arXiv subjects

Martin Dressel

Publications and source records attributed to Martin Dressel.

At least 109 records · Page 6Linked to original sources

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

The effect of Coulomb interactions on the disorder driven superconductor-insulator transition: THz versus tunneling spectroscopy

The interplay between disorder and superconductivity has intrigued physicists for decades. Of particular interest is the influence of disorder on the superconducting energy gap $Δ$. In the absence of Coulomb interactions between electrons, disorder leads to emergent granularity of the local order parameter resulting in a pseudogap at temperatures above the critical temperature $T_c$, as well as a finite gap $Δ$ on the insulating side of the disorder-driven superconductor-insulator transition (SIT). At the same time, disorder also enhances the Coulomb interactions, which subsequently may influence $Δ$ in a manner that is still not fully understood. Here we investigate the evolution of the energy gap through the SIT by two different experimental methods: tunneling spectroscopy, in which a metallic electrode is placed close to the studied sample thus screening the Coulomb interactions, and terahertz (THz) spectroscopy, which probes the unscreened sample. The comparison between the two methods illustrates the role played by electronic interactions in determining the nature of the phases across the SIT and sheds light on the mechanisms involved in the destruction 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

Search for quantum electronic dipoles in the dimerized κ-(BEDT-TTF)2Cu[N(CN)2]Cl salt

The Mott insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl consists of molecular dimers arranged on an anisotropic triangular lattice and develops a canted antiferromagnetic ground state. It has recently been suggested that this system features purely electronic ferroelectricity which requires an electric dipole moment. Optical spectroscopy clearly rules out charge imbalance in this system, which excludes the existence of quantum electric dipoles on the dimers and subsequently a dipolar spin coupling. We suggest that the prominent in-plane dielectric response in κ-(BEDT-TTF)2Cu[N(CN)2]Cl is due to short-range discommensurations of the antiferromagnetic phase in the temperature range 30 < T < 50 K, and domain wall relaxations at lower temperatures.

cond-mat.str-el↗

arXiv2012

Absence of charge order in the dimerized κ-phase BEDT-TTF salts

Utilizing infrared vibrational spectroscopy we have investigated dimerized two-dimensional organic salts in order to search for possible charge redistribution that might cause electronic dipoles and ferroelectricity: the quantum spin liquid κ-(BEDT-TTF)2Cu2(CN)3, the antiferromagnetic Mott insulator κ-(BEDT-TTF)2$Cu[N(CN)2]Cl, and the superconductor κ-(BEDT-TTF)2$Cu[N(CN)2]Br. None of them exhibit any indication of charge disproportionation upon cooling down to low temperatures. No modification in the charge distribution is observed around T = 6 K where a low-temperature anomaly has been reported for the spin-liquid κ-(BEDT-TTF)2$Cu2(CN)3. In this compound the in-plane optical response and vibrational coupling are rather anisotropic, indicating that the tilt of the BEDT-TTF molecules in c-direction and their coupling to the anion layers has to be considered in the explanation of the electromagnetic properties.

cond-mat.str-el↗

arXiv2012

Power-law dependence of the optical conductivity observed in the quantum spin-liquid compound κ-(BEDT-TTF)2Cu2(CN)3

The Mott-insulator κ-(BEDT-TTF)_2Cu_2(CN)_3 is the prime candidate of a quantum spin liquid with puzzling magnetic properties. Our THz and infrared investigations reveal that also the charge dynamics does not follow the expectations for a Mott insulator. The frequency-dependent conductivity exhibits a power-law behavior $σ_1(ω)\propto ω^n$ that grows stronger as the temperature decreases and extends all the way through the far-infrared. With $n\approx 0.8$ to 1.5 we obtain a significantly smaller exponent than predicted by Ng and Lee [Phys. Rev. Lett. {\bf 99}, 156402 (2007)]. We suggest fluctuations becomes important in the spin-liquid state and couple to the electrodynamic properties differently compared to the antiferromagnetic Mott insulator κ-(BEDT-TTF)_2Cu[N(CN)_2]Cl. We discuss the various possibilities of how charge fluctuations are influenced by the presence or absence of magnetic order.

cond-mat.str-el↗

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

Electrodynamics of Correlated Electron Materials

We review studies of the electromagnetic response of various classes of correlated electron materials including transition metal oxides, organic and molecular conductors, intermetallic compounds with $d$- and $f$-electrons as well as magnetic semiconductors. Optical inquiry into correlations in all these diverse systems is enabled by experimental access to the fundamental characteristics of an ensemble of electrons including their self-energy and kinetic energy. Steady-state spectroscopy carried out over a broad range of frequencies from microwaves to UV light and fast optics time-resolved techniques provide complimentary prospectives on correlations. Because the theoretical understanding of strong correlations is still evolving, the review is focused on the analysis of the universal trends that are emerging out of a large body of experimental data augmented where possible with insights from numerical studies.

cond-mat.str-el↗

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

Varying Eu$^{2+}$ magnetic order in EuFe$_2$As$_2$ by chemical pressure

Among iron 122 pnictide superconductors, the EuFe$_2$As$_2$ series draws particular interest because, in addition to superconductivity or the long-range spin-density-wave order in the Fe subsystem, the localized Eu$^{2+}$ magnetic moments order at low temperatures. Here we present a novel scheme of how the spins align in the Eu compounds when pressure varies the coupling; we explain magnetization measurements on EuFe$_2$(As$_{1-x}$P$_x$)$_2$ single crystals as well as other observations of the Eu$^{2+}$ ordering previously reported in literature. The magnetic moments of the Eu$^{2+}$ ions are slightly canted even in the parent compound EuFe$_2$As$_2$, yielding a ferromagnetic contribution along the $c$-direction that becomes stronger with pressure. Reducing the interlayer distance even further, the antiferromagnetic coupling of the $ab$ planes finally turns ferromagnetic.

cond-mat.supr-con↗

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↗

arXiv2010

Looking at the superconducting gap of iron pnictides

THz and infrared spectroscopy is widely utilized to investigate the electrodynamic properties of the novel iron-based superconductors in the normal and superconducting states. Besides electronic excitations and correlations, electron-phonon coupling and the influence of magnetism, the experiments yield important information on low-lying excitations and help to clarify the number and symmetry of superconducting gaps. While the experimental data of different groups converge, the interpretation is still under debate. Here we review the status of optical investigations on the superconducting state for the 122 and 11 family of iron pnictides.

cond-mat.supr-con↗

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↗

arXiv2009

Dielectric properties of ultrathin metal films around the percolation threshold

We report on optical reflection measurements of thin Au films at and around the percolation threshold (film thickness 3 to 10 nm) in an extremely broad spectral range from 500 to 35000 cm-1 (0.3 - 20 *10-6 m). Combining spectroscopic ellipsometry and Fourier-transform infrared spectroscopy, the dielectric properties of the films can be described over the whole frequency range by Kramers-Kronig consistent effective dielectric functions. The optical conductivity of the films is dominated by two contributions: by a Drude-component starting at the percolation threshold in the low frequency range and a plasmon in the near-infrared region, which shifts down in frequency with increasing film thickness. The interplay of both components leads to a dielectric anomaly in the infrared region with a maximum of the dielectric constant at the insulator-to-metal transition.

cond-mat.mtrl-sci↗