arXiv subjects

G. N. Goltsman

Publications and source records attributed to G. N. Goltsman.

17 recordsLinked to original sources

arXiv2026

Correlated Insulator Moiré Bolometer

Light incident on an insulator is generally not expected to turn it into a metal without invoking intense ultrafast excitation that leads to transient structural transitions. Here we show that magic-angle twisted bilayer graphene tuned to half filling of the moiré band provides a notable exception to this expectation. We find that weak beam of long-wavelength photons, with energies comparable to the flat-band width, selectively heat the low-heat-capacity electronic subsystem, thereby suppressing the correlated gap. This produces a giant resistance change governed not by a persistent photocarrier population, but by the extreme sensitivity of a many-body correlated gap to weak electronic heating. The resulting photon-driven insulator-to-metal transition produces a broadband low-noise photoresponse with voltage responsivity exceeding millivolts per nW of absorbed power. The mechanism is dual to superconducting hot-electron response: radiation-heated electrons suppress a many-body order, but in reverse the correlated insulator melts into a metal, providing robustness to magnetic fields of several tesla and a sharp insulator-to-metal resistive contrast. Our results establish correlated flat-band systems as a platform for ultra-sensitive detection of faint long-wavelength radiation.

cond-mat.mes-hall↗

arXiv2025

Low-temperature electron dephasing rates indicate magnetic disorder in superconducting TiN films

We investigate electron transport and phase-breaking processes in thin titanium nitride (TiN) films of epitaxial quality. Previous studies show that a minute surface magnetic disorder significantly reduces the critical temperature ($T_\mathrm{c}$) and broadens the superconducting transition as the film thickness and device size decrease. We measure electron dephasing rates via magnetoresistance from $T_\mathrm{c}$ to $\sim 4T_\mathrm{c}$ in various-thickness TiN films. Electron dephasing occurs on the picosecond timescale and is nearly independent of temperature, differing from the expected inelastic scattering due to the electron-phonon and electron-electron interactions near $T_\mathrm{c}$, which occur over a nanosecond timescale. We propose spin-flip scattering as a possible additional phase-breaking mechanism. The significant increase in the dephasing rate for the thinnest film indicates that magnetic disorder resides near the surface of naturally oxidized films. Our research suggests that magnetic disorder may be a significant contributor to RF dissipation in superconducting devices based on TiN.

cond-mat.supr-con↗

arXiv2025

Accurate measurement of energy relaxation via flux-flow instability

In this paper, we investigate flux-flow instability (FFI) in a superconducting single-crystal titanium nitride (TiN) film with negligible volume pinning. By studying the critical current density in 12-nm thick TiN strips of varying widths, we accurately identify the experimental parameters at which the FFI regime occurs. A comprehensive analysis of critical velocity measurements allows us to determine the quasiparticle energy relaxation time, $τ_E$. By comparing our results with the $τ_E$ values obtained from other experimental methods, we gain insight into the dominant microscopic process that governs quasiparticle relaxation within the vortex core in TiN. This mechanism is driven by an increase in quasiparticle temperature relative to phonons, rather than by quasiparticles leaving the core. Our findings indicate that $τ_E$ can be accurately determined through FFI measurements.

cond-mat.supr-con↗

arXiv2024

Natural width of the superconducting transition in epitaxial TiN films

We investigate the effect of various fluctuation mechanisms on the DC resistance in superconducting devices based on epitaxial titanium nitride (TiN) films. The samples we studied show a relatively steep resistive transition (RT), with a transition width $ΔT/T_\mathrm{c} \sim 0.002-0.025$, depending on the film thickness (20 nm, 9 nm, and 5 nm) and device dimensions. This value is significantly broader than expected due to conventional superconducting fluctuations ($ΔT/T_\mathrm{c} \ll 10^{-3}$). The shape and width of the RT can be perfectly described by the well-known effective medium theory, which allows us to understand the origin of the inhomogeneity in the superconducting properties of TiN films. We propose that this inhomogeneity can have both dynamic and static origins. The dynamic mechanism is associated with spontaneous fluctuations in electron temperature (T-fluctuations), while the static mechanism is due to a random spatial distribution of surface magnetic disorder (MD). Our analysis has revealed clear correlations between the transition width and material parameters as well as device size for both proposed mechanisms. While T-fluctuations may contribute significantly to the observed transition width, our findings suggest that the dominant contribution comes from the MD mechanism. Our results provide new insights into the microscopic origin of broadening of the superconducting transition and inhomogeneity in thin superconducting films.

cond-mat.supr-con↗

arXiv2024

Slow electron-phonon relaxation controls the dynamics of the superconducting resistive transition

We investigate the temporal and spatial scales of resistance fluctuations ($R$-fluctuations) at the superconducting resistive transition accessed through voltage fluctuations measurements in thin epitaxial TiN films. This material is characterized by a slow electron-phonon relaxation, which puts it far beyond the applicability range of the textbook scenario of superconducting fluctuations. The measured Lorentzian spectrum of the $R$-fluctuations identifies their correlation time, which is nearly constant across the transition region and has no relation to the conventional Ginzburg-Landau time scale. Instead, the correlation time coincides with the energy relaxation time determined by a combination of the electron-phonon relaxation and the relaxation via diffusion into reservoirs. Our data is quantitatively consistent with the model of spontaneous temperature fluctuations and highlight the lack of understanding of the resistive transition in materials with slow electron-phonon relaxation.

cond-mat.supr-con↗

arXiv2024

Anomalous terahertz photoconductivity caused by the superballistic flow of hydrodynamic electrons in graphene

Light incident upon materials can induce changes in their electrical conductivity, a phenomenon referred to as photoresistance. In semiconductors, the photoresistance is negative, as light-induced promotion of electrons across the band gap enhances the number of charge carriers participating in transport. In superconductors, the photoresistance is positive because of the destruction of the superconducting state, whereas in normal metals it is vanishing. Here we report a qualitative deviation from the standard behavior in metallic graphene. We show that Dirac electrons exposed to continuous wave (CW) terahertz (THz) radiation can be thermally decoupled from the lattice by 50~K which activates hydrodynamic electron transport. In this regime, the resistance of graphene constrictions experiences a decrease caused by the THz-driven superballistic flow of correlated electrons. We analyze the dependencies of the negative photoresistance on the carrier density, and the radiation power and show that our superballistic devices operate as sensitive phonon-cooled bolometers and can thus offer a picosecond-scale response time. Beyond their fundamental implications, our findings underscore the practicality of electron hydrodynamics in designing ultra-fast THz sensors and electron thermometers.

cond-mat.mes-hall↗

arXiv2023

Fundamental limits of few-layer NbSe$_2$ microbolometers at terahertz frequencies

The rapid development of infrared spectroscopy, observational astronomy, and scanning near-field microscopy has been enabled by the emergence of sensitive mid- and far-infrared photodetectors. Owing to their exceptional signal-to-noise ratio and fast photoresponse, superconducting hot-electron bolometers (HEBs) have become a critical component in these applications. While superconducting HEBs are traditionally made from sputtered superconducting thin films like Nb or NbN, the potential of layered van der Waals (vdW) superconductors is untapped at THz frequencies. Here, we report the fabrication of superconducting HEBs out of few-layer NbSe$_2$ microwires. By improving the interface between NbSe$_2$ and metal leads connected to a broadband antenna, we overcome the impedance mismatch between this vdW superconductor and the radio frequency (RF) readout circuitry that allowed us to achieve large responsivity THz detection over the range from 0.13 to 2.5 THz with minimum noise equivalent power of 7~pW$\sqrt{Hz}$. Using the heterodyne sub-THz mixing technique, we reveal that NbSe$_2$ superconducting HEBs are relatively fast and feature a characteristic response time in the nanosecond range limited by the slow heat escape to the bath through a SiO$_2$ layer, on which they are assembled, in agreement with energy relaxation model. Our work expands the family of materials for superconducting HEBs technology, reveals NbSe$_2$ as a promising platform, and offers a reliable protocol for the in-lab production of custom bolometers using the vdW assembly technique.

cond-mat.supr-con↗

arXiv2022

Evidence of the disorder-independent electron-phonon scattering time in thin NbN films

We report on experimental study of the effect of disorder on electronic parameters and inelastic scattering mechanisms in ultrathin superconducting NbN films, which are commonly used in single-photon detectors. An increase in disorder in the studied 2.5 nm thick NbN films characterized by Ioffe-Regel parameter from 6.3 to 1.6 is accompanied by a decrease in the critical temperature $T_c$ from 11.5 K to 3.4 K. By measuring magnetoconductance in the range from $T_c$ to $\sim3T_c$, we extract the inelastic scattering rates of electrons, including electron-phonon (e-ph) scattering rates $τ_{e-ph}^{-1}$. We observe that $τ_{e-ph}^{-1}$ and their temperature dependencies are insensitive to disorder that is not described by the existing models of the e-ph scattering in disordered metals and can be due to the presence of weakly disordered metal grains. As the temperature decreases the temperature dependence $τ_{e-ph}^{-1}$ changes from $T^3$ to $T^2$, which can be result of a decrease in the dimension of the phonons involved in the e-ph scattering. The obtained values of material parameters of ultrathin NbN films can be useful for optimization of performance of NbN-based electronic devices.

cond-mat.supr-con↗

arXiv2021

Thermal relaxation in metal films limited by diffuson lattice excitations of amorphous substrates

Here we examine the role of the amorphous insulating substrate in the thermal relaxation in thin NbN, InO$_x$, and Au/Ni films at temperatures above 5 K. The studied samples are made up of metal bridges on an amorphous insulating layer lying on or suspended above a crystalline substrate. Noise thermometry was used to measure the electron temperature $T_e$ of the films as a function of Joule power per unit of area $P_{2D}$. In all samples, we observe the dependence $P_{2D}\propto T_e^n$ with the exponent $n\simeq 2$, which is inconsistent with both electron-phonon coupling and Kapitza thermal resistance. In suspended samples, the functional dependence of $P_{2D}(T_e)$ on the length of the amorphous insulating layer is consistent with the linear $T$-dependence of the thermal conductivity, which is related to lattice excitations (diffusons) for the phonon mean free path smaller than the dominant phonon wavelength. Our findings are important for understanding the operation of devices embedded in amorphous dielectrics.

cond-mat.mtrl-sci↗

arXiv2019

Sub-shot-noise-limited fiber-optic quantum receiver

We experimentally demonstrate a quantum receiver based on Kennedy scheme for discrimination between two phase-modulated weak coherent states. The receiver is assembled entirely from the standard fiber-optic elements and operates at the conventional telecom wavelength 1.55 microns. The local oscillator and the signal are transmitted through different optical fibers, and the displaced signal is measured with a high-efficiency superconducting nanowire single-photon detector. We show the discrimination error rate two times below the shot-noise limit.

quant-ph↗

arXiv2019

Superconductivity behavior in epitaxial TiN films points at surface magnetic disorder

We analyze the evolution of the normal and superconducting electronic properties in epitaxial TiN films, characterized by high Ioffe-Regel parameter values, as a function of the film thickness. As the film thickness decreases, we observe an increase of in the residual resistivity, which becomes dominated by diffusive surface scattering for $d\leq20\,$nm. At the same time, a substantial thickness-dependent reduction of the superconducting critical temperature is observed compared to the bulk TiN value. In such a high quality material films, this effect can be explained by a weak magnetic disorder residing in the surface layer with a characteristic magnetic defect density of $\sim10^{12}\,\mathrm{cm}^{-2}$. Our results suggest that surface magnetic disorder is generally present in oxidized TiN films.

cond-mat.mtrl-sci↗

arXiv2019

Thermal properties of NbN single-photon detectors

We investigate thermal properties of a NbN single-photon detector capable of unit internal detection efficiency. Using an independent calibration of the coupling losses we determine the absolute optical power absorbed by the NbN film and, via a resistive superconductor thermometry, the thermal resistance Z(T) of the NbN film in dependence of temperature. In principle, this approach permits a simultaneous measurement of the electron-phonon and phonon-escape contributions to the energy relaxation, which in our case is ambiguous for their similar temperature dependencies. We analyze the Z(T) within the two-temperature model and impose an upper bound on the ratio of electron and phonon heat capacities in NbN, which is surprisingly close to a recent theoretical lower bound for the same quantity in similar devices.

cond-mat.supr-con↗

arXiv2018

Optical single photon detection in micron-scaled NbN bridges

We demonstrate experimentally that single photon detection can be achieved in micron-wide NbN bridges, with widths ranging from 0.53 $μ$m to 5.15 $μ$m and for photon-wavelengths from 408 nm to 1550 nm. The microbridges are biased with a dc current close to the experimental critical current, which is estimated to be about 50 % of the theoretically expected depairing current. These results offer an alternative to the standard superconducting single-photon detectors (SSPDs), based on nanometer scale nanowires implemented in a long meandering structure. The results are consistent with improved theoretical modelling based on the theory of non-equilibrium superconductivity including the vortex-assisted mechanism of initial dissipation.

cond-mat.supr-con↗

arXiv2016

Relaxation of the resistive superconducting state in boron-doped diamond films

We report a study of the relaxation time of the restoration of the resistive superconducting state in single crystalline boron-doped diamond using amplitude-modulated absorption of (sub-)THz radiation (AMAR). The films grown on an insulating diamond substrate have a low carrier density of about 2.5x10^{21} cm^{-3} and a critical temperature of about 2 K. By changing the modulation frequency we find a high-frequency rolloff which we associate with the characterstic time of energy relaxation between the electron and the phonon systems or the relaxation time for nonequilibrium superconductivity. Our main result is that the electron-phonon scattering time varies clearly as T^{-2}, over the accessible temperature range of 1.7 to 2.2 K. In addition, we find, upon approaching the critical temperature T_c, evidence for an increasing relaxation time on both sides of T_c.

cond-mat.supr-con↗

arXiv2015

Vortex assisted mechanism of photon counting in superconducting nanowire single photon detector revealed by external magnetic field

We use external magnetic field to probe the detection mechanism of superconducting nanowire single photon detector. We argue that the hot belt model (which assumes partial suppression of the superconducting order parameter $Δ$ across the whole width of the superconducting nanowire after absorption of the single photon) does not explain observed weak field dependence of the photon count rate (PCR) for photons with $λ$=450 nm and noticeable {\it decrease} of PCR (with increasing the magnetic field) in some range of the currents for photons with wavelengths $λ$ =450-1200 nm. Found experimental results for all studied wavelengths $λ= 450-1550$ nm could be explained by the vortex hot spot model (which assumes partial suppression of $Δ$ in the area with size smaller than the width of the nanowire) if one takes into account nucleation and entrance of the vortices to the photon induced hot spot and their pinning by the hot spot with relatively large size and strongly suppressed $Δ$.

cond-mat.supr-con↗

arXiv2014

Superconducting single-photon detector made of MoSi film

We fabricated and characterised nanowire superconducting single-photon detectors (SSPDs) made of 4 nm thick amorphous Mo$_x$Si$_{1-x}$ films. At 1.7 K the best devices exhibit a detection efficiency up to 18% at 1.2 um wavelength of unpolarised light, a characteristic response time of about 6 ns and timing jitter of 120 ps. The detection efficiency was studied in wavelength range from 650 nm to 2500 nm. At wavelengths below 1200 nm these detectors reach their maximum detection efficiency limited by photon absorption in the thin MoSi film.

cond-mat.supr-con↗

arXiv2012

High Speed and High Efficiency Travelling Wave Single-Photon Detectors Embedded in Nanophotonic Circuits

Ultrafast, high quantum efficiency single photon detectors are among the most sought-after elements in modern quantum optics and quantum communication. High photon detection efficiency is essential for scalable measurement-based quantum computation, quantum key distribution, and loophole-free Bell experiments. However, imperfect modal matching and finite photon absorption rates have usually limited the maximum attainable detection efficiency of single photon detectors. Here we demonstrate a superconducting nanowire detector atop nanophotonic waveguides which allows us to drastically increase the absorption length for incoming photons. When operating the detectors close to the critical current we achieve high on-chip single photon detection efficiency up to 91% at telecom wavelengths, with uncertainty dictated by the variation of the waveguide photon flux. We also observe remarkably low dark count rates without significant compromise of detection efficiency. Furthermore, our detectors are fully embedded in a scalable silicon photonic circuit and provide ultrashort timing jitter of 18ps. Exploiting this high temporal resolution we demonstrate ballistic photon transport in silicon ring resonators. The direct implementation of such a detector with high quantum efficiency, high detection speed and low jitter time on chip overcomes a major barrier in integrated quantum photonics.

physics.optics↗