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W. J. Padilla

Publications and source records attributed to W. J. Padilla.

14 recordsLinked to original sources
arXiv2010

Towards a two-dimensional superconducting state of La$_{2-x}$Sr$_{x}$CuO$_{2}$ in a moderate external magnetic field

We report a novel aspect of the competition and coexistence between magnetism and superconductivity in the high-$T_{c}$ cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$ (La214). With a modest magnetic field applied $H \parallel c$-axis, we monitored the infrared signature of pair tunneling between the CuO$_2$ planes and discovered the complete suppression of interlayer coupling in a series of underdoped La214 single crystals. We find that the in-plane superconducting properties remain intact, in spite of enhanced magnetism in the planes.

cond-mat.supr-con↗
arXiv2008

Highly-flexible wide angle of incidence terahertz metamaterial absorber

We present the design, fabrication, and characterization of a metamaterial absorber which is resonant at terahertz frequencies. We experimentally demonstrate an absorptivity of 0.97 at 1.6 terahertz. Importantly, this free-standing absorber is only 16 microns thick resulting in a highly flexible material that, further, operates over a wide range of angles of incidence for both transverse electric and transverse magnetic radiation.

cond-mat.mtrl-sci↗
arXiv2008

Terahertz metamaterials on free-standing highly-flexible polyimide substrates

We have fabricated resonant terahertz metamaterials on free standing polyimide substrates. The low-loss polyimide substrates can be as thin as 5.5 micron yielding robust large-area metamaterials which are easily wrapped into cylinders with a radius of a few millimeters. Our results provide a path forward for creating multi-layer non-planar metamaterials at terahertz frequencies.

cond-mat.mtrl-sci↗
arXiv2008

A Perfect Metamaterial Absorber

We present the design for an absorbing metamaterial element with near unity absorbance. Our structure consists of two metamaterial resonators that couple separately to electric and magnetic fields so as to absorb all incident radiation within a single unit cell layer. We fabricate, characterize, and analyze a metamaterial absorber with a slightly lower predicted absorbance of 96%. This achieves a simulated full width at half maximum (FWHM) absorbance of 4% thus making this material ideal for imaging purposes. Unlike conventional absorbers, our metamaterial consists solely of metallic elements. The underlying substrate can therefore be chosen independently of the substrate's absorptive qualities and optimized for other parameters of interest. We detail the design and simulation process that led to our metamaterial, and our experiments demonstrate a peak absorbance greater than 88% at 11.5 GHz.

cond-mat.mes-hall↗
arXiv2007

Disparities in the Josephson vortex state electrodynamics of high-Tc cuprates

We report on far infrared measurements of interplane conductivity for underdoped single-crystal YBa2Cu3Oy in magnetic field and situate these new data within earlier work on two other high-Tc cuprate superconductors, La(2-x)SrxCuO4 and Bi2Sr2CaCu2O(8+d). The three systems have displayed apparently disparate electrodynamic responses in the Josephson vortex state formed when magnetic field H is applied parallel to the CuO2 planes. Specifically, there is discrepancy in the number and field dependence of longitudinal modes observed. We compare and contrast these findings with several models of the electrodynamics in the vortex state and suggest that most differences can be reconciled through considerations of the Josephson vortex lattice ground state as well as the c-axis and in-plane quasiparticle dissipations.

cond-mat.supr-con↗
arXiv2006

Infrared probe of the anomalous magnetotransport of highly oriented pyrolytic graphite in the extreme quantum limit

We present a systematic investigation of the magnetoreflectance of highly oriented pyrolytic graphite in magnetic field B up to 18 T . From these measurements, we report the determination of lifetimes tau associated with the lowest Landau levels in the quantum limit. We find a linear field dependence for inverse lifetime 1/tau(B) of the lowest Landau levels, which is consistent with the hypothesis of a three-dimensional (3D) to 1D crossover in an anisotropic 3D metal in the quantum limit. This enigmatic result uncovers the origin of the anomalous linear in-plane magnetoresistance observed both in bulk graphite and recently in mesoscopic graphite samples.

cond-mat.mes-hall↗
arXiv2006

Novel electrically resonant terahertz metamaterials

We present a new class of artificial materials which exhibit a tailored response to the electrical component of electromagnetic radiation. These electric metamaterials (EM-MMs) are investigated theoretically, computationally, and experimentally using terahertz time-domain spectroscopy. These structures display a resonant response including regions of negative permittivity (epsilon < 0) ranging from ~500 GHz to 1 THz. Conventional electric media such as distributed wires are difficult to incorporate into metamaterials. In contrast, these new localized structures will simplify the construction of future metamaterials - including those with negative index of refraction - and will enhance the design and fabrication of functional THz devices.

cond-mat.mtrl-sci↗
arXiv2005

Strong-coupling Effects in cuprate High-$T_{c}$ Superconductors by magnetooptical studies

Signatures of strong coupling effects in cuprate high-$T_{c}$ superconductors have been authenticated through a variety of spectroscopic probes. However, the microscopic nature of relevant excitations has not been agreed upon. Here we report on magneto-optical studies of the CuO$_{2}$ plane carrier dynamics in a prototypical high-$T_{c}$ superconductor YBa$%_{2} $Cu$_{3}$O$_{y}$ (YBCO). Infrared data are directly compared with earlier inelastic neutron scattering results by Dai \textit{et al}. [Nature (London) \textbf{406}, 965 (2000)] revealing a characteristic depression of the magnetic resonance in H $\parallel $ \textit{c} field less than 7 T. This analysis has allowed us to critically assess the role of magnetic degrees of freedom in producing strong coupling effects for YBCO system.

cond-mat.supr-con↗
arXiv2005

Constant effective mass across the phase diagram of high-T$_{c}$ cuprates

We investigate the hole dynamics in two prototypical high temperature superconducting systems: La$_{2-x}$Sr$_{x}$CuO$_{4}$ and YBa$_{2}$Cu$_{3}% $O$_{y}$ using a combination of DC transport and infrared spectroscopy. By exploring the effective spectral weight obtained with optics in conjunction with DC Hall results we find that the transition to the Mott insulating state in these systems is of the "vanishing carrier number" type since we observe no substantial enhancement of the mass as one proceeds to undoped phases. Further, the effective mass remains constant across the entire underdoped regime of the phase diagram. We discuss the implications of these results for the understanding of both transport phenomena and pairing mechanism in high-T$_{c}$ systems.

cond-mat.str-el↗
arXiv2005

Electrodynamics of the Nodal Metal in Weakly Doped High-$T_{c}$ Cuprates

We report on the detailed analysis of the infrared (IR) conductivity of two prototypical high-$T_{c}$ systems YBa$_{2}$Cu$_{3}$O$_{y}$ and La$_{2-x}$Sr$%_{x}$CuO$_{4}$ throughout the complex phase diagram of these compounds. Our focus in this work is to thoroughly document the electromagnetic response of the nodal metal state which is initiated with only few holes doped in parent antiferromagnetic systems and extends up to the pseudogap boundary in the phase diagram. The key signature of the nodal metal is the two-component conductivity: the Drude mode at low energies followed by a resonance in mid-IR. The Drude component can be attributed to the response of coherent quasiparticles residing on the Fermi arcs detected in photoemission experiments. The microscopic origin of the mid-IR band is yet to be understood. A combination of transport and IR data uncovers fingerprints of the Fermi liquid behavior in the response of the nodal metal. The comprehensive nature of the data sets presented in this work allows us to critically re-evaluate common approaches to the interpretation of the optical data. Specifically we re-examine the role of magnetic excitations in generating electronic self energy effects through the analysis of the IR data in high magnetic field.

cond-mat.supr-con↗
arXiv2005

Group theoretical description of artificial magnetic metamaterials utilized for negative index of refraction

Group theoretical methods are used to determine the electromagnetic properties of artificial magnetic meta-materials, based solely upon the symmetries of the underlying constituent particles. Point groups for such materials are determined. From the transformation properties of an electromagnetic (EM) basis under symmetries of the particles, it is possible to determine, (i) the EM modes of the particles, (ii) the form of constitutive relations (iii) magneto-optical response of a meta-material or lack thereof. These methods are shown to be useful for determination of the isotropic or bi-anisotropic nature of artificial magnetic particles. The results for several artificial magnetic metamaterials are given. Based upon these methods, we predict an ideal planar artificial meta-material, which eliminates an undesirable electric resonance while still exhibiting a magnetic response. Further we determine the subset of point groups of which particles must belong to in order to yield an isotropic 3D magnetic response, and we show an example.

cond-mat.other↗
arXiv2005

Infrared signatures of hole and spin stripes in La{2-x}Sr{x}CuO{4}

We investigate the hole and lattice dynamics in a prototypical high temperature superconducting system La{2-x}Sr{x}CuO{4} using infrared spectroscopy. By exploring the anisotropy in the electronic response of CuO2 planes we show that our results support the notion of stripes. Nevertheless, charge ordering effects are not apparent in the phonon spectra. All crystals show only the expected infrared active modes for orthorhombic phases without evidence for additional peaks that may be indicative of static charge ordering. Strong electron-phonon interaction manifests itself through the Fano lineshape of several phonon modes. This analysis reveals anisotropic electron-phonon coupling across the phase diagram, including superconducting crystals. Due to the ubiquity of the CuO2 plane, these results may have implications for other high Tc superconductors.

cond-mat.supr-con↗
arXiv2002

Searching for the Slater Transition in the Pyrochlore Cd$_{2}$Os$_{2}$O$_{7}$ with Infrared Spectroscopy

Infrared reflectance measurements were made on the single crystal pyrochlore Cd$_{2}$Os$_{2}$O$_{7}$ in order to examine the transformations of the electronic structure and crystal lattice across the boundary of the metal insulator transition at $T_{MIT}=226K$. All predicted IR active phonons are observed in the conductivity over all temperatures and the oscillator strength is found to be temperature independent. These results indicate that charge ordering plays only a minor role in the MIT and that the transition is strictly electronic in nature. The conductivity shows the clear opening of a gap with $2Δ=5.2k_{B}T_{MIT}$. The gap opens continuously, with a temperature dependence similar to that of BCS superconductors, and the gap edge having a distinct $σ(ω)\thicksimω^{1/2}$ dependence. All of these observables support the suggestion of a Slater transition in Cd$_{2}% $Os$_{2}$O$_{7}$.

cond-mat.str-el↗