arXiv subjects

Thomas R. Lemberger

Publications and source records attributed to Thomas R. Lemberger.

At least 19 recordsLinked to original sources

arXiv2014

Doping-dependent critical Cooper-pair momentum in thin, underdoped cuprate films

We apply a recently-developed low-field technique to inductively measure the critical pair momentum $p_c$ in thin, underdoped films of Y$_{1-x}$Ca$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-δ}$ and Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ reflecting a wide range of hole doping. We observe that $p_c \propto \hbar/ξ$ scales with $T_c$ and therefore superfluid density $n_s(T\rightarrow0)$ in our two-dimensional cuprate films. This relationship was famously predicted by a universal model of the cuprates with a \textit{doping-independent} superconducting gap, but has not been observed by high field measurements of the coherence length $ξ$ due to field-induced phenomena not included in the theory.

cond-mat.supr-con↗

arXiv2014

Measuring the superconducting coherence length in thin films using a two-coil experiment

We present measurements of the superconducting coherence length ξ in thin (d < 100 Å) films of MoGe alloy and Nb using a combination of linear and nonlinear mutual inductance techniques. As the alternating current in the drive coil is increased at fixed temperature, we see a crossover from linear to nonlinear coupling to the pickup coil, consistent with the unbinding of vortex-antivortex pairs as the peak pair momentum nears \hbar\/ξ and the unbinding barrier vanishes. We compare measurements of ξ made by this mutual inductance technique to values determined from the films' upper critical fields, thereby confirming the applicability of a recent calculation of the upper limit on a vortex-free state in our experiment.

cond-mat.supr-con↗

arXiv2013

Theory of the lower critical magnetic field for a two-dimensional superconducting film in a non-uniform field

We consider the first appearance of vortices in a two-dimensional (2-D) superconducting film exposed to a non-uniform magnetic field, $\mathbf{B_a}$, produced by a nearby coil. The film has "infinite" radius, $R_f$, and thickness $t$ about equal to the coherence length, $ξ$. The coil is approximated as a point dipole. We find that the first vortex-bearing state to appear has both a vortex and an antivortex. The Gibbs free energy of this state is lower than the vortex-free state when the applied perpendicular field, $B_0$, at the origin exceeds the external critical field: $B_{c1}^0 = \frac{4\sqrt{2}Λ}{R}\frac{Φ_0}{4πΛ^2}ln\left(\fracΛξ\right)$, where $\frac{Φ_0}{4πΛ^2}ln\left(\fracΛξ\right) \equiv B_{c1}^{2D}$ is the intrinsic critical field in 2-D, $Λ\equiv 2λ^2/t$ the 2-D penetration depth introduced by Pearl, and $λ$ is the bulk penetration depth. The prefactor, $4\sqrt{2}Λ/R$, is calculated in the strong-screening regime, $Λ/R \ll 1$. $R$ is the radial distance at which the applied perpendicular field, $B_{a,z}(ρ)$, changes sign. In the lab, the onset of vortex effects generally occurs at a field much higher than $B_{c1}^0$, indicating that vortices are inhibited by the vortex-antivortex unbinding barrier, or by pinning.

cond-mat.supr-con↗

arXiv2007

Superfluid density of superconductor-ferromagnet bilayers

We report the first measurements of the effective superfluid density n_S(T) \propto λ^{-2}(T) of Superconductor-Ferromagnet (SC/FM) bilayers, where λis the effective magnetic field penetration depth. Thin Nb/Ni bilayers were sputtered in ultrahigh vacuum in quick succession onto oxidized Si substrates. Nb layers are 102 A thick for all samples, while Ni thicknesses vary from 0 to 100 A. T_C determined from λ^{-2}(T) decreases rapidly as Ni thickness d_Ni increases from zero to 15 A, then it has a shallow minimum at d_Ni \approx 25 A. λ^{-2}(0) behaves similarly, but has a minimum several times deeper. In fact, λ^{-2}(0) continues to increase with increasing Ni thickness long after T_C has stopped changing. We argue that this indicates a substantial superfluid density inside the ferromagnetic Ni films.

cond-mat.supr-con↗

arXiv2007

Quantum critical behaviour in the superfluid density of strongly underdoped ultrathin cuprate films

A central issue in the physics of high temperature superconductors is to understand superconductivity within a single copper-oxide layer or bilayer, the fundamental structural unit in the cuprates, and how it is lost with underdoping. As mobile holes are removed from the CuO_2 planes, the transition temperature T_C and superfluid density n_S decrease in a surprisingly correlated fashion in crystals and thick films. We seek to elucidate the intrinsic physics of bilayers in the strongly underdoped regime, near the critical doping level where superconductivity disappears. We report measurements of n_S(T) in films of Y_{1-x}Ca_xBa_2Cu_3O_{7-δ} as thin as two copper-oxide bilayers with T_C's as low as 3 K. In addition to seeing the two-dimensional (2D) Kosterlitz-Thouless-Berezinski transition at T_C, we observe a remarkable scaling of T_C with n_S(0) that demonstrates that the disappearance of superconductivity with underdoping is due to quantum fluctuations near a T = 0 2D quantum critical point.

cond-mat.supr-con↗

arXiv2004

Correlation between superfluid density and Tc of underdoped YBa2Cu3O6+x near the superconductor-insulator transition

We report measurements of the ab-plane superfluid density Ns (magnetic penetration depth, λ) of severely underdoped films of YBa2Cu3O6+x, with Tc's from 6 to 50 K. Tc is not proportional to Ns(0); instead, we find Tc ~ Ns^{1/2.3 +/- 0.4}. At the lowest dopings, Tc is as much as 5 times larger than the upper limit set by the KTB transition temperature of individual CuO2 bilayers.

cond-mat.supr-con↗

arXiv2004

The Role of Thermal Phase Fluctuations in Underdoped YBCO Films

The effect of thermal phase fluctuations (TPF's) on the ab-plane penetration depth, lambda(T), of thin YBa2Cu3O(7-delta) (YBCO) films is found to be much smaller than expected from the paradigm of cuprates as weakly-coupled 2D superconducting layers. A 2D vortex-pair-unbinding transition is observed, but the effective thickness for fluctuations is the film thickness, not a CuO bilayer thickness. In a strongly underdoped YBCO film, Tc= 34 K, TPF's suppress Tc by only about 3 K. They cannot be a significant factor in the suppression of Tc and emergence of the pseudogap with underdoping.

cond-mat.supr-con↗

arXiv2003

Magnetic Penetration Depth Measurements of Pr$_{2-x}$Ce$_x$CuO$_{4-δ}$ Films on Buffered Substrates: Evidence for a Nodeless Gap

We report measurements of the inverse squared magnetic penetration depth, $λ^{-2}(T)$, in Pr$_{2-x}$Ce$_{x}$CuO$_{4-δ}$ ($0.115 \leq x \leq 0.152$) superconducting films grown on SrTiO$_3$ (001) substrates coated with a buffer layer of insulating Pr$_{2}$CuO$_{4}$. $λ^{-2}(0)$, $T_c$ and normal-state resistivities of these films indicate that they are clean and homogeneous. Over a wide range of Ce doping, $0.124\leq x \leq 0.144$, $λ^{-2}(T)$ at low $T$ is flat: it changes by less than 0.15% over a factor of 3 change in $T$, indicating a gap in the superconducting density of states. Fits to the first 5% decrease in $λ^{-2}(T)$ produce values of the minimum superconducting gap in the range of $0.29\leqΔ_{\rm min}/k_BT_c\leq1.01$.

cond-mat.supr-con↗

arXiv2003

Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La_{2-x}Ce_xCuO_{4-y} and Pr_{2-x}Ce_xCuO_{4-y}

From measurements of the magnetic penetration depth, $λ(T)$, from 1.6 K to $T_c$ in films of electron-doped cuprates La$_{2-x}$Ce$_x$CuO$_{4-y}$ and Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ we obtain the normalized density of states, $N_s(E)$ at T=0 by using a simple model. In this framework, the flat behavior of $λ^{-2}(T)$ at low $T$ implies $N_s(E)$ is small, possibly gapped, at low energies. The upward curvature in $λ^{-2}(T)$ near $T_c$ seen in overdoped films implies that superfluid comes from an anomalously small energy band within about $3k_BT_c$ of the Fermi surface.

cond-mat.supr-con↗

arXiv2002

Reflection of two-gap nature in penetration depth measurements of MgB$_2$ film

The magnetic penetration depth, $λ^{-2}(T)$, in the basal plane of a magnesium diboride (MgB$_2$) film was measured using a two-coil mutual inductance technique at 50 kHz. This film has $T_c\simeq 38$ K, $ΔT_c \leq 1$ K, and $λ(0)\sim 1500$ Å. At low temperatures, $λ^{-2}(T)$ shows a clear exponential temperature dependence, indicating s-wave superconducting order parameter symmetry. However, the data are not quantitatively well described by theory assuming a single gap. From the data fit by the full BCS calculation assuming a double gap, the values of the two distinct gaps were obtained: $Δ_S(0)=2.61\pm0.41$ meV and $Δ_L(0)=6.50\pm0.33$ meV. The contributions of the small and the large gaps to the total superfluid density at T=0 were estimated to be 21% and 79%, respectively. Finally, we consider the effect of gap anisotropy on the penetration depth measurements, and find that the anisotropy does not play a significant role in determining the temperature dependence of the penetration depth.

cond-mat.supr-con↗

arXiv2002

Evidence for a Nodeless Gap from the Superfluid Density of Optimally Doped Pr_{1.855}Ce_{0.145}CuO_{4-y} Films

We present measurements of the ab-plane magnetic penetration depth, λ(T), in five optimally doped Pr_{1.855}Ce_{0.145}CuO_{4-y} films for 1.6 K \leq T \leq T_c \sim 24 K. Low resistivities, high superfluid densities n_s(T)\propto λ^{-2}(T), high T_c's, and small transition widths are reproducible and indicative of excellent film quality. For all five films, λ^{-2}(T)/λ^{-2}(0) at low T is well fitted by an exponential temperature dependence with a gap, Δ_{min}, of 0.85 k_B T_c. This behavior is consistent with a nodeless gap and is incompatible with d-wave superconductivity.

cond-mat.supr-con↗

arXiv2002

Evidence for a Transition in the Pairing Symmetry of the Electron-Doped Cuprates La_{2-x}Ce_xCuO_{4-y} and Pr_{2-x}Ce_xCuO_{4-y}

We present measurements of the magnetic penetration depth, λ^{-2}(T), in Pr_{2-x}Ce_{x}CuO_{4-y} and La_{2-x}Ce_{x}CuO_{4-y} films at three Ce doping levels, x, near optimal. Optimal and overdoped films are qualitatively and quantitatively different from underdoped films. For example, λ^{-2}(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, λ^{-2}(T) at low T is exponential at optimal and overdoping but is quadratic at underdoping. In light of other studies that suggest both d- and s-wave pairing symmetry in nominally optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.

cond-mat.supr-con↗

arXiv2000

Dynamic Impedance of Two-Dimensional Superconducting Films Near the Superconducting Transition

The sheet impedances, Z(w,T), of several superconducting a-Mo77Ge23 films and one In/InOx film have been measured in zero field using a two-coil mutual inductance technique at frequencies from 100 Hz to 100 kHz. Z(w,T) is found to have three contributions: the inductive superfluid, renormalized by nonvortex phase fluctuations; conventional vortex-antivortex pairs, whose contribution turns on very rapidly just below the usual Kosterlitz-Thouless-Berezinskii unbinding temperature; and an anomalous contribution. The latter is predominantly resistive, persists well below the KTB temperature, and is weakly dependent on frequency down to remarkably low frequencies, at least 100 Hz. It increases with T as e-U'(T)/kT, where the activation energy, U'(T), is about half the energy to create a vortex-antivortex pair, indicating that the frequency dependence is that of individual excitations, rather than critical behavior.

cond-mat.supr-con↗

arXiv1999

Metallic nonsuperconducting phase and d-wave superconductivity in Zn-substituted LaSrCuO

Measurements of the resistivity, magnetoresistance and penetration depth were made on films of LaSrCuO with up to 12 at.% of Zn substituted for the Cu. The results show that the quadratic temperature dependence of the inverse square of the penetration depth, indicative of d-wave superconductivity, is not affected by doping. The suppression of superconductivity leads to a metallic nonsuperconducting phase, as expected for a pairing mechanism related to spin fluctuations. The metal-insulator transition occurs in the vicinity of kFl~1, and appears to be disorder-driven, with the carrier concentration unaffected by doping.

cond-mat.supr-con↗

arXiv1999

Thermodynamics of Magnets

Thermodynamics of magnetic materials is discussed in practical, lab-oriented terms. In the common experimental configuration in which the external magnetic field comes from a solenoidal coil connected to a power supply, magnetic work is identified unambiguously as the flow of electromagnetic field energy from the power supply into the system via the connecting wires. A simple algebraic expression is derived for the "magnetic energy" of microscopic dipoles which interact with the magnetic fields produced by each other, by an external coil, or by a permanent magnet. The discussion delineates the important distinction between induced magnetic moments, which are diamagnetic, and permanent microscopic moments, which are paramagnetic. The practicality of these ideas is illustrated by calculations of the magnetic properties of several idealized magnetic solids via minimization of the appropriate free energy.

physics.class-ph↗

arXiv1999

Effect of the pseudogap on the mean-field magnetic penetration depth of YBCO thin films

We report measurements of the magnetic penetration depth in YBCO films at various oxygen concentrations. At optimal doping, critical fluctuation effects are absent, and the penetration depth from 4 K to 0.99 Tc is well described by d-wave, BCS, strong-coupling theory with a gap, Delta0/ kT ~ 3.3. This implies that the T-dependence of the penetration depth comes largely from single-particle excitations. As in crystals, underdoping reduces the zero temperature superfluid density without affecting the low-T slope or curvature of the penetration depth. We show that these results, as well as heat capacity measurements, are well described by an ad hoc model in which superfluid is lost from regions of the Fermi surface occupied by the pseudogap while the low lying excitations near the nodes remain unaffected.

cond-mat.supr-con↗

arXiv1999

Experimental Study of the Inductance of Pinned Vortices in Superconducting YBa2Cu3O7-d Films

Using a two-coil mutual inductance method, we have measured the complex resistivity, rho_v(T,Be), of pinned vortices in c-axis pulsed laser deposited YBa2Cu3O7-d films with magnetic field Be applied perpendicular to the film. At low frequencies, (<100 kHz), rho_v is inductive and is inversely proportional to the Labusch parameter, the average vortex pinning force constant, kappa_exp. The observed weakening of kappa_exp with Be is consistent with a simple model based on linear pinning defects. Adding classical thermal fluctuations to the model in a simple way describes the observed linear T dependence of rho_v, below ~15 K and provides reasonable values for the effective radius (.3 nm to >.8 nm) of the defects and the depth of the pinning potential. The success of this model implies that thermal supercurrent (phase) fluctuations have their full classical amplitude down to 5 K for frequencies below the characteristic depinning frequency. To date, no sufficient theory exists to explain the data between ~15 K and the vortex glass melting temperature.

cond-mat.supr-con↗

arXiv1999

Effect of thermal phase fluctuations on the superfluid density of two-dimensional superconducting films

High precision measurements of the complex sheet conductivity of superconducting Mo77Ge23 thin films have been made from 0.4 K through Tc. A sharp drop in the inverse sheet inductance, 1/L(T), is observed at a temperature, Tc, which lies below the mean-field transition temperature, Tco. Just below Tc, the suppression of 1/L(T) below its mean-field value indicates that longitudinal phase fluctuations have nearly their full classical amplitude, but they disappear rapidly as T decreases. We argue that there is a quantum crossover at about 0.94 Tco, below which classical phase fluctuations are suppressed.

cond-mat.supr-con↗