arXiv subjects

Martin Dressel

Publications and source records attributed to Martin Dressel.

At least 91 records · Page 5Linked to original sources

arXiv2016

Electrically-induced phase transition in $α$-(BEDT-TTF)$_2$I$_3$: Indications for high-mobility hot charge carriers

The two-dimensional organic conductor $α$-(BEDT-TTF)$_2$I$_3$ undergoes a metal-insulator transition at $T_{\rm CO}=135$ K due to electronic charge ordering. We have conducted time-resolved investigations of its electronic properties in order to explore the field- and temperature-dependent dynamics. At a certain threshold field, the system switches from low-conducting to a high-conducting state, accompanied by a negative differential resistance. Our time-dependent infrared investigations indicate that close to $T_{\rm CO}$ the strong electric field pushes the crystal into a metallic state with optical properties similar to the one for $T>T_{\rm CO}$. Well into the insulating state, however, at $T=80$ K, the spectral response evidences a completely different electronically-induced high-conducting state. Applying a two-state model of hot electrons explains the observations by excitation of charge carriers with a high mobility. They resemble the Dirac-like charge-carriers with a linear dispersion of the electronic bands found in $α$-(BEDT-TTF)$_2$I$_3$ at high-pressure. Extensive numerical simulations quantitatively reproduce our experimental findings in all details.

cond-mat.str-el↗

arXiv2016

Tracing the $s_\pm$-symmetry in iron pnictides by controlled disorder

Consecutive proton irradiation of iron pnictides reduces the superconducting transition temperature to decrease monotonically. Our systematic optical THz investigations of Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ thin films reveal, however, that the low-energy superconducting gap is first suppressed, but recovers for higher irradiation doses. At the same time the decrease of the superfluid density with disorder comes to a halt. These observations are in excellent agreement with theoretical calculations, which predict a disorder-induced transition of the order parameter from $s_{\pm}$ to $s_{++}$ symmetry. Thereby we solve the longstanding puzzle of the iron pnictides gap symmetry in favor of $s_{\pm}$-symmetry and demonstrate THz spectroscopy as a tool to determine the symmetry of the superconducting order parameter.

cond-mat.supr-con↗

arXiv2016

Optical properties of VO$_2$ films at the phase transition: influence of substrate and electronic correlations

Thin films of VO$_2$ on different substrates, Al$_2$O$_3$ and SiO$_2$/Si, have been prepared and characterized from room temperature up to 360 K. From the band structure in the rutile metallic phase and in the monoclinic insulating phase the optical properties are calculated and compared with reflection measurements as a function of temperatures. Various interband transitions can be assigned and compared with previous speculations. We extract the parameters of the metallic charge carriers that evolve upon crossing the insulator-to-metal phase transition and find effects by the substrate. The effect of electronic correlations becomes obvious at the phase transition.

cond-mat.str-el↗

arXiv2016

Molecular Dynamics at Electrical- and Optical-Driven Phase Transitions Time-Resolved Infrared Studies Using Fourier-Transform Spectrometers

The time-dependent optical properties of molecular systems are investigated by step-scan Fourier-transform spectroscopy in order to explore the dynamics at phase transitions and molecular orientation in the milli- and microsecond range. The electrical switching of liquid crystals traced by vibrational spectroscopy reveals a rotation of the molecules with a relaxation time of 2 ms. The photo-induced neutral-ionic transition in TTF-CA takes place by a suppression of the dimerization in the ionic phase and creation of neutral domains. The time-dependent infrared spectra depend on temperature and laser pulse intensity; the relaxation of the spectra follows a stretched-exponential decay with relaxation times in the microsecond range strongly dependent on temperature and laser intensity. We present all details of the experimental setups and thoroughly discuss the technical challenges.

cond-mat.mtrl-sci↗

arXiv2016

Signatures of phase transitions in the microwave response of YbRh2Si2

We used a spectroscopic microwave technique utilizing superconducting stripline resonators at frequencies between 3 GHz and 15 GHz to examine the charge dynamics of YbRh2Si2 at temperatures and magnetic fields close to the quantum critical point. The different electronic phases of this heavy-fermion compound, in particular the antiferromagnetic, Fermi-liquid, and non-Fermi-liquid regimes, were probed with temperature-dependent microwave measurements between 40 mK and 600 mK at a set of different magnetic fields up to 140 mT. Signatures of phase transitions were observed, which give information about the dynamic response of this peculiar material that exhibits field-tuned quantum criticality and pronounced deviations from Fermi-liquid theory.

cond-mat.str-el↗

arXiv2016

THz conductivity of Sr$_{1-x}$Ca$_x$RuO$_3$

We investigate the optical conductivity of Sr$_{1-x}$Ca$_x$RuO$_3$ across the ferromagnetic to paramagnetic transition that occurs at $x=0.8$. The thin films were grown by metalorganic aerosol deposition with $0 \leq x \leq 1$ onto NdGaO$_3$ substrates. We performed THz frequency domain spectroscopy in a frequency range from 3~cm$^{-1}$ to 40~cm$^{-1}$ (100~GHz to 1.4~THz) and at temperatures ranging from 5~K to 300~K, measuring transmittivity and phase shift through the films. From this we obtained real and imaginary parts of the optical conductivity. The end-members, ferromagnetic SrRuO$_3$ and paramagnetic CaRuO$_3$, show a strongly frequency-dependent metallic response at temperatures below 20~K. Due to the high quality of these samples we can access pronounced intrinsic electronic contributions to the optical scattering rate, which at 1.4~THz exceeds the residual scattering rate by more than a factor of three. Deviations from a Drude response start at about 0.7~THz for both end-members in a remarkably similar way. For the intermediate members a higher residual scattering originating in the compositional disorder leads to a featureless optical response, instead. The relevance of low-lying interband transitions is addressed by a calculation of the optical conductivity within density functional theory in the local density approximation (LDA).

cond-mat.str-el↗

arXiv2015

Charge Carrier Dynamics of the Heavy Fermion Metal CeCoIn$_5$ Probed by THz Spectroscopy

We discuss the charge carrier dynamics of the heavy-fermion compound CeCoIn$_5$ in the metallic regime measured by means of quasi-optical THz spectroscopy. The transmittance of electromagnetic radiation through a CeCoIn$_5$ thin film on a dielectric substrate is analyzed in the single-particle Drude framework. We discuss the temperature dependence of the electronic properties, such as the scattering time and dc-conductivity and compare with transport measurements of the sheet resistance. Towards low temperatures, we find an increasing mismatch between the results from transport and Drude-analyzed optical measurements and a growing incapability of the simple single-particle picture describing the charge dynamics, likely caused by the evolving heavy-fermion nature of the correlated electron system.

cond-mat.str-el↗

arXiv2015

Shaping a superconducting dome: Enhanced Cooper-pairing versus suppressed phase coherence in coupled aluminum nanograins

Deterministic enhancement of the superconducting (SC) critical temperature $T_c$ is a long-standing goal in material science. One strategy is engineering a material at the nanometer scale such that quantum confinement strengthens the electron pairing, thus increasing the superconducting energy gap $Δ$, as was observed for individual nanoparticles. A true phase-coherent SC condensate, however, can exist only on larger scales and requires a finite phase stiffness $J$. In the case of coupled aluminium (Al) nanograins, $T_c$ can exceed that of bulk Al by a factor of three, but despite several proposals the relevant mechanism at play is not yet understood. Here we use optical spectroscopy on granular Al to disentangle the evolution of the fundamental SC energy scales, $Δ$ and $J$, as a function of grain coupling. Starting from well-coupled arrays, $Δ$ grows with progressive grain decoupling, causing the increasing of $T_c$. As the grain-coupling is further suppressed, $Δ$ saturates while $T_c$ decreases, concomitantly with a sharp decline of $J$. This crossover to a phase-driven SC transition is accompanied by an optical gap persisting above $T_c$. These findings identify granular Al as an ideal playground to test the basic mechanisms that enhance superconductivity by nano-inhomogeneity.

cond-mat.supr-con↗

arXiv2015

Observing electron spin resonance between 0.1 and 67 GHz at temperatures between 50 mK and 300 K using broadband metallic coplanar waveguides

We describe a fully broadband approach for electron spin resonance (ESR) experiments where it is possible to not only tune the magnetic field but also the frequency continuously over wide ranges. Here a metallic coplanar transmission line acts as compact and versatile microwave probe that can easily be implemented in different cryogenic setups. We perform ESR measurements at frequencies between 0.1 and 67 GHz and at temperatures between 50 mK and room temperature. Three different types of samples (Cr3+ ions in ruby, organic radicals of the nitronyl-nitroxide family, and the doped semiconductor Si:P) represent different possible fields of application for the technique. We demonstrate that an extremely large phase space in temperature, magnetic field, and frequency for ESR measurements, substantially exceeding the range of conventional ESR setups, is accessible with metallic coplanar lines.

cond-mat.mes-hall↗

arXiv2014

The Higgs Mode in Disordered Superconductors Close to a Quantum Phase Transition

The concept of mass-generation via the Higgs mechanism was strongly inspired by earlier works on the Meissner-Ochsenfeld effect in superconductors. In quantum field theory, the excitations of longitudinal components of the Higgs field manifest as massive Higgs bosons. The analogous Higgs mode in superconductors has not yet been observed due to its rapid decay into particle-hole pairs. Following recent theories, however, the Higgs mode should decrease below the pairing gap $2Δ$ and become visible in two-dimensional systems close to the superconductor-insulator transition (SIT). For experimental verification, we measured the complex terahertz transmission and tunneling density of states (DOS) of various thin films of superconducting NbN and InO close to criticality. Comparing both techniques reveals a growing discrepancy between the finite $2Δ$ and the threshold energy for electromagnetic absorption which vanishes critically towards the SIT. We identify the excess absorption below $2Δ$ as a strong evidence of the Higgs mode in two dimensional quantum critical superconductors.

cond-mat.supr-con↗

arXiv2014

Optimization of Coplanar Waveguide Resonators for ESR Studies on Metals

We present simulations and analytic calculations of the electromagnetic microwave fields of coplanar waveguide (CPW) resonators in the vicinity of highly conducting metallic samples. The CPW structures are designed with the aim of investigating electron spin resonance (ESR) in metallic heavy-fermion systems, in particular YbRh$_2$Si$_2$, close to the quantum critical point. Utilizing CPW resonators for ESR experiments allows for studies at mK temperatures and a wide range of freely selectable frequencies. It is therefore of great interest to evaluate the performance of resonant CPW structures with nearby metallic samples. Here we study the microwave fields at the sample surface as a function of sample distance from the waveguide structure and analyze the implications of the sample on the performance of the resonator. The simulation results reveal an optimum sample distance for which the microwave magnetic fields at the sample are maximized and thus best suited for ESR studies.

cond-mat.mes-hall↗

arXiv2014

Niobium stripline resonators for microwave studies on superconductors

Microwave spectroscopy is a powerful experimental tool to reveal information on the intrinsic properties of superconductors. Superconducting stripline resonators, where the material under study constitutes one of the ground planes, offer a high sensitivity to investigate superconducting bulk samples. In order to improve this measurement technique, we have studied stripline resonators made of niobium, and we compare the results to lead stripline resonators. With this technique we are able to determine the temperature dependence of the complex conductivity of niobium and the energy gap $Δ(0)=2.1$ meV. Finally we show measurements at the superconducting transition of a tantalum bulk sample using niobium stripline resonators.

cond-mat.supr-con↗

arXiv2014

Rabi oscillations of solitons in spin-chains: a new route to quantum computation and communication

We provide the first evidence for coherence and Rabi oscillations of spin-solitons pinned by the local breaking of translational symmetry in isotropic Heisenberg chains (simple antiferromagnetic-Néel or spin-Peierls).We show that these correlated spin systems made of hundreds of coupled spin bear an overall spin S=1/2 and can be manipulated as a single spin. This is clearly contrary to all known spin-qubits which are paramagnetic centres, highly diluted to prevent decoherence. These results offer an alternative approach for spin-qubits paving the way for the implementation of a new type of quantum computer.

cond-mat.str-el↗

arXiv2014

Charge-order fluctuations and superconductivity in two-dimensional organic metals

We report comprehensive Raman and infrared investigations of charge-order (CO) fluctuations in the organic metal $β^{\prime\prime}$-(BEDT-TTF)$_2$SF$_5$CHFSO$_3$ and superconductor $β^{\prime\prime}$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$. The charge-sensitive vibrational bands have been analyzed through an extension of the well-known Kubo model for the spectral signatures of an equilibrium between two states. At room temperature, both salts exhibit charge fluctuations between two differently charged molecular states with an exchange frequency of about $6\times10^{11} {\rm s}^{-1}$. The exchange rate of the metallic salt remains roughly constant down to 10 K, while in the superconductor the exchange velocity starts to decrease below 200 K, and a "frozen" charge-ordered state emerges, and coexists with the charge-order fluctuation state down to the superconducting temperature. These findings are confronted with other existing spectroscopic experiments, and a tentative phase diagram is proposed for the $β^{\prime\prime}$ BEDT-TTF quarter-filled salts.

cond-mat.supr-con↗

arXiv2014

Vibrational states of a water molecule in a nano-cavity of beryl crystal lattice

Low-energy excitations of a single water molecule are studied when confined within a nano-size cage formed by the ionic crystal lattice. Optical spectra are measured of manganese doped beryl single crystal Mn:Be3Al2Si6O18, that contains water molecules individually isolated in 0.51 nm diameter voids within the crystal lattice. Two types of orientation are distinguished: water-I molecules have their dipole moments aligned perpendicular to the c axis, and dipole moments of water-II molecules are parallel to the c-axis. The optical conductivity and permittivity spectra are recorded in terahertz and infrared ranges, at frequencies from several wavenumbers up to 7000cm-1, at temperatures from 5K to 300K and for two polarizations, when the electric vector E of the radiation is parallel and perpendicular to the c-axis. Comparative experiments on as-grown and on dehydrated samples allow to identify the conductivity and permittivity spectra caused exclusively by water molecules. In the infrared range well-known internal modes nu1, nu2 and nu3 of the H2O molecule are observed for both polarizations, indicating the presence of water-I and water-II molecules in the crystal. Spectra recorded below 1000cm-1 reveal a rich set of highly anisotropic features in the low-energy response of H2O molecule in a crystalline nanocage. While for E parallel to c only two absorption peaks are detected, at 90cm-1 and 160cm-1, several absorption bands are discovered for E perpendicular to c, each consisting of narrower resonances. The bands are assigned to librational and translational vibrations of water-I molecule that is weakly, via hydrogen bonds, coupled to the nanocage walls. A model is presented that explains the fine structure of the bands by splitting of the energy levels due to quantum tunneling between the minima in a six-well potential relief felt by a molecule within the cage.

cond-mat.mes-hall↗

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

Quantum Behavior of Water Molecules Confined to Nanocavities in Gemstone

When water is confined to nanocavities, its quantum-mechanical behavior can be revealed by terahertz spectroscopy. We place H2O molecules in the nanopores of a beryl crystal-lattice and observe a rich and highly anisotropic set of absorption lines in the terahertz spectral range. Two bands can be identified, which originate from translational and librational motions of the water molecule isolated within the cage; they correspond to the analogous broad bands in liquid water and ice. In the present case of well-defined and highly symmetric nanocavities, the observed fine structure can be explained by macroscopic tunneling of the H2O molecules within a six-fold potential caused by the interaction of the molecule with the cavity walls.

cond-mat.mes-hall↗