arXiv subjects

G. Sambandamurthy

Publications and source records attributed to G. Sambandamurthy.

32 records · Page 2Linked to original sources

arXiv2006

Melting of a 2D Quantum Electron Solid in High Magnetic Field

The melting temperature ($T_m$) of a solid is generally determined by the pressure applied to it, or indirectly by its density ($n$) through the equation of state. This remains true even for helium solids\cite{wilk:67}, where quantum effects often lead to unusual properties\cite{ekim:04}. In this letter we present experimental evidence to show that for a two dimensional (2D) solid formed by electrons in a semiconductor sample under a strong perpendicular magnetic field\cite{shay:97} ($B$), the $T_m$ is not controlled by $n$, but effectively by the \textit{quantum correlation} between the electrons through the Landau level filling factor $ν$=$nh/eB$. Such melting behavior, different from that of all other known solids (including a classical 2D electron solid at zero magnetic field\cite{grim:79}), attests to the quantum nature of the magnetic field induced electron solid. Moreover, we found the $T_m$ to increase with the strength of the sample-dependent disorder that pins the electron solid.

cond-mat.str-el↗

arXiv2006

Angular dependence of the magnetic-field driven superconductor-insulator transition in thin films of amorphous indium-oxide

A significant anisotropy of the magnetic-field driven superconductor-insulator transition is observed in thin films of amorphous indium-oxide. The anisotropy is largest for more disordered films which have a lower transition field. At higher magnetic field the anisotropy reduces and even changes sign beyond a sample specific and temperature independent magnetic field value. The data are consistent with the existence of more that one mechanism affecting transport at high magnetic fields.

cond-mat.supr-con↗

arXiv2005

Nanowire Acting as a Superconducting Quantum Interference Device

We present the results from an experimental study of the magneto-transport of superconducting wires of amorphous Indium-Oxide, having widths in the range 40 - 120 nm. We find that, below the superconducting transition temperature, the wires exhibit clear, reproducible, oscillations in their resistance as a function of magnetic field. The oscillations are reminiscent of those which underlie the operation of a superconducting quantum interference device.

cond-mat.supr-con↗

arXiv2005

Experimental evidence for a collective insulating state in two-dimensional superconductors

We present the results of an experimental study of the current-voltage characteristics in strong magnetic field ($B$) of disordered, superconducting, thin-films of amorphous Indium-Oxide. As the $B$ strength is increased superconductivity degrades, until a critical field ($B_c$) where the system is forced into an insulating state. We show that the differential conductance measured in the insulating phase vanishes abruptly below a well-defined temperature, resulting in a clear threshold for conduction. Our results indicate that a new collective state emerges in two-dimensional superconductors at high $B$.

cond-mat.supr-con↗

arXiv2004

Superconductivity-Related Insulating Behavior

We present the results of an experimental study of superconducting, disordered, thin-films of amorphous Indium Oxide. These films can be driven from the superconducting phase to a reentrant insulating state by the application of a perpendicular magnetic field ($B$). We find that the high-$B$ insulator exhibits activated transport with a characteristic temperature, $T_I$. $T_I$ has a maximum value ($T_{I}^p$) that is close to the superconducting transition temperature ($T_c$) at $B$ = 0, suggesting a possible relation between the conduction mechanisms in the superconducting and insulating phases. $T_{I}^p$ and $T_c$ display opposite dependences on the disorder strength.

cond-mat.supr-con↗

arXiv2002

Insulator superconductor transition on solid inert gas substrates

We present observations of the insulator-superconductor transition in ultrathin films of Bi on solid xenon condensed on quartz and on Ge on quartz. The relative permeability $ε_{r}$ ranges from 1.5 for Xe to 15 for Ge. Though we find screening effects as expected, the I-S transition is robust, and unmodified by the substrate. The resistance separatrix is found to be close to $h/4e^2$ and the crossover thickness close to 25 $Å$ for all substrates. I-V studies and Aslamazov-Larkin analyses indicate superconductivity is inhomogeneous. The transition is best described in terms of a percolation model.

cond-mat.supr-con↗

arXiv2002

Vortices and the mixed state of ultrathin Bi films

Current-voltage (I-V) characteristics of quench condensed, superconducting, ultrathin Bi films in a magnetic field are reported. These show hysteresis for all films, grown both with and without thin Ge underlayers. Films on Ge underlayers, close to superconductor-insulator transition, show a peak in the critical current, indicating a structural transformation of the vortex solid. These underlayers, used to make the films more homogeneous, are found to be more effective in pinning the vortices. The upper critical fields ($B_{c2}$) of these films are determined from the resistive transitions in perpendicular magnetic field. The temperature dependence of the upper critical field is found to differ significantly from Ginzburg-Landau theory, after modifications for disorder.

cond-mat.supr-con↗

arXiv2002

Critical currents and vortex-unbinding transitions in quench-condensed ultrathin films of Bismuth and Tin

We have investigated the I-V characteristics of strongly disordered ultra-thin films of {\it Bi} and {\it Sn} produced by quench-condensation. Our results show that both these sytems can be visualized as strongly disordered arrays of Josephson junctions. The experimentally observed I-V characteristics of these films is hysteretic, when the injected current is ramped from zero to critical current and back. These are remarkably similar to the hysteretic I-V of an underdamped single junction. We show by computer simulations that hysteresis can persist in a very strongly disordered array. It is also possible to estimate the individual junction parameters ($R$, $C$ and $I_c$) from the experimental I-Vs of the film using this model. The films studied are in a regime where the Josephson-coupling energy is larger than the charging energy. We find that a simple relation $I_c(T)=I_c(0)(1-(T/T_c)^4)$ describes the temperature dependence of the critical current quite accurately for films with sheet resistance $\sim$ 500$Ω$ or lower. We also find evidence of a vortex-unbindi

cond-mat.supr-con↗

arXiv2001

Vortex dynamics and upper critical fields in ultrathin Bi films

Current-voltage (I-V) characteristics of quench condensed, superconducting, ultrathin $Bi$ films in a magnetic field are reported. These I-V's show hysteresis for all films, grown both with and without thin $Ge$ underlayers. Films on Ge underlayers, close to superconductor-insulator transition (SIT), show a peak in the critical current, indicating a structural transformation of the vortex solid (VS). These underlayers, used to make the films more homogeneous, are found to be more effective in pinning the vortices. The upper critical fields (B$_{c2}$) of these films are determined from the resistive transitions in perpendicular magnetic field. The temperature dependence of the upper critical field is found to differ significantly from Ginzburg-Landau theory, after modifications for disorder.

cond-mat.supr-con↗

arXiv2001

Effect of granularity on the insulator-superconductor transition in ultrathin Bi films

We have studied the insulator-superconductor transition (IST) by tuning the thickness in quench-condensed $Bi$ films. The resistive transitions of the superconducting films are smooth and can be considered to represent "homogeneous" films. The observation of an IST very close to the quantum resistance for pairs, $R_{\Box}^N \sim h/4e^2$ on several substrates supports this idea. The relevant length scales here are the localization length, and the coherence length. However, at the transition, the localization length is much higher than the superconducting coherence length, contrary to expectation for a "homogeneous" transition. This suggests the invalidity of a purely fermionic model for the transition. Furthermore, the current-voltage characteristics of the superconducting films are hysteretic, and show the films to be granular. The relevant energy scales here are the Josephson coupling energy and the charging energy. However, Josephson coupling energies ($E_J$) and the charging energies ($E_c$) at the IST, they are found to obey the relation $E_J < E_c$. This is again contrary to expectation, for the IST in a granular or inhomogeneous, system. Hence, a purely bosonic picture of the transition is also inconsistent with our observations. We conclude that the IST observed in our experiments may be either an intermediate case between the fermioinc and bosonic mechanisms, or in a regime of charge and vortex dynamics for which a quantitative analysis has not yet been done.

cond-mat.supr-con↗

arXiv2000

Possible robust insulator-superconductor transition on solid inert gas and other substrates

We present observations of the insulator-superconductor transition in ultrathin films of Bi on amorphous quartz, quartz coated with Ge, and for the first time, solid xenon condensed on quartz. The relative permeability $ε_r$ ranges from 1.5 for Xe to 15 for Ge. Though we find screening effects as expected, the I-S transition is robust, and unmodified by the substrate. The resistance separatrix is found to be close to h/4e^2 and the crossover thickness close to $\rm 25 Å$ for all substrates. I-V studies and Aslamazov-Larkin analyses indicate superconductivity is inhomogeneous. The transition can be understood in terms of a percolation model.

cond-mat.supr-con↗

arXiv2000

Josephson junction array type I-V characteristics of quench-condensed ultra thin films of Bi

In this communication we report studies of d.c current-voltage (I-V) characteristics of ultra thin films of Bi, quench condensed on single crystal sapphire substrates at T = 15K. The hysteretic I-V characteristics are explained using a resistively and capacitively shunted junction (RCSJ) model of Josephson junction arrays. The Josephson coupling energy($E_J$) and the charging energy($E_c$) are calculated for different thickness($d$) values. A low resistance state is found in the low current regime below the critical current, $I_c$. This resistance $R_0$ is found to have a minimum at a particular thickness ($d_c$) value. Reflection High Energy Electron Diffraction (RHEED) studies are done on these films. A distinct appearance of a diffuse ring near $d_c$ is observed in the diffraction images, consistent with the recent STM studies(Ekinci and Valles, PRL {\bf 82}(1999) 1518). These films show an irreversible annealing when temperature is increased. The annealing temperature ($T_a$) also has a maximum at the same thickness. Althoguh the R$_s$ vs T of quench condensed Bi films suggest that the films are uniform, our results indicate that even in thick films, the order parameter is not fully developed over the complete area of the film. These results are discussed qualitatively.

cond-mat.supr-con↗

arXiv1999

Magnetoresistance and conductivity exponents of quench-condensed ultra-thin films of Bi

We have studied the magnetoresistance (MR) and evolution of conductivity with thickness of quench-condensed Bismuth films on substrates of various dielectric constants. Our results indicate a negative intial MR proportional to the square of the magnetic field. The conductance shows a power-law kind of dependence on thickness, with an exponent close to 1.33, characterisitic of a 2-D percolating system, only when the films are grown on a thin ($\sim 10${\rm Å} Germanium underlayer but not otherwise.

cond-mat.supr-con↗

arXiv1999

Electrical transport properties of ultrathin disordered films

We report an experimental study of quench condensed ($2K\le T \le 15K$) disordered ultrathin films of {\rm Bi} where localisation effects and superconductivity compete. Experiments are done with different substrates and/or different underlayers. Quasi-free standing films of {\rm Bi}, prepared by quenching {\rm Bi} vapours onto solid {\rm Xe}, are also studied. The results show a dependence of the transport properties both on the dielectric constant of the substrate/underlayer as well as the temperature of quench condensation. RHEED studies indicate that quantum size effects are important in these systems. In this paper, we try to correlate the structure of the films to the transport properties obtained.

cond-mat.supr-con↗