arXiv subjects

M. Dressel

Publications and source records attributed to M. Dressel.

At least 127 records · Page 7Linked to original sources

arXiv2004

Evidence for a Bulk Complex Order-Parameter in Y0.9Ca0.1Ba2Cu3O7-delta Thin Films

We have measured the penetration depth of overdoped Y0.9Ca0.1Ba2Cu3O7-delta (Ca-YBCO) thin films using two different methods. The change of the penetration depth as a function of temperature has been measured using the parallel plate resonator (PPR), while its absolute value was obtained from a quasi-optical transmission measurements. Both sets of measurements are compatible with an order parameter of the form: Delta*dx2-y2+i*delta*dxy, with Delta=14.5 +- 1.5 meV and delta=1.8 meV, indicating a finite gap at low temperature. Below 15 K the drop of the scattering rate of uncondensed carriers becomes steeper in contrast to a flattening observed for optimally doped YBCO films. This decrease supports our results on the penetration depth temperature dependence. The findings are in agreement with tunneling measurements on similar Ca-YBCO thin films.

cond-mat.supr-con↗

arXiv2004

In-plane optical conductivity of La$_{2-x}$Sr$_x$CuO$_4$: Reduced superconducting condensate and residual Drude-like response

Temperature dependences of the optical spectra of La$_{2-x}$Sr$_x$CuO$_4$ with $x$=0.12 and 0.15 were carefully examined for a polarization parallel to the CuO$_2$-plane over a wide frequency range down to 8 cm$^{-1}$. Selection of well-characterized crystals enabled us to measure purely in-plane polarized spectra without any additional peak. The weight of superconducting (SC) condensate estimated from the missing area in $σ_1(ω$) well agrees with the estimate from the slope of $σ_2(ω$) vs 1/$ω$ plot, showing no evidence that the Ferrell-Glover-Tinkham sum-rule is violated in the optical spectrum. We demonstrate that the optically estimated SC condensate is much smaller than the value obtained from the $μ$SR measurement of magnetic penetration depth. We also find an anomalous increase of conductivity in sub-millimeter region towards $ω$=0 below $T_c$, which suggests the microscopic inhomogeneity in the superconducting state. Both observations are discussed in relation with the inhomogeneous electronic state that might be inherent to high-$T_c$ cuprates.

cond-mat.supr-con↗

arXiv2003

Suppression of Superconductivity in YBCO/LCMO Superlattices

The competition of superconductivity and magnetism in superlattices composed of alternating YBa$_2$Cu$_3$O$_{7-d}$ and La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films is investigated using low-energy optical spectroscopy. The thickness of the superconducting YBCO layers is varied from 30 nm to 20 nm while the thickness of the magnetic LCMO layers is kept constant at 20 nm. We clearly observe that the superconducting condensate density in the superconducting state of superlattice is drastically reduced by the magnetic subsystem which may be connected with proximity effects that distort the gap symmetry and thus suppress superconductivity.

cond-mat.supr-con↗

arXiv2003

Low-Frequency Optical Properties of $β^{\prime\prime}$-(BEDO- TTF)$_5$[CsHg(SCN)$_4$]$_2$: Indications of Electronic Correlations in a 1/5-Filled Two-Dimensional Conductor

The polarized reflectivity of $β^{\prime\prime}$-(BEDO-TTF)$_5$[CsHg(SCN)$_4$]$_2$ is studied in the infrared range between 60 cm-1 and 6000 cm-1 from room temperature down to 10 K. Already at T=300 K a pseudogap in the optical conductivity is present of about 300 cm-1; the corresponding maximum in the spectrum shifts to lower frequencies as the temperature decreases. In contrast to quarter-filled BEDT-TTF-based conductors of the $β^{\prime\prime}$-phase a robust Drude component in the conductivity spectra is observed which we ascribe to the larger fraction of charge carriers associated with the 1/5-filling of the conduction band. This observation is corroborated by exact diagonalization calculations on an extended Hubbard model on a square lattice for different fillings. A broad band at 4000 cm-1 which appears for the electric field polarized parallel to the stacks of the BEDO-TTF molecules is associated to structural modulations in the stacks; these modulations lead to a rise of the dc and microwave resistivity in 100 to 30 K temperature range.

cond-mat.str-el↗

arXiv2003

Asymmetric Lineshape due to Inhomogeneous Broadening of the Crystal-Field Transitions in Mn12ac Single Crystals

The lineshape of crystal-field transitions in single crystals of Mn12ac molecular magnets is determined by the magnetic history. The absorption lines are symmetric and Gaussian for the non-magnetized state obtained by zero-field cooling (zfc). In the magnetized state which is reached when the sample is cooled in a magnetic field (fc), however, they are asymmetric even in the absence of an external magnetic field. These observations are quantitatively explained by inhomogeneous symmetrical (Gaussian) broadening of the crystal-field transitions combined with a contribution of off-diagonal components of the magnetic susceptibility to the effective magnetic permeability.

cond-mat.str-el↗

arXiv2003

Suppression of the charge-density-wave state in Sr_14Cu_24O_41 by calcium doping

The charge response in the spin chain/ladder compound Sr_14-xCa_xCu_24O_41 is characterized by DC resistivity, low-frequency dielectric spectroscopy and optical spectroscopy. We identify a phase transition below which a charge-density wave (CDW) develops in the ladder arrays. Calcium doping suppresses this phase with the transition temperature decreasing from 210 K for x=0 to 10 K for x=9, and the CDW gap from 130 meV down to 3 meV, respectively. This suppression is due to the worsened nesting originating from the increase of the inter-ladder tight-binding hopping integrals, as well as from disorder introduced at the Sr sites. These results altogether speak in favor of two-dimensional superconductivity under pressure.

cond-mat.supr-con↗

arXiv2003

Electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ

We study the electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ by means of density-functional band theory, Hubbard model calculations, and angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant quantitative and qualitative discrepancies to band theory. We demonstrate that the dispersive behavior as well as the temperature-dependence of the spectra can be consistently explained by the finite-energy physics of the one-dimensional Hubbard model at metallic doping. The model description can even be made quantitative, if one accounts for an enhanced hopping integral at the surface, most likely caused by a relaxation of the topmost molecular layer. Within this interpretation the ARPES data provide spectroscopic evidence for the existence of spin-charge separation on an energy scale of the conduction band width. The failure of the one-dimensional Hubbard model for the {\it low-energy} spectral behavior is attributed to interchain coupling and the additional effect of electron-phonon interaction.

cond-mat.str-el↗

arXiv2003

Variable-range hopping conductivity in the copper-oxygen chains of La_3Sr_3Ca_8Cu_24O_41

We show that the spin chain/ladder compound La_3Sr_3Ca_8Cu_24O_41 is an insulator with hopping transport along the chains. In the temperature range 35 - 280 K, DC conductivity sigma_{DC}(T) follows Mott's law of variable-range hopping conduction; the frequency dependence has the form sigma(ν, T) = σ_{DC}(T) + A(T)ν^{s}, where s \approx 1. The conduction mechanism changes from variable-range hopping to nearest-neighbor hopping around T_{c} =300 K. The chain array thus behaves like a one-dimensional disordered system. Disorder is due to random structural distortions of chains induced by irregular coordination of the La/Sr/Ca ions.

cond-mat.supr-con↗

arXiv2003

Signatures of polaronic excitations in quasi-one-dimensional LaTiO$_{3.41}$

The optical properties of quasi-one-dimensional metallic LaTiO$_{3.41}$ are studied for the polarization along the $a$ and $b$ axes. With decreasing temperature modes appear along both directions suggestive for a phase transition. The broadness of these modes along the conducting axis might be due to the coupling of the phonons to low-energy electronic excitations across an energy gap. We observe a pronounced midinfrared band with a temperature dependence consistent with (interacting) polaron models. The polaronic picture is corroborated by the presence of strong electron-phonon coupling and the temperature dependence of the dc conductivity.

cond-mat↗

arXiv2003

Extremely Small Energy Gap in the Quasi-One-Dimensional Conducting Chain Compound SrNbO$_{3.41}$

Resistivity, optical, and angle-resolved photoemission experiments reveal unusual one-dimensional electronic properties of highly anisotropic SrNbO$_{3.41}$. Along the conducting chain direction we find an extremely small energy gap of only a few meV at the Fermi level. A discussion in terms of typical 1D instabilities (Peierls, Mott-Hubbard) shows that neither seems to provide a satisfactory explanation for the unique properties of SrNbO$_{3.41}$.

cond-mat.str-el↗

arXiv2003

Surface characterization and surface electronic structure of organic quasi-one-dimensional charge transfer salts

We have thoroughly characterized the surfaces of the organic charge-transfer salts TTF-TCNQ and (TMTSF)2PF6 which are generally acknowledged as prototypical examples of one-dimensional conductors. In particular x-ray induced photoemission spectroscopy turns out to be a valuable non-destructive diagnostic tool. We show that the observation of generic one-dimensional signatures in photoemission spectra of the valence band close to the Fermi level can be strongly affected by surface effects. Especially, great care must be exercised taking evidence for an unusual one-dimensional many-body state exclusively from the observation of a pseudogap.

cond-mat↗

arXiv2002

Direct Observation of Quantum Tunneling and Relaxation in Mn12ac

The spin relaxation in the molecular magnet \Mn12 is investigated by a novel type of high-frequency magnetic spectroscopy. By reversing the external magnetic field the intensities of the $|\pm 10>\to |\pm 9>$ transitions within the ground multiplet redistribute; below $T\approx 2$ K resonant quantum tunneling is directly observed as {\em magnetic hole burning} in the absorption spectra of a polycrystal. We quantitatively describe our observations by taking into account inhomogeneous line broadening, the time dependence of the level population during the relaxation process, and the resonance behavior of the relaxation rate due to quantum tunneling near the avoided level crossing.

cond-mat.str-el↗

arXiv2002

Competition between Charge Ordering and Superconductivity in Layered Organic Conductors $α$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ (M = K, NH$_4$)

While the optical properties of the superconducting salt $α$-(BEDT-TTF)$_2$NH$_4$Hg(SCN)$_4$ remain metallic down to 2 K, in the non-superconducting K-analog a pseudogap develops at frequencies of about 200 cm$^{-1}$ for temperatures T < 200 K. Based on exact diagonalisation calculations on an extended Hubbard model at quarter-filling we argue that fluctuations associated with short range charge ordering are responsible for the observed low-frequency feature. The different ground states, including superconductivity, are a consequence of the proximity of these compounds to a quantum phase charge-ordering transition driven by the intermolecular Coulomb repulsion.

cond-mat.str-el↗

arXiv2002

Evidence of the superconducting energy gap in the optical spectra of $α_t$-(BEDT-TTF)$_{2}$I$_{3}$

The far-infrared in-plane reflectivity of the organic superconductor $α_t$-(BEDT-TTF)$_{2}$I$_{3}$ is measured down to frequencies of 10 cm$^{-1}$; the optical conductivity is obtained by a Kramers-Kronig procedure. In the superconducting state an energy gap opens at 25 cm$^{-1}$ which corresponds to $2Δ/k_BT_c = 4.4$ in agreement with moderate coupling. Even well below $T_c=8$ K a considerable absorption remains for $\hbarω<2Δ$. We discuss different posibilities to explain this behaviour.

cond-mat.supr-con↗

arXiv2002

Correlation gap in the heavy-fermion antiferromagnet UPd_2Al_3

The optical properties of the heavy-fermion compound UPd$_2$Al$_3$ have been measured in the frequency range from 0.04 meV to 5 meV (0.3 to 40 cm$^{-1}$) at temperatures $2 {\rm K}<T< 300$ K. Below the coherence temperature $T^*\approx 50$ K, the hybridization gap opens around 10 meV. As the temperature decreases further ($T\leq 20$ K), a well pronounced pseudogap of approximately 0.2 meV develops in the optical response; we relate this to the antiferromagnetic ordering which occurs below $T_N\approx 14$ K. The frequency dependent mass and scattering rate give evidence that the enhancement of the effective mass mainly occurs below the energy which is associated to the magnetic correlations between the itinerant and localized 5f electrons. In addition to this correlation gap, we observe a narrow zero-frequency conductivity peak which at 2 K is less than 0.1 meV wide, and which contains only a fraction of the delocalized carriers. The analysis of the spectral weight infers a loss of kinetic energy associated with the superconducting transition.

cond-mat.str-el↗

arXiv2002

Microspectroscopy and Imaging in the THz Range Using Coherent CW Radiation

A novel THz near-field spectrometer is presented which allows to perform biological and medical studies with high spectral resolution combined with a spatial resolution down to l/100. In the setup an aperture much smaller than the used wavelength is placed in the beam very close to the sample. The sample is probed by the evanescent wave behind the aperture. The distance is measured extremely accurate by a confocal microscope. We use monochromatic sources which provide powerful coherent cw radiation tuneable from 50 GHz up to 1.5 THz. Transmission and reflection experiments can be performed which enable us to study solids and molecules in aqueous solution. Examples for spectroscopic investigations on biological tissues are presented.

physics.bio-ph↗

arXiv2002

Spectroscopic signatures of spin-charge separation in the quasi-one-dimensional organic conductor TTF-TCNQ

The electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ is studied by angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant discrepancies to band theory. We demonstrate that the measured dispersions can be consistently mapped onto the one-dimensional Hubbard model at finite doping. This interpretation is further supported by a remarkable transfer of spectral weight as function of temperature. The ARPES data thus show spectroscopic signatures of spin-charge separation on an energy scale of the conduction band width.

cond-mat.str-el↗

arXiv2002

Charge-density wave formation in Sr_{14}Cu_{24}O_{41}

The electrodynamic response of the spin-ladder compound Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ ($x=0, 3, 9$) has been studied from radiofrequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around 12 cm$^{-1}$ in Sr$_{14}$Cu$_{24}$O$_{41}$ for the radiation polarized along the chains/ladders (${\bf E}\parallel {\bf c}$). In addition a strongly temperature dependent dielectric relaxation is observed in the kHz - MHz range. We explain this behavior by a charge density wave which develops in the ladders sub-system and produces a mode pinned at 12 cm$^{-1}$. With increasing Ca doping the mode shifts up in frequency and eventually disappears for $x=9$ because the dimensionality of the system crosses over from one to two dimensions, giving way to the superconducting ground state under pressure.

cond-mat.str-el↗