arXiv subjects

Ivar Martin

Publications and source records attributed to Ivar Martin.

115 records · Page 7Linked to original sources

arXiv2000

Visualizing the particle-hole dualism in high-temperature superconductors

Recent Scanning Tunneling Microscope (STM) experiments offer a unique insight into the inner workings of the superconducting state of high-Tc superconductors. Deliberately placed inside the material impurities perturb the coherent state and produce additional excitations. Superconducting excitations - quasiparticles - are the quantum mechanical mixture of negatively charged electron (-e) and positively charged hole (+e). Depending on the applied voltage bias in STM one can sample the particle and hole content of a superconducting excitation. We argue that the complimentary cross-shaped patterns observed on the positive and negative biases are the manifestation of the particle-hole dualism of the quasiparticles.

cond-mat.supr-con↗

arXiv2000

Finite Temperature Density Instability at High Landau Level Occupancy

We study here the onset of charge density wave instabilities in quantum Hall systems at finite temperature for Landau level filling $ν>4$. Specific emphasis is placed on the role of disorder as well as an in-plane magnetic field. Beyond some critical value, disorder is observed to suppress the charge density wave melting temperature to zero. In addition, we find that a transition from perpendicular to parallel stripes (relative to the in-plane magnetic field) exists when the electron gas thickness exceeds $\approx 60$Å. The perpendicular alignment of the stripes is in agreement with the experimental finding that the easy conduction direction is perpendicular to the in-plane field.

cond-mat.mes-hall↗

arXiv2000

Probing Pseudogap by Josephson Tunneling

We propose here an experiment aimed to determine whether there are superconducting pairing fluctuations in the pseudogap regime of the high-$T_c$ materials. In the experimental setup, two samples above $T_c$ are brought into contact at a single point and the differential AC conductivity in the presence of a constant applied bias voltage between the samples, $V$, should be measured. We argue the the pairing fluctuations will produce randomly fluctuating Josephson current with zero mean, however the current-current correlator will have a characteristic frequency given by Josephson frequency $ω_J = 2 e V /\hbar$. We predict that the differential AC conductivity should have a peak at the Josephson frequency with the width determined by the phase fluctuations time.

cond-mat.supr-con↗

arXiv1998

Superconductivity in a two-dimensional Electron Gas

In a series of recent experiments, Kravchenko and colleagues observed unexpectedly that a two-dimensional electron gas in zero magnetic field can be a conductor. The two-dimensionality was imposed by confining the electron gas to move laterally at the interface between two semiconductors. The observation of a conductor in two dimensions (2D) is surprising as the conventional theory of metals precludes the presence of a metallic state at zero temperature in 2D. Nonetheless, there are now several experiments confirming the existence of the new conducting phase in a dilute two-dimensional electron gas in zero magnetic field. Here we argue based on an analysis of the experiments and general theoretical grounds that this phase is a zero-temperature superconductor with an inhomogeneous charge density.

cond-mat.str-el↗

arXiv1997

Local Pairing at U-impurities in BCS Superconductors

We analyse here the role electrons on Anderson-U impurities play in superconductivity in a metal alloy. We find that phonon coupling at impurities counteracts the traditional effects which dominate T_c suppression in the non-magnetic limit. In some cases, we findt hat non-magnetic impurities can enhance T_c. In the Kondo limit, a Fermi liquid analysis reveals that it is the enhancement in the density of states arising from the Kondo resonancethat counteracts pair-weakening.

cond-mat.str-el↗

arXiv1997

Positive Magneto-Resistance in Quasi-1D Conductors

We present here a simple qualitative model that interpolates between the high and low temperature properties of quasi-1D conductors. At high temperatures we argue that transport is governed by inelastic scattering whereas at low temperatures the conductance decays exponentially with the electron dephasing length. The crossover between these regimes occurs at the temperature at which the elastic and inelastic scattering times become equal. This model is shown to be in quantitative agreement with the organic conductor $TTT_2I_{3-δ}$. Within this model, we also show that on the insulating side, the positive magnetoresistance of the form $(H/T)^2$ observed in $TTT_2I_{3-δ}$ and other quasi-1D conductors can be explained by the role spin-flip scattering plays in the electron dephasing rate.

cond-mat.dis-nn↗

arXiv1996

Size Dependence In The Disordered Kondo Problem

We study here the role randomly-placed non-magnetic scatterers play on the Kondo effect. We show that spin relaxation effects (with time $τ_s^o$)in the vertex corrections to the Kondo self-energy lead to an exact cancellation of the singular temperature dependence arising from the diffusion poles. For a thin film of thickness $L$ and a mean-free path $\ell$, disorder provides a correction to the Kondo resistivity of the form $τ_s^o/(k_FL\ell^2)\ln T$ that explains both the disorder and sample-size depression of the Kondo effect observed by Blachly and Giordano (PRB {\bf 51}, 12537 (1995)).

cond-mat↗