arXiv subjects

Guy Deutscher

Publications and source records attributed to Guy Deutscher.

15 recordsLinked to original sources

arXiv2024

Even-Balanced States Excitation in Two-Qubits System

An essential step in building a quantum computer system is combining two qubits. Previous theoretical analyses of this element have been based primarily on the short schematic matrix approaches. In the present work we extend the investigation to the combination of two qubits by implementing the Dirac picture method as a theoretical framework. We conduct simulations, using parameters of grAl qubits, to demonstrate the feasibility of driving two qubits into an even-balanced quantum state. The research highlights the effectiveness of the Dirac picture method in achieving the desired state, enabling improved control of entangled states and advanced quantum computing research. The simulation results and the theoretical calculations agree within of 0.2%, and the two examples at section IV, show a good agreement to the level of 6% error from the even balanced state

quant-ph↗

arXiv2021

Tunneling study in granular aluminum near the Mott metal-to-insulator transition

We find excellent agreement between tunneling and optical conductivity gap values in superconducting granular aluminum films, up to the metal-to-insulator transition. This behavior, in strong contrast with that recently reported in atomically disordered samples for which the optical gap becomes smaller than the tunneling gap, suggests that disorder is not at the origin of the transition. The large increase seen in the strong coupling ratio and a finite value of the gap at T_c near the metal-to-insulator transition are consistent with a BCS to BEC crossover.

cond-mat.supr-con↗

arXiv2020

Granular superconductors for high kinetic inductance and low loss quantum devices

Granular aluminum is a promising material for high kinetic inductance devices such as qubit circuits. It has the advantage over atomically disordered materials such as NbN_x, to maintain a high kinetic inductance concomitantly with a high quality factor. We show that high quality nano-scale granular aluminum films having a sharp superconducting transition with normal state resistivity values of the order of 1x10^5 μΩcm and kinetic inductance values of the order of 10 nH/sq can be obtained, surpassing state of the art values. We argue that this is a result of the different nature of the metal-to-insulator transition, being electronic correlations driven (Mott type) in the former and disorder driven (Anderson type) in the latter.

cond-mat.supr-con↗

arXiv2019

Optical conductivity of granular aluminum films near the Mott metal-to-insulator transition

We report measurements of the energy gap of granular aluminum films by THz spectroscopy. We find that as the grains progressively decouple, the coupling ratio $2Δ(0)/k_{B}T_{c}$ increases above the BCS weak coupling ratio $3.53$, and reaches values consistent with an approach to BCS-BEC crossover for the high resistivity samples, expected from the short coherence length. The Mattis-Bardeen theory describes remarkably well the behavior of $σ_{1,s}/σ_{1,n}$ for all samples up to very high normal state resistivities.

cond-mat.supr-con↗

arXiv2018

Kubo spins in nano-scale aluminum grains: A muon spin relaxation study

We report muon spin relaxation rate measurements on films composed of aluminum grains having a size of a few nm, with a large energy level splitting of the order of 100 K. The films range from weakly metallic to insulating. In the insulating case the low temperature relaxation rate is consistent with the presence of single electron spins in grains having an odd number of electrons. The relaxation rate temperature dependence follows an activation law having an energy scale in agreement with the average level splitting. In weakly metallic films the relaxation rate is smaller and decreases faster with temperature. Overall our observations are in line with the presence of a Kubo spin in Al nano-size grains due to quantum size effects.

cond-mat.str-el↗

arXiv2017

Optical signatures of the superconducting Goldstone mode in granular aluminum: experiments and theory

Recent advances in the experimental growth and control of disordered thin films, heterostructures, and interfaces provide a fertile ground for the observation and characterisation of the collective superconducting excitations emerging below $T_c$ after breaking the $U(1)$ gauge symmetry. Here we combine THz experiments in a nano-structured granular Al thin film and theoretical calculations to demonstrate the existence of optically-active phase modes, which represent the Goldstone excitations of the broken gauge symmetry. By measuring the complex transmission trough the sample we identify a sizeable and temperature-dependent optical sub-gap absorption, which cannot be ascribed to quasiparticle excitations. A quantitative modelling of this material as a disordered Josephson array of nano-grains allows us to determine, with no free parameters, the structure of the spatial inhomogeneities induced by shell effects. Besides being responsible for the enhancement of the critical temperature with respect to bulk Al, already observed in the past, this spatial inhomogeneity provides a mechanism for the optical visibility of the Goldstone mode. By computing explicitly the optical spectrum of the superconducting phase fluctuations we obtain a good quantitative description of the experimental data. Our results demonstrate that nanograins arrays are a promising setting to study and control the collective superconducting excitations via optical means.

cond-mat.supr-con↗

arXiv2015

Shaping a superconducting dome: Enhanced Cooper-pairing versus suppressed phase coherence in coupled aluminum nanograins

Deterministic enhancement of the superconducting (SC) critical temperature $T_c$ is a long-standing goal in material science. One strategy is engineering a material at the nanometer scale such that quantum confinement strengthens the electron pairing, thus increasing the superconducting energy gap $Δ$, as was observed for individual nanoparticles. A true phase-coherent SC condensate, however, can exist only on larger scales and requires a finite phase stiffness $J$. In the case of coupled aluminium (Al) nanograins, $T_c$ can exceed that of bulk Al by a factor of three, but despite several proposals the relevant mechanism at play is not yet understood. Here we use optical spectroscopy on granular Al to disentangle the evolution of the fundamental SC energy scales, $Δ$ and $J$, as a function of grain coupling. Starting from well-coupled arrays, $Δ$ grows with progressive grain decoupling, causing the increasing of $T_c$. As the grain-coupling is further suppressed, $Δ$ saturates while $T_c$ decreases, concomitantly with a sharp decline of $J$. This crossover to a phase-driven SC transition is accompanied by an optical gap persisting above $T_c$. These findings identify granular Al as an ideal playground to test the basic mechanisms that enhance superconductivity by nano-inhomogeneity.

cond-mat.supr-con↗

arXiv2014

Onset of Nernst Effect Beyond the Coherence Critical Field of a Nano-Scale Granular Superconductor

We report measurements of the Nernst effect and of the magneto-resistance of granular aluminum films near the metal to insulator transition. These films show sharp transitions as a function of temperature and magnetic field. At low temperatures the Nernst signal displays a sharp peak at a field where more than 90% of the normal state resistance has been restored, suggesting a transition involving entropy transport after superconducting coherence has been destroyed. At temperatures higher than the critical temperature the fluctuation paraconductivity scales with the Nernst signal, in agreement with a description in terms of fluctuations of the order parameter.

cond-mat.supr-con↗

arXiv2011

Coexistence of a triplet nodal order-parameter and a singlet order-parameter at the interfaces of ferromagnet-superconductor Co/CoO/In junctions

We present differential conductance measurements of Cobalt / Cobalt-Oxide / Indium planar junctions, 500nm x 500nm in size. The junctions span a wide range of barriers, from very low to a tunnel barrier. The characteristic conductance of all the junctions show a V-shape structure at low bias instead of the U-shape characteristic of a s-wave order parameter. The bias of the conductance peaks is, for all junctions, larger than the gap of indium. Both properties exclude pure s-wave pairing. The data is well fitted by a model that assumes the coexistence of s-wave singlet and equal spin p-wave triplet fluids. We find that the values of the s-wave and p-wave gaps follow the BCS temperature dependance and that the amplitude of the s-wave fluid increases with the barrier strength.

cond-mat.supr-con↗

arXiv2010

Origin of Weak-Link Behavior of Grain Boundaries in Superconducting Cuprates and Pnictides

Superconducting cuprates and pnictides composed of CuO2 or AsFe planes respectively with intercalated insulating layers, are at the crossroads of three families of crystalline solids: metals, doped Mott insulators, and ferroelectrics. In the latter atomic displacements play a key role. Both the metallic and the doped insulator approaches to high temperature superconductivity are essentially electronic ones and do not directly involve the lattice. By contrast, in a recently proposed Bond Contraction Pairing (BCP) model, contraction of in-plane Cu-O (or As-Fe) bonds plays an essential role in the pairing mechanism. Here we apply it to low angle grain boundaries and show that their reduced critical current is due to tensile deformation generated by dislocations. The model also explains why interface misfit dislocations, which can result in a dead layer in the case of ferro-electrics, may improve vortex pinning in the cuprates.

cond-mat.supr-con↗

arXiv2008

A spatial interpretation of emerging superconductivity in lightly doped cuprates

The formation of domains comprising alternating 'hole rich' and 'hole poor' ladders recently observed by Scanning Tunneling Microscopy by Kohsaka et al., on lightly hole doped cuprates, is interpreted in terms of an attractive mechanism which favors the presence of doped holes on Cu sites located each on one side of an oxygen atom. This mechanism leads to a geometrical pattern of alternating hole-rich and hole-poor ladders with a periodicity equal to 4 times the lattice spacing in the CuO plane, as observed experimentally. To cite this article: G. Deutscher, P.-G. de Gennes, C. R. Physique 8 (2007).

cond-mat.supr-con↗

arXiv2004

Andreev - Saint James reflections: a probe of cuprate superconductors

Electrical transport through a normal metal / superconductor contact at biases smaller than the energy gap can occur via the reflection of an electron as a hole of opposite wave vector. The same mechanism of electron-hole reflection gives rise to low energy states at the surface of unconventional superconductors having nodes in their order parameter. The occurrence of electron-hole reflections at normal metal / superconductor interfaces was predicted independently by Saint James and de Gennes and by Andreev, and their spectroscopic features discussed in detail by Saint James in the early sixties. They are generally called Andreev reflections but, for that reason, we call them Andreev - Saint James (ASJ) reflections. We present a historical review of ASJ reflections and spectroscopy in conventional superconductors, and review their application to the High $T_c$ cuprates. The occurrence of ASJ reflections in all studied cuprates is well documented for a broad range of doping levels, implying that there is no large asymmetry between electrons and holes near the Fermi level in the superconducting state. In the underdoped regime, where the pseudo-gap phenomenon has been observed by other methods such as NMR, ARPES and Giaever tunneling, gap values obtained from ASJ spectroscopy are smaller than pseudo-gap values, indicating a lack of coherence in the pseudo-gap energy range.

cond-mat.supr-con↗

arXiv2004

High Quality a-axis outgrowth on c-axis YlCa1-xBa2Cu3O7-d

The large amplitude of the high Tc (HTS) superconducting gap is attractive for improved electronic applications. However, the study of such HTS cuprates has uncovered that unlike the s-wave order parameter of the low Tc, an angle dependent dx2-y2 wave function is the dominant order parameter in such compounds. This symmetry causes low energy surface bound states, detrimental for applications, except at (100) oriented surfaces. It is therefore essential to have a smooth and well oriented surface of the crystallographic a-axis (100). In this work we present a study of an unconventional way to attain such surfaces in the form of a-axis outgrowth on a c-axis surface of sputtered Y1-xCaxBa2Cu3O7-d thin film. The grains topography was tested using X-ray, SEM and AFM together with Point Contact and Tunnel Junctions measurements.

cond-mat.supr-con↗

arXiv2004

Determination of the critical current density in the d-wave superconductor YBCO under applied magnetic fields by nodal tunneling

We have studied nodal tunneling into YBa2Cu3O7-x (YBCO) films under magnetic fields. The films' orientation was such that the CuO2 planes were perpendicular to the surface with the a and b axis at 450 form the normal. The magnetic field was applied parallel to the surface and perpendicular to the CuO2 planes. The Zero Bias Conductance Peak (ZBCP) characteristic of nodal tunneling splits under the effect of surface currents produced by the applied fields. Measuring this splitting under different field conditions, zero field cooled and field cooled, reveals that these currents have different origins. By comparing the field cooled ZBCP splitting to that taken in decreasing fields we deduce a value of the Bean critical current superfluid velocity, and calculate a Bean critical current density of up to 3*10^7 A/cm2 at low temperatures. This tunneling method for the determination of critical currents under magnetic fields has serious advantages over the conventional one, as it avoids having to make high current contacts to the sample.

cond-mat.supr-con↗

arXiv2001

Origin of time reversal symmetry breaking in Y(1-y)Ca(y)Ba(2)Cu(3)O(7-x)

We have studied the Zero Bias Conductance Peak (ZBCP) of the tunneling conductance measured on (1,1,0) oriented Y(1-y)Ca(y)Ba(2)Cu(3)O(7-x) thin films as a function of doping and of magnetic field. A spontaneous (zero field) split of the ZBCP was observed only in overdoped samples (either by O or by Ca). The magnitude of this split was found to be linear in doping. All samples exhibited a magnetic field splitting, also strongly doping dependent. The field susceptibility chi=d(delta)/dH diverges at the point at which spontaneous ZBCP splitting occurs, its inverse value, chi^(-1), following a linear doping dependence on both the underdoped and overdoped sides. We discuss these results in terms of recent theoretical models of Time Reversal Symmetry Breaking (TRSB).

cond-mat.supr-con↗