arXiv subjects

Marc Scheffler

Publications and source records attributed to Marc Scheffler.

53 records · Page 3Linked to original sources

arXiv2014

Surface-resistance measurements using superconducting stripline resonators

We present a method to measure the absolute surface resistance of conductive samples at a set of GHz frequencies with superconducting lead stripline resonators at temperatures 1- 6K. The stripline structure can easily be applied for bulk samples and allows direct calculation of the surface resistance without the requirement of additional calibration measurements or sample reference points. We further describe a correction method to reduce experimental background on high-Q resonance modes by exploiting TEM-properties of the external cabling. We then show applications of this method to the reference materials gold, tantalum, and tin, which include the anomalous skin effect and conventional superconductivity. Furthermore, we extract the complex optical conductivity for an all-lead stripline resonator to find a coherence peak and the superconducting gap of lead.

cond-mat.mes-hall↗

arXiv2013

Anomalous Microwave Surface Resistance of CeCu6

We present surface resistance measurements of the archetypical heavy-fermion compound CeCu6 for frequencies between 3.7 and 18 GHz and temperatures from 1.2 to 6 K. The measurements were performed with superconducting stripline resonators that allow simultaneous measurements at multiple frequencies. The surface resistance of CeCu6 exhibits a pronounced decrease below 3 K, in consistence with dc resistivity. The low-temperature frequency dependence of the surface resistance follows a power law with exponent 2/3. While for conventional metals this would be consistent with the anomalous skin effect, we discuss the present situation of a heavy-fermion metal, where this frequency dependence might instead stem from the influence of electronic correlations.

cond-mat.str-el↗

arXiv2013

Electrodynamics of the Superconducting State in Ultra-Thin Films at THz Frequencies

We report on terahertz frequency-domain spectroscopy (THz-FDS) experiments in which we measure charge carrier dynamics and excitations of thin-film superconducting systems at low temperatures in the THz spectral range. The characteristics of the set-up and the experimental procedures are described comprehensively. We discuss the single-particle density of states and a theory of electrodynamic absorption and optical conductivity of conventional superconductors. We present the experimental performance of the setup at low temperatures for a broad spectral range from 0.1 - 1.1 THz by the example of ultra-thin films of weakly disordered superconductors niobium nitride (NbN) and tantalum nitride (TaN) with different values of critical temperatures. Furthermore, we analyze and interpret our experimental data within the framework of conventional Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity.

cond-mat.supr-con↗

arXiv2013

Terahertz Conductivity of the Heavy-Fermion State in CeCoIn5

The optical properties of thin films of the heavy-fermion compound CeCoIn5, which were deposited by molecular beam epitaxy onto MgF2 substrates, have been studied at frequencies 7 to 45 cm^{-1} (corresponding to 0.2 to 1.3 THz) and temperatures 2 to 300 K. We observe an electrodynamic behavior which is typical for heavy fermions, namely Drude-like conductivity with a relaxation rate at rather low frequencies. This relaxation rate increases almost linearly with temperature up to at least 30 K. The coherent heavy-fermion state, characterized by an increase of the effective mass, continuously evolves upon cooling and is not fully developed for temperatures as low as 5 K.

cond-mat.str-el↗

arXiv2013

Microwave spectroscopy on heavy-fermion systems: probing the dynamics of charges and magnetic moments

Investigating solids with light gives direct access to charge dynamics, electronic and magnetic excitations. For heavy fermions, one has to adjust the frequency of the probing light to the small characteristic energy scales, leading to spectroscopy with microwaves. We review general concepts of the frequency-dependent conductivity of heavy fermions, including the slow Drude relaxation and the transition to a superconducting state, which we also demonstrate with experimental data taken on UPd2Al3. We discuss the optical response of a Fermi liquid and how it might be observed in heavy fermions. Microwave studies with focus on quantum criticality in heavy fermions concern the charge response, but also the magnetic moments can be addressed via electron spin resonance (ESR). We discuss the case of YbRh2Si2, the open questions concerning ESR of heavy fermions, and how these might be addressed in the future. This includes an overview of the presently available experimental techniques for microwave studies on heavy fermions, with a focus on broadband studies using the Corbino approach and on planar superconducting resonators.

cond-mat.str-el↗

arXiv2012

Direct observation of the superconducting gap in thin film of titanium nitride using terahertz spectroscopy

We report on the charge carrier dynamics of superconducting titanium nitride (TiN) in the frequency range 90 - 510 GHz (3 - 17 cm-1). The experiments were perfomed on a 18 nm thick TiN film with a critical temperature of 3.4 K. Measurements were carried out from room temperature down to 2 K, and in magnetic fields up to 7 T. We extract the real and imaginary parts of the complex conductivity as a function of frequency and temperature, directly providing the superconducting energy gap. Further analysis yields the superconducting London penetration depth. The findings as well as the normal state properties strongly suggest conventional BCS superconductivity, underlined by the BCS ratio 3.44. Detailed analysis of the charge carrier dynamics of the silicon substrate is also discussed.

cond-mat.supr-con↗

arXiv2012

The phase boundary of superconducting niobium thin films with antidot arrays fabricated with microsphere photolithography

The experimental investigation of the $I_c(B)$--$T_c(B)$ phase boundary of superconducting niobium films with large area quasihexagonal hole arrays is reported. The hole arrays were patterned with microsphere photolithography. We investigate the perforated niobium films by means of electrical directed current transport measurements close to the transition temperature $T_c$ in perpendicularly applied magnetic fields. We find pronounced modulations of the critcal current with applied magnetic field, which we interpret as a consequence of commensurable states between the Abrikosov vortex lattice and the quasihexagonal pinning array. Furthermore, we observe Little-Parks oscillations in the critical temperature vs magnetic field.

cond-mat.supr-con↗

arXiv2011

Terahertz conductivity of the heavy-fermion compound UNi2Al3

We have studied the optical properties of the heavy-fermion compound UNi2Al3 at frequencies between 100 GHz and 1 THz (3 cm^-1 and 35 cm^-1), temperatures between 2 K and 300 K, and magnetic fields up to 7 T. From the measured transmission and phaseshift of radiation passing through a thin film of UNi2Al3, we have directly determined the frequency dependence of the real and imaginary parts of the optical conductivity (or permittivity, respectively). At low temperatures the anisotropy of the optical conductivity along the a- and c-axes is about 1.5. The frequency dependence of the real part of the optical conductivity shows a maximum at low temperatures, around 3 cm^-1 for the a-axis and around 4.5 cm^-1 for the c-axis. This feature is visible already at 30 K, much higher than the Neel temperature of 4.6 K, and it does not depend on external magnetic fields as high as 7 T. We conclude that this feature is independent of the antiferromagnetic order for UNi2Al3, and this might also be the case for UPd2Al3 and UPt3, where a similar maximum in the optical conductivity was observed previously.

cond-mat.str-el↗

arXiv2011

Magnetoresistance and Phase Diagram of Thin-Film UNi2Al3

We study the dc resistivity of UNi2Al3 thin films as a function of temperature and magnetic field. We focus on the temperature range around the antiferromagnetic transition (TN \approx 4 K in zero applied field). From a clear signature of TN in the dc resistance along the crystallographic a-direction, we extract the shape of the magnetic phase diagram. Here we find quantitative differences in comparison to previous studies on bulk crystals.

cond-mat.str-el↗

arXiv2010

Microwave inductance of thin metal strips

We have measured the frequency-dependent, complex impedance of thin metal strips in a broad range of microwave frequencies (45~MHz to 20~GHz). The spectra are in good agreement with theoretical predictions of an RCL model. The resistance, inductance, and capacitance, which govern the microwave response, depend on the strip width and thickness as well as on the strip and substrate materials. While the strip resistance scales inversely with the cross section, the inductance depends on the width of the strip, but not on the thickness (in the limit of small thickness).

cond-mat.other↗

arXiv2010

Microwave conductivity of heavy fermions in UPd2Al3

Heavy-fermion compounds are characterized by electronic correlation effects at low energies which can directly be accessed with optical spectroscopy. Here we present detailed measurements of the frequency- and temperature-dependent conductivity of the heavy-fermion compound UPd2Al3 using broadband microwave spectroscopy in the frequency range 45 MHz to 40 GHz at temperatures down to 1.7 K. We observe the full Drude response with a relaxation time up to 50 ps, proving that the mass enhancement of the heavy charge carriers goes hand in hand with an enhancement of the relaxation time. We show that the relaxation rate as a function of temperature scales with the dc resistivity. We do not find any signs of a frequency-dependent relaxation rate within the addressed frequency range.

cond-mat.str-el↗

arXiv2009

Diameter-dependent conductance of InAs nanowires

Electrical conductance through InAs nanowires is relevant for electronic applications as well as for fundamental quantum experiments. Here we employ nominally undoped, slightly tapered InAs nanowires to study the diameter dependence of their conductance. Contacting multiple sections of each wire, we can study the diameter dependence within individual wires without the need to compare different nanowire batches. At room temperature we find a diameter-independent conductivity for diameters larger than 40 nm, indicative of three-dimensional diffusive transport. For smaller diameters, the resistance increases considerably, in coincidence with a strong suppression of the mobility. From an analysis of the effective charge carrier density, we find indications for a surface accumulation layer.

cond-mat.mes-hall↗

arXiv2009

Observing the anisotropic optical response of the heavy-fermion compound UNi2Al3

The optical conductivity of heavy fermions can reveal fundamental properties of the charge carrier dynamics in these strongly correlated electron systems. Here we extend the conventional techniques of infrared optics on heavy fermions by measuring the transmission and phase shift of THz radiation that passes through a thin film of UNi2Al3, a material with hexagonal crystal structure. We deduce the optical conductivity in a previously not accessible frequency range, and furthermore we resolve the anisotropy of the optical response (parallel and perpendicular to the hexagonal planes). At frequencies around 7cm^-1, we find a strongly temperature-dependent and anisotropic optical conductivity that - surprisingly - roughly follows the dc behavior.

cond-mat.str-el↗

arXiv2008

Quasi-Particle Dynamics in Superconducting Aluminum

The response of superconducting aluminum to electromagnetic radiation is investigated in a broad frequency (45 MHz to 40 GHz) and temperature range ($T>T_c/2$), by measuring the complex conductivity. While the imaginary part probes the superfluid density (Cooper-pairs), the real part monitors the opening of the superconducting energy gap and -- most important here -- the zero-frequency quasi-particle response. Varying the mean free path gives some insight into the dynamics, scattering and coherence effects of the quasi-particles in the superconducting state.

cond-mat.supr-con↗

arXiv2007

Signature of Electronic Correlations in the Optical Conductivity of the Doped Semiconductor Si:P

Electronic transport in highly doped but still insulating silicon at low temperatures is dominated by hopping between localized states; it serves as a model system of a disordered solid for which the electronic interaction can be investigated. We have studied the frequency-dependent conductivity of phosphorus-doped silicon in the THz frequency range (30 GHz to 3 THz) at low temperatures $T\geq 1.8$ K. The crossover in the optical conductivity from a linear to a quadratic frequency dependence as predicted by Efros and Shklovskii is observed qualitatively; however, the simple model does not lead to a quantitative agreement. Covering a large range of donor concentration, our temperature- and frequency-dependent investigations reveal that electronic correlation effects between the localized states play an important and complex role at low temperatures. In particular we find a super-linear frequency dependence of the conductivity that highlights the influence of the density of states, i.e. the Coulomb gap, on the optical conductivity. When approaching the metal-to-insulator transition by increasing doping concentration, the dielectric constant and the localization length exhibit critical behavior.

cond-mat.str-el↗

arXiv2000

The frequency, temperature, and magnetic field dependence of ferromagnetic resonance and anti-resonance in La$_{0.8}$Sr$_{0.2}$MnO$_3$

Employing a broadband microwave reflection configuration, we have measured the complex surface impedance, $Z_S(ω,T,H)$, of single crystal La$_{0.8}$Sr$_{0.2}$MnO$_3$, as a function of frequency (0.045-45 GHz), temperature (250-325 K), and magnetic field (0-1.9 kOe). The microwave surface impedance depends not only on the resistivity of the material, but also on the magnetic permeability, $\hatμ(ω,T,H)$, which gives rise to ferromagnetic resonance (FMR) and ferromagnetic anti-resonance (FMAR). The broadband nature of this experiment allows us to follow the FMR to low frequency and to deduce the behavior of both the local internal fields and the local magnetization in the sample.

cond-mat.str-el↗

arXiv1999

Determination of the magnetization scaling exponent for single crystal La$_{0.8}$Sr$_{0.2}$MnO$_3$ by broadband microwave surface impedance measurements

Employing a broadband microwave reflection configuration, we have measured the complex surface impedance, $Z_S(ω,T)$, of single crystal La$_{0.8}$Sr$_{0.2}$MnO$_3$, as a function of frequency (0.045-45 GHz) and temperature (250-325 K). Through the dependence of the microwave surface impedance on the magnetic permeability, $\hatμ(ω,T)$, we have studied the local magnetic behavior of this material, and have extracted the spontaneous magnetization, $M_0(T)$, in {\em zero applied field}. The broadband nature of these measurements and the fact that no external field is applied to the material provide a unique opportunity to analyze the critical behavior of the spontaneous magnetization at temperatures very close to the ferromagnetic phase transition. We find a Curie temperature $T_C=305.5\pm 0.5$ K and scaling exponent $β=0.45\pm 0.05$, in agreement with the prediction of mean-field theory. We also discuss other recent determinations of the magnetization critical exponent in this and similar materials and show why our results are more definitive.

cond-mat.str-el↗