Quasiparticle Density Of States Research Articles

Microwave Properties of Superconducting Atomic-Layer Deposited TiN Films

We have grown superconducting TiN films by atomic layer deposition with thicknesses ranging from 6 to 89 nm. This deposition method allows us to tune the resistivity and critical temperature by controlling the film thickness. The microwave properties are measured, using a coplanar-waveguide resonator, and we find internal quality factors above a million, high sheet inductances (5.2-620 pH), and pulse response times up to 100 \mu s. The high normal state resistivity of the films (> 100 \mu\Omega cm) affects the superconducting state and thereby the electrodynamic response. The microwave response is modeled using a quasiparticle density of states modified with an effective pair-breaker,consistently describing the measured temperature dependence of the quality factor and the resonant frequency.

P-concentration dependence of the quasiparticle density of states in BaFe2(As1-xPx)2

31P-NMR measurements were performed in the iron pnictides BaFe2(As1−xPx)2 for 0.07 ≤ x ≤ 0.64 in order to investigate evolution of electronic state with isovalent substitution. In general, the spin part of the Knight shift Kspin and (T1T)−1/2 are proportional to the quasiparticle density of states at the Fermi level N(EF) at high temperature where the Korringa relation holds. We found that Kspin and (T1T)−1/2 are almost unaffected by the P-substitution, indicating that N(EF) is nearly constant. These results are quantitatively consistent with our band calculation. We claim that the slight changes of N(EF) by P-substitution in BaFe2(As1−xPx)2 is not enough to explain the Tc variation ranging from 30 K to 0 K, and thus suggest that antiferromagnetic correlations related to the quantum critical character are essential for the high Tc superconductivity.

Strongly Disordered TiN and NbTiNs-Wave Superconductors Probed by Microwave Electrodynamics

We probe the effects of strong disorder (2.4<k(F)l<8.6) on superconductivity in thin films of niobium titanium nitride and titanium nitride by measuring the microwave electrodynamics in coplanar waveguide resonators. We find a gradual evolution of the electromagnetic response with disorder, deviating from BCS theory, for both materials. This result can be understood as due to changes in the quasiparticle density of states, induced by the short elastic scattering length. The observations are consistent with a model using an effective pair breaker, dependent on the level of disorder.

Doping dependence of femtosecond quasiparticle relaxation dynamics in Ba(Fe,Co)2As2single crystals: Evidence for normal-state nematic fluctuations

We systematically investigate the photoexcited (PE) quasiparticle (QP) relaxation and low-energy electronic structure in electron doped Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ single crystals as a function of Co doping, $0\ensuremath{\le}x\ensuremath{\le}0.11$. The evolution of the photoinduced reflectivity transients with $x$ proceeds with no abrupt changes. In the orthorhombic spin-density-wave (SDW) state, a bottleneck associated with a partial charge-gap opening is detected, similar to previous results in different SDW iron pnictides. The relative charge gap magnitude $2\ensuremath{\Delta}(0)/{k}_{\mathrm{B}}{T}_{\mathrm{s}}$ decreases with increasing $x$. In the superconducting (SC) state, an additional relaxational component appears due to a partial (or complete) destruction of the SC state proceeding on a sub-0.5-picosecond timescale. From the SC component saturation behavior the optical SC-state destruction energy, ${U}_{\mathrm{p}}/{k}_{\mathrm{B}}=0.3$ K/Fe, is determined near the optimal doping. The subsequent relatively slow recovery of the SC state indicates clean SC gaps. The $T$ dependence of the transient reflectivity amplitude in the normal state is consistent with the presence of a pseudogap in the QP density of states. The polarization anisotropy of the transients suggests that the pseudogap-like behavior might be associated with a broken fourfold rotational symmetry resulting from nematic electronic fluctuations persisting up to $T\ensuremath{\simeq}200$ K at any $x$. The second moment of the Eliashberg function, obtained from the relaxation rate in the metallic state at higher temperatures, indicates a moderate electron phonon coupling, $\ensuremath{\lambda}\ensuremath{\lesssim}0.3$, that decreases with increasing doping.

Ferromagnetic ordering in CeIr2B2: Transport, magnetization, specific heat, and NMR studies

We present a complete characterization of the ferromagnetic system CeIr${}_{2}$B${}_{2}$ using powder x-ray diffraction (XRD), magnetic susceptibility $\ensuremath{\chi}(T)$, isothermal magnetization $M(H)$, specific heat $C(T)$, electrical resistivity $\ensuremath{\rho}(T,H)$, and thermoelectric power $S(T)$ measurements. Furthermore, ${}^{11}$B NMR study was performed to probe the magnetism on a microscopic scale. Rietveld refinement of powder XRD data confirms that CeIr${}_{2}$B${}_{2}$ crystallizes in CaRh${}_{2}$B${}_{2}$-type orthorhombic structure (space group fddd). The $\ensuremath{\chi}(T)$, $C(T)$, and $\ensuremath{\rho}$(T) data confirm bulk ferromagnetic ordering with ${T}_{c}=5.1$ K. Ce ions in CeIr${}_{2}$B${}_{2}$ are in a stable trivalent state. Our low-temperature $C(T)$ data measured down to 0.4 K yield a Sommerfeld coefficient $\ensuremath{\gamma}$ = 73(4) mJ/mol K${}^{2}$, which is much smaller than the previously reported value of $\ensuremath{\gamma}$ = 180 mJ/mol K${}^{2}$ deduced from the specific heat measurement down to 2.5 K. For LaIr${}_{2}$B${}_{2}$, $\ensuremath{\gamma}$ = 6(1) mJ/mol K${}^{2}$, which implies the density of states at the Fermi level $\mathcal{D}({E}_{F})=2.54$ states/(eV f.u.) for both spin directions. The renormalization factor for quasiparticle density of states and hence for quasiparticle mass due to $4\phantom{\rule{-0.16em}{0ex}}f$ correlations in CeIr${}_{2}$B${}_{2}$ is $\ensuremath{\approx}$12. The Kondo temperature ${T}_{\mathrm{K}}\ensuremath{\sim}4$ K is estimated from the jump in specific heat of CeIr${}_{2}$B${}_{2}$ at ${T}_{c}$. Both $C(T)$ and $\ensuremath{\rho}(T)$ data exhibit a gapped-magnon behavior in the magnetically ordered state with an energy gap ${E}_{g}\ensuremath{\sim}3.5$ K. The $\ensuremath{\rho}$ data as a function of magnetic field $H$ indicate a large negative magnetoresistance (MR) which is highest for $T=5$ K. While at 5 K the negative MR keeps on increasing up to 10 T, at 2 K an upturn is observed near $H=3.5$ T. On the other hand, the thermoelectric power data have small absolute values ($S\ensuremath{\sim}7\ensuremath{\mu}$V/K), indicating a weak Kondo interaction. A shoulder in $S(T)$ at about 30 K, followed by a minimum at $\ensuremath{\sim}$10 K, is attributed to crystal electric field effects and the onset of magnetic ordering. ${}^{11}$B NMR line broadening provides strong evidence of ferromagnetic correlations below 40 K.

Interplay of optical potential and condensate properties for bosons in different optical lattice geometries

Motivated by the availability of optical lattices with tunable geometries in experiments, we compute different physical properties such as condensate fraction, fluctuation and depletion of the condensate density and quasi-particle velocity as a function of the interparticle interaction strength for bosons in different 2D optical potentials that correspond to square, triangular and honeycomb geometries. Our results demonstrate an interplay of these features with that of the coordination number and underlying lattice geometries. A triangular lattice, which has a coordination number z = 6, shows larger condensate fraction and fluctuation of the condensate density, along with a low depletion of the ground state occupancy. The other candidates, namely the square lattice (z = 4) and honeycomb lattice (z = 3) occupy second and third places, respectively, with regard to these properties. The quasi-particle velocities for these geometries follow a similar pattern as that of the condensate fraction data. The observations are explained by the behaviour of the low-energy quasi-particle density of states. Finally, to make the role of lattice geometry obvious, we include a discussion on a kagome lattice, which in spite of having the same coordination number as that of a square lattice, i.e. z = 4, yields a significantly low condensate fraction than that of the square lattice for all values of the interacting strength.

Ferromagnetism and nonlocal correlations in the Hubbard model

We study the possibility and stability of band ferromagnetism in the single-band Hubbard model for the simple cubic (sc) lattice. A nonlocal self-energy is derived within a modified perturbation theory. Results for the spectral density and quasiparticle density of states are shown with special attention to the effects of $\mathbf{k}$ dependence. The importance of nonlocal correlations for the fulfillment of the Mermin-Wagner theorem is our main result. A phase diagram showing regions of ferromagnetic order is calculated for the three-dimensional lattice. Besides, we show results for the optical conductivity and prove that the renormalized one-loop contribution to the conductivity already cancels the Drude peak exactly in case of a local self-energy which is not true anymore for a nonlocal self-energy.

Vanishing quasiparticle density in a hybrid Al/Cu/Al single-electron transistor

The achievable fidelity of many nanoelectronic devices based on superconducting aluminum is limited by either the density of residual nonequilibrium quasiparticles n_qp or the density of quasiparticle states in the gap, characterized by Dynes parameter \gamma. We infer upper bounds n_qp < 0.033 um^-3 and \gamma < 1.6*10^-7 from transport measurements performed on Al/AlOx/Cu single-electron transistors, improving previous results by an order of magnitude. Owing to efficient microwave shielding and quasiparticle relaxation, typical number of quasiparticles in the superconducting leads is zero.

SIS Junctions for Millimeter and Submillimeter Wave Mixers

We have developed a process for the fabrication of high-quality Nb/AlOx/Nb tunnel junctions with small area and high current densities for the heterodyne mixers at millimeter and submillimeter wavelengths. Their dc I-V curves are numerically studied, including the broadening of quasiparticle density of states resulting from the existence of an imaginary part of the gap energy of Nb. We have found both experimentally and numerically that the subgap current is strongly dependent on bias voltage at temperatures below 4.2K unlike the prediction of the BCS tunneling theory. It is shown that calculated dc I-V curves taking into account the complex number of the gap energy agree well with those of Nb/AlOx/Nb junctions measured at temperatures from 0.4 to 4.2K. We have successfully built receivers at millimeter and submillimeter wavelengths with the noise temperature as low as 4 times the quantum photon noise, employing those high-quality Nb/AlOx/Nb junctions. Those low-noise receivers are to be installed in the ALMA (Atacama Large Millimeter/Submillimeter Array) telescope and they are going into series production now.

Numerical study of Kondo impurity models with strong potential scattering: Reverse Kondo effect and antiresonance

Accurate numerical results are derived for transport properties of Kondo impurity systems with potential scattering and orbital degeneracy. Using the continuous-time quantum Monte Carlo (CT-QMC) method, static and dynamic physical quantities are derived in a wide temperature range across the Kondo temperature T_K. With strong potential scattering, the resistivity tends to decrease with decreasing temperature, in contrast to the ordinary Kondo effect. Correspondingly, the quasi-particle density of states obtains the antiresonance around the Fermi level. Thermopower also shows characteristic deviation from the standard Kondo behavior, while magnetic susceptibility follows the universal temperature dependence even with strong potential scattering. It is found that the t-matrix in the presence of potential scattering is not a relevant quantity for the Friedel sum rule, for which a proper limit of the f-electron Green's function is introduced. The optical theorem is also discussed in the context of Kondo impurity models with potential scattering. It is shown that optical theorem holds not only in the Fermi-liquid range but also for large energies, and therefore is less restrictive than the Friedel sum rule.

Inducing Odd-Frequency Triplet Superconducting Correlations in a Normal Metal

This work discusses theoretically the interplay between the superconducting and ferromagnetic proximity effects, in a diffusive normal metal strip in contact with a superconductor and a nonuniformly magnetized ferromagnetic insulator. The quasiparticle density of states of the normal metal shows clear qualitative signatures of triplet correlations with spin one (TCS1). When one goes away from the superconducting contact, TCS1 focus at zero energy under the form of a peak surrounded by dips, which show a typical spatial scaling behavior. This effect can coexist with a focusing of singlet correlations and triplet correlations with spin zero at finite but subgap energies. The simultaneous observation of both effects would enable an unambiguous characterization of TCS1.

Evidence for attractive pair interaction in diffusive gold films deduced from studies of the superconducting proximity effect with aluminum

An experimental study of the proximity effect of superconductor-normal metal films with the help of lowtemperature scanning tunneling spectroscopy is reported. The behaviors of bilayers of the noble metals gold and silver in contact with the superconductor aluminum are compared for various thicknesses of the normal metal. Although the normal conducting properties of Au and Ag are very similar to each other, the measured differential conductance spectra from which the quasiparticle density of states is deduced differ markedly. While the behavior of the Al/Ag system follows the quasiclassical theory of the proximity effect for diffusive systems, differences exist for the Al/Au system. The absolute value of the induced minigap in Au is larger than predicted by theory, and its suppression with increasing temperature is weaker. These observations are quantitatively accounted for by including a finite interaction parameter for Au of (N0V)Au = 0.10 ± 0.03. The thirdinvestigatedmetalispalladium,whichisclosetoferromagnetism.Themethodpresentedhereenables oneto detect small superconducting correlations by investigating a spectroscopic property rather than the supercurrent or the critical temperature.

Molecular-beam epitaxy and robust superconductivity of stoichiometric FeSe crystalline films on bilayer graphene

We report on molecular beam epitaxy growth of stoichiometric and superconducting FeSe crystalline thin films on double-layer graphene. Layer-by-layer growth of high-quality films has been achieved in a well-controlled manner by using Se-rich condition, which allow us to investigate the thickness-dependent superconductivity of FeSe. In situ low-temperature scanning tunneling spectra reveal that the local superconducting gap in the quasiparticle density of states is visible down to two triple layers for the minimum measurement temperature of 2.2 K, and that the transition temperature ${T}_{\mathrm{c}}$ scales inversely with film thickness.

Doping dependent tunneling conductance in SDW ordered copper oxide superconductors

It is observed that doping suppresses the long range anti-ferromagnetic order and induces superconducting phase for a suitable doping. In order to study this effect, we present a model study of the doping dependence of the tunneling conductance in high- T c systems. The system is described by the Hamiltonian consisting of spin density wave (SDW) and s-wave type superconducting interaction in presence of varying impurity concentrations. The gap equations are calculated by using Green’s functions technique of Zubarev. The gap equations and the chemical potential are solved self-consistently. The imaginary part of the electron Green’s functions shows the quasi-particle density of states which represent the tunneling conductance observed by the scanning tunneling microscopy (STM). We investigate the effect of impurity on the gap equations as well as on the tunneling conductance. The results will be discussed based on the experimental observations.

Heat capacity through the magnetic-field-induced resistive transition in an underdoped high-temperature superconductor

The underlying physics of the magnetic-field-induced resistive state in high temperature cuprate superconductors remains a mystery. One interpretation is that the application of magnetic field destroys the d-wave superconducting gap to uncover a Fermi surface that behaves like a conventional (i.e.Fermi Liquid) metal (1). Another view is that an applied magnetic field destroys long range superconducting phase coherence, but the superconducting gap amplitude survives (2, 3). By measuring the specific heat of ultra-clean YBa2Cu3O6.56 (YBCO 6.56), we obtain a measure of the quasi-particle density of states from the superconducting state well into the magnetic-field-induced resistive state. We have found that at very high magnetic fields the specific heat exhibits both the conventional temperature dependence and quantum oscillations expected for a Fermi Liquid. On the other hand, the magnetic field dependence of the quasi-particle density of states follows a \sqrt{H} behavior that persists right through the zero-resistance transition, evidencing the fully developed d-wave superconducting gap over the entire magnetic field range measured. The coexistence of these two phenomena pose a rigorous thermodynamic constraint on theories of high-magnetic-field resistive state in the cuprates.

Magnetic Rare-Earth Impurity Resonance Bound States in Iron-Based Superconductors

Starting from an Anderson model for two-orbital model of electron and hole conduction bands which hybridized with the localized impurity spin, we investigate the effect of magnetic impurities on the local quasiparticle density of states (LDOS) in iron-based superconductors. We consider extended s-wave (s+−) superconducting gap symmetry with higher harmonic correction. The impurity-induced bound states are a probe for the nodal structure of the extended s-wave symmetry in ferropnictides.

Renormalized parameters and perturbation theory for ann-channel Anderson model with Hund’s rule coupling: Asymmetric case

We explore the predictions of the renormalized perturbation theory for the $n$-channel Anderson model, both with and without Hund's rule coupling, in the regime away from particle-hole symmetry. For the model with $n=2$ we deduce the renormalized parameters from numerical renormalization-group calculations and plot them as a function of the local occupation of the impurity site ${n}_{d}$. From the exact relations in terms of the renormalized parameters, we calculate the orbital, spin and charge susceptibilities, Wilson ratios, and quasiparticle density of states at $T=0$, in the different parameter regimes, which gives a comprehensive overview of the low-energy behavior of the model. We compare the difference in Kondo behaviors at the points, where ${n}_{d}=1$ and ${n}_{d}=2$. Some unexpected features of the results are that a strong on-site interaction gives significant renormalization and suppression charge susceptibility in the intermediate valence regimes, and the peaks in the spin susceptibility away from the particle-hole symmetric case do not occur at integer values ${n}_{d}=1,3$

Density of state and effect of external magnetic field on co-existence state of SC and AFM in high- [formula omitted] cuprates

A mean field Hamiltonian model is taken for our system to study density of states and the effect of external magnetic field on superconductivity (SC) and antiferromagnetism (AFM) at the co-existence stage in high- T c cuprates. The order parameters corresponding to SC and AFM long range orders are determined. The variation of dimensionless superconducting gap ( z ) and AFM gap ( h ̃ ) with temperature are investigated at different magnetic fields. The gap parameters are found to be strongly coupled to each other through their coupling constants. The interplay displays a BCS type of two gaps in the quasi-particle density of states which resemble the tunneling conductance of the STM experiment. The two gap edges in the density of states(DOS) appear at ± ( h ̃ 2 + z ) and ± ( h ̃ 2 − z ) . The effect of external magnetic field on DOS is also studied.

Thermal conductivity tensor of single crystalline Co-doped BaFe 2As 2

Thermal conductivity tensor has been measured using single crystalline Co-doped BaFe 2As 2 down to 0.1 K and under magnetic fields up to 7 T. We observe peak anomalies both in the thermal conductivity and the thermal Hall conductivity in the superconducting state as an indication of enhancement of the quasiparticle mean-free path. Furthermore, we find a residual T-linear term in the thermal conductivity possibly due to a finite quasiparticle density of states in the superconducting gap induced by impurity pair-breaking.

Signature of pseudogap formation in the density of states of underdoped cuprates

The resonating valence bond spin liquid model for the underdoped cuprates has as an essential element, the emergence of a pseudogap. This energy scale introduces asymmetry in the quasiparticle density of states because it is associated with the antiferromagnetic Brillouin zone. By contrast, superconductivity develops on the Fermi surface and this largely restores the particle-hole symmetry for energies below the superconducting energy-gap scale. In the highly underdoped regime, these two scales can be separately identified in the density of states and also partial density of states for each fixed angle in the Brillouin zone. From the total density of states, we find that the pseudogap energy scale manifests itself differently as a function of doping for positive and negative biases. Furthermore, we find evidence from recent scanning tunneling spectroscopy data for asymmetry in the positive and negative biases of the extracted $\ensuremath{\Delta}(\ensuremath{\theta})$ which is in qualitative agreement with this model. Likewise, the slope of the linear low-energy density of states is nearly constant in the underdoped regime while it increases significantly with overdoping in agreement with the data.