTiO$_3$

We demonstrate top-gate tunable Josephson junction like behavior in the two dimensional electron gas at the LaAlO$_3$-SrTiO$_3$ interface. A combination of global back-gating and local top-gating is used to define the junctions, providing an efficient way for much finer spatial control over the properties of the interface, as compared to back-gating alone. The variation of critical currents and zero bias resistances with temperature shows that the junctions behave like short, overdamped weak links. This technique could be an important tool to illuminate the nature of superconductivity in the LaAlO$_3$-SrTiO$_3$ interface system.

cond-mat.supr-con↗