arXiv subjects

Arindam Ghosh

Publications and source records attributed to Arindam Ghosh.

108 records · Page 6Linked to original sources

arXiv2008

Electrochemical fabrication of ultralow noise metallic nanowires with hcp crystalline lattice

We experimentally demonstrate that low-frequency electrical noise in silver nanowires is heavily suppressed when the crystal structure of the nanowires is hexagonal closed pack (hcp) rather than face centered cubic (fcc). Using a low-potential electrochemical method we have grown single crystalline silver nanowires with hcp crystal structure, in which the noise at room temperature is two to six orders of magnitude lower than that in the conventional fcc nanowires of the same diameter. We suggest that motion of dislocations is probably the primary source of electrical noise in metallic nanowires, which is strongly diminished in hcp crystals.

cond-mat.mtrl-sci↗

arXiv2008

Signature of Martensite transformation on conductivity noise in thin films of NiTi shape memory alloys

Slow time-dependent fluctuations, or noise, in the electrical resistance of dc magnetron sputtered thin films of Nickel Titanium shape memory alloys have been measured. Even in equilibrium, the noise was several orders of magnitude larger than that of simple diffusive metallic films, and was found to be non-monotonic around the martensitic transformation regime. The results are discussed in terms of dynamics of structural defects, which also lay foundation to a new noise-based characterization scheme of martensite transformation.

cond-mat.other↗

arXiv2008

Criticality of tuning in athermal phase transitions

We experimentally address the importance of tuning in athermal phase transitions, which are triggered only by a slowly varying external field acting as tuning parameter. Using higher order statistics of fluctuations, a singular critical instability is detected for the first time in spite of an apparent universal self-similar kinetics over a broad range of driving force. The results as well as the experimental technique are likely to be of significance to many slowly driven non-equilibrium systems from geophysics to material science which display avalanche dynamics.

cond-mat.dis-nn↗

arXiv2007

Quantisation of Hopping Magnetoresistance Prefactor in Strongly Correlated Two-Dimensional Electron Systems

We report an universal behaviour of hopping transport in strongly interacting mesoscopic two-dimensional electron systems (2DES). In a certain window of background disorder, the resistivity at low perpendicular magnetic fields follows the expected relation $ρ(B_\perp) = ρ_{\rm{B}}\exp(αB_\perp^2)$. The prefactor $ρ_{\rm{B}}$ decreases exponentially with increasing electron density but saturates to a finite value at higher densities. Strikingly, this value is found to be universal when expressed in terms of absolute resistance and and shows quantisation at $R_{\rm{B}}\approx h/e^2$ and $R_{\rm{B}}\approx 1/2$ $ h/e^2$. We suggest a strongly correlated electronic phase as a possible explanation.

cond-mat.mes-hall↗

arXiv2007

Observation of a two-dimensional spin-lattice in non-magnetic semiconductor heterostructures

Tunable magnetic interactions in high-mobility nonmagnetic semiconductor heterostructures are centrally important to spin-based quantum technologies. Conventionally, this requires incorporation of "magnetic impurities" within the two-dimensional (2D) electron layer of the heterostructures, which is achieved either by doping with ferromagnetic atoms, or by electrostatically printing artificial atoms or quantum dots. Here we report experimental evidence of a third, and intrinsic, source of localized spins in high-mobility GaAs/AlGaAs heterostructures, which are clearly observed in the limit of large setback distance (=80 nm) in modulation doping. Local nonequilibrium transport spectroscopy in these systems reveals existence of multiple spins, which are located in a quasi-regular manner in the 2D Fermi sea, and mutually interact at temperatures below 100 milliKelvin via the Ruderman-Kittel-Kasuya-Yosida (RKKY) indirect exchange. The presence of such a spin-array, whose microscopic origin appears to be disorder-bound, simulates a 2D lattice-Kondo system with gate-tunable energy scales.

cond-mat.mes-hall↗

arXiv2005

Experimental Measurement of Mixed State Geometric Phase by Quantum Interferometry using NMR

Geometric phase has been proposed as one of the promising methodologies to perform fault tolerant quantum computations. However, since decoherence plays a crucial role in such studies, understanding of mixed state geometric phase has become important. While mixed state geometric phase was first introduced mathematically by Uhlmann, recently Sjoqvist et al. [Phys. Rev. Lett. 85(14), 2845 (2000)] have described the mixed state geometric phase in the context of quantum interference and shown theoretically that the visibility as well as the shift of the interference pattern are functions of geometric phase and the purity of the mixed state. Here we report the first experimental study of the dependence of interference visibility and shift of the interference pattern on the mixed state geometric phase by Nuclear Magnetic Resonance.

quant-ph↗

arXiv2005

Experimental implementation of local adiabatic evolution algorithms by an NMR quantum information processor

Quantum adiabatic algorithm is a method of solving computational problems by evolving the ground state of a slowly varying Hamiltonian. The technique uses evolution of the ground state of a slowly varying Hamiltonian to reach the required output state. In some cases, such as the adiabatic versions of Grover's search algorithm and Deutsch-Jozsa algorithm, applying the global adiabatic evolution yields a complexity similar to their classical algorithms. However, using the local adiabatic evolution, the algorithms given by J. Roland and N. J. Cerf for Grover's search [ Phys. Rev. A. {\bf 65} 042308(2002)] and by Saurya Das, Randy Kobes and Gabor Kunstatter for the Deutsch-Jozsa algorithm [Phys. Rev. A. {\bf 65}, 062301 (2002)], yield a complexity of order $\sqrt{N}$ (where N=2$^{\rm n}$ and n is the number of qubits). In this paper we report the experimental implementation of these local adiabatic evolution algorithms on a two qubit quantum information processor, by Nuclear Magnetic Resonance.

quant-ph↗

arXiv2005

Relaxation of Pseudo pure states : The Role of Cross-Correlations

In Quantum Information Processing by NMR one of the major challenges is relaxation or decoherence. Often it is found that the equilibrium mixed state of a spin system is not suitable as an initial state for computation and a definite initial state is required to be prepared prior to the computation. As these preferred initial states are non-equilibrium states, they are not stationary and are destroyed with time as the spin system relaxes toward its equilibrium, introducing error in computation. Since it is not possible to cut off the relaxation processes completely, attempts are going on to develop alternate strategies like Quantum Error Correction Codes or Noiseless Subsystems. Here we study the relaxation behavior of various Pseudo Pure States and analyze the role of Cross terms between different relaxation processes, known as Cross-correlation. It is found that while cross-correlations accelerate the relaxation of certain pseudo pure states, they retard that of others.

quant-ph↗

arXiv2005

Zero-bias anomaly and Kondo-assisted quasi-ballistic 2D transport

Nonequilibrium transport measurements in mesoscopic quasi-ballistic 2D electron systems show an enhancement in the differential conductance around the Fermi energy. At very low temperatures, such a zero-bias anomaly splits, leading to a suppression of linear transport at low energies. We also observed a scaling of the nonequilibrium characteristics at low energies which resembles electron scattering by two-state systems, addressed in the framework of two-channel Kondo model. Detailed sample-to-sample reproducibility indicates an intrinsic phenomenon in unconfined 2D systems in the low electron-density regime.

cond-mat.mes-hall↗

arXiv2004

A set-up for measurement of low frequency conductance fluctuation (noise) using digital signal processing techniques

We describe a set up for measurements of low frequency (1 mHz < f < 20 Hz) conductance fluctuations in a solid conductor. The set-up uses a five probe a.c. measurement technique and extensive digital signal processing to reach a noise floor down to $S_{v}(f) \leq 10^{-20}$ V$^{2}$Hz$^{-1}$. The set up also allows measurement of noise using an a.c. in presence of a superimposed direct current. This feature is desirable in studies of electromigration damage or in systems that show non-linear conductivity. In addition, we describe a scheme which allows us to obtain the probability density function of the conductance fluctuation after subtracting the extraneous noise contributions (background) from the observed noise. The set-up has been used for conductance fluctuation measurement in the temperature range 1.5 K < T < 500 K in the presence of magnetic fields. We present some representative data obtained by this system.

cond-mat.other↗

arXiv2003

Quantum Information Processing by NMR using strongly coupled spins

The enormous theoretical potential of Quantum Information Processing (QIP) is driving the pursuit for its practical realization by various physical techniques. Currently Nuclear Magnetic Resonance (NMR) has been the forerunner by demonstrating a majority of quantum algorithms. In NMR, spin systems consisting of coupled nuclear spins are utilized as qubits. In order to carry out QIP, a spin system has to meet two major requirements: (i) qubit addressability and (ii) mutual coupling among the qubits. It has been demonstrated that the magnitude of the mutual coupling among qubits can be increased by orienting the spin-systems in a liquid crystal matrix and utilizing the residual dipolar couplings. While utilizing residual dipolar couplings may be useful to increase the number of qubits, nuclei of same species (homonuclei) might become strongly coupled. In strongly coupled spin-systems, spins loose their individual identity of being qubits. We propose that even such strongly coupled spin-systems can be used for QIP and the qubit-manipulation can be achieved by transition-selective pulses. We demonstrate experimental preparation of pseudopure states, creation of maximally entangled states, implementation logic gates and implementation of Deutsch-Jozsa (DJ) algorithm in strongly coupled 2,3 and 4 spin systems. The energy levels of the strongly coupled 3 and 4 spin systems were obtained by using a Z-COSY experiment.

quant-ph↗

arXiv2002

Observation of non-gaussian conductance fluctuations at low temperatures in Si:P(B) at the metal-insulator transition

We report investigations of conductance fluctuations (noise) in doped silicon at low temperatures (T$<20$K) as it is tuned through the metal-insulator transition (MIT). The scaled magnitude of noise, $γ_H$, increases with decrease in T following an approximate power law $γ_H \sim T^{-β}$. At low T, $γ_H$ diverges as $n/n_c$ crosses 1 from the metallic side. We find that the distribution function and second spectrum of the fluctuations show strong non-gaussian behavior below 20K as $n/n_c$ decreases through 1. In particular, the observed distribution function which is gaussian for $n/n_c >> 1$, develops a log-normal tail as the transition is approached from the metallic side and eventually it dominates in the critical region.

cond-mat.dis-nn↗

arXiv1999

Electric field induced suppression of universal conductance fluctuations and dephasing in disordered systems

We report a novel phenomenon that the universal conductance fluctuations (UCF) can be suppressed by a small electric field E. The experiment has been carried out on single crystals of Si doped heavily with P and B beyond the critical composition of insulator to metal transition. The phenomenon is identified as a consequence of electric field induced dephasing of the electron wavefunction. Over the range of measurements, the observed dephasing rate (τ_ϕ^{-1}) varied as τ_ϕ^{-1} = aT + bE^q with q \approx 1.3 and for E >> E^*, a cross-over field, τ_ϕ^{-1} \sim E^q, independent of T. This experiment also establishes that the UCF can be utilized as a sensitive electron "interferometer" to measure dephasing rate.

cond-mat.mes-hall↗

arXiv1999

Universal conductance fluctuations in three dimensional metallic single crystals of Si

In this paper we report the measurement of conductance fluctuations in single crystals of Si made metallic by heavy doping (n \approx 2-2.5n_c, n_c being critical composition at Metal-Insulator transition). Since all dimensions (L) of the samples are much larger than the electron phase coherent length L_ϕ(L/L_ϕ\sim 10^3), our system is truly three dimensional. Temperature and magnetic field dependence of noise strongly indicate the universal conductance fluctuations (UCF) as predominant source of the observed magnitude of noise. Conductance fluctuations within a single phase coherent region of L_ϕ^3 was found to be saturated at <(δG_ϕ)^2> \approx (e^2/h)^2. An accurate knowledge of the level of disorder, enables us to calculate the change in conductance δG_1 due to movement of a single scatterer as δG_1 \sim e^2/h, which is \sim 2 orders of magnitude higher than its theoretically expected value in 3D systems.

cond-mat.dis-nn↗

arXiv1999

Non-linear electrical conduction and broadband noise in charge-ordered rare earth manganate Nd_0.5Ca_0.5MnO_3

Measurements of the dc transport properties and the low-frequency conductivity noise in films of charge ordered Nd_0.5Ca_0.5MnO_3 grown on Si subtrate reveal the existence of a threshold field in the charge ordered regime beyond which strong non linear conduction sets in along with a large broad band conductivity noise. Threshold-dependent conduction disappears as T -> T_{CO}, the charge ordering temperature. This observation suggests that the charge ordered state gets depinned at the onset of the non-linear conduction.

cond-mat.str-el↗

arXiv1999

Conductance fluctuations near Anderson transition

In this paper we report measurements of conductance fluctuations in single crystal samples of Si doped with P and B close to the critical composition of the metal insulator transition (n_c \approx 4 x 10^18 /cc). The measurements show that the noise, which arises from bulk sources, does not diverge as the Ioffe-Regal limit (k_{F}l -> 1) is approached from the metallic side. At room temperatures, the magnitude of the noise shows a shallow maximum around k_{F}l \approx 1.5 and drops sharply as the insulating state is approached.

cond-mat.dis-nn↗

arXiv1998

Low energy excitations in crystalline perovskite oxides: Evidence from noise experiments

In this paper we report measurements of 1/f noise in a crystalline metallic oxide with perovskite structure down to 4.2K. The results show existence of localized excitations with average activation energy $\approx$ 70-80 meV which produce peak in the noise at T $\approx$ 35-40K. In addition, it shows clear evidence of tunnelling type two-level-systems (as in glasses) which show up in noise measurements below 30K.

cond-mat.dis-nn↗

arXiv1998

Kinetics of electric field induced oxygen ion migration in epitaxial metallic oxide films

In this paper we report the observation of curent induced change of resistance of thin metallic oxide films. The resistance changes at a very low current (current density $J \geq 10^{3}$ A/cm$^{2}$). We find that the time dependence associated with the processes (increase of resistance) show a streched exponential type dependence at lower temperature, which crosses over to a creep type behavior at $T \geq$ 350 K. The time scale associated shows a drastic drop in the magnitude at $T \approx$ 350 K, where a long range diffusion sets in increasing the conductivity noise. The phenomena is like a "glass-transition" in the random lattice of oxygen ions.

cond-mat.dis-nn↗