P. Haas

Universität Stuttgart, Stuttgart, Baden-Württemberg, Germany

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Publications (22)57.76 Total impact

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    ABSTRACT: Evidence of the superconducting energy gap in the optical spectra of αt-(BEDT-TTF)2I3
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    ABSTRACT: Zeeman spectroscopy of hydrogen-like shallow impurities in elemental semiconductors has revealed a splitting of the degenerated excited states with magnetic quantum number m = +-1 at low magnetic fields (B<3T). The frequency shifts of impurity absorption transitions originating from the unsplitted ground state depend linearly on the magnetic field. Spectral analysis of the Si:Bi laser emission lines (originating from intracenter transitions 2p+- to 1s(E) and 2p+- to 1s(T2) at a temperature of ~5K, indicate a deviation of the magnetic field dependence from the linear Zeeman splitting at low B < 0.4-0.6 T which is not related to resonator mode features. In order to investigate this further absorption spectroscopy of the 1s(E)-2p+- and 1s(T2)-2p+- transitions has been carried out at temperatures around 90 K, i.e. under conditions of thermal population of the orbit-splitted excited bismuth states.
    HMF 20; 01/2012
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    ABSTRACT: Noncascade relaxation of photoexcited electrons on ionized donor centers has been observed in silicon doped by arsenic (Si:As) at low temperatures. Emission spectra of the Si:As terahertz intracenter laser give evidence of specific channels for the electron relaxation through low-lying donor states. The dominating relaxation channels strongly depend on the initial energy distribution of the nonequilibrium carriers. A relaxation step may exceed not only the energy gap to an adjacent lower-lying donor level but also the characteristic energy step as set by the energy and momentum conservation requirements for intravalley acoustic phonons.
    Physical review. B, Condensed matter 10/2008; 78(16). DOI:10.1103/PhysRevB.78.165201 · 3.66 Impact Factor
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    ABSTRACT: Using far-infrared spectroscopy, we have studied the magnetic field and temperature dependence of the spin gap modes in the chains of Sr14Cu24O41 . Two triplet modes T1 and T2 were found in the center of the Brillouin zone at Delta1=9.65meV and Delta2=10.86meV in zero magnetic field. The T1 mode was excited when the electric field vector E of the light was polarized along the b axis (perpendicular to the planes of chains and ladders) and T2 was excited for E||a (perpendicular to the chains and along the rungs). Up to the maximum magnetic field of 18T , applied along the chains, the electron g factors of these two modes were similar, g1c=2.049 and g2c=2.044 . Full linewidth at half maximum for both modes was 1cm-1 (0.12meV) at 4K and increased with T . The temperature dependence of mode energies and line intensities was in agreement with the inelastic neutron scattering results from two groups [M. Matsuda , Phys. Rev. B 59, 1060 (1999); L. P. Regnault , ibid 59, 1055 (1999)]. The T1 mode has not been observed by inelastic neutron scattering in the points of the k space equivalent to the center of the Brillouin zone. Our study indicates that the zone structure model of magnetic excitations of Sr14Cu24O41 must be modified to include a triplet mode at 9.65meV in the center of the magnetic Brillouin zone.
    Physical review. B, Condensed matter 10/2007; 76(13). DOI:10.1103/PhysRevB.76.134418 · 3.66 Impact Factor
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    ABSTRACT: We studied using far-infrared spectroscopy, magnetic field and temperature dependence of the spin gap modes in the chains of Sr14Cu24O41. Two triplet modes T1 and T2 were found in the center of the Brillouin zone at E1=77.8 cm-1 (9.65 meV) and E2=87.7 cm-1 (10.86 meV) in zero magnetic field. Both excitations are electric dipole active modes. T1 mode is excited when the light E-vector is along the b crystallographic axis and T2 is excited when the light E-vector is along the a-axis, both perpendicular to the chain direction. The selection rules of the transitions are compatible with dynamic Dzyaloshinskii-Moria interaction mechanism. Up to the field of 18T the electron g-factors of two modes are similar, g1c=2.049 and g2c=2.055 with magnetic field applied along the chains. Linewidth of both modes is 1 cm-1 (0.12 meV) at 4K and increases with T. The temperature dependence of the mode energies is in agreement with the inelastic neutron scattering (INS) results from other groups. However the T1 mode has not been observed by INS. The zone structure model of magnetic excitations in the chains is not complete and must include a triplet mode at 9.65 meV in the center of the magnetic Brillouin zone.
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    ABSTRACT: The optical properties of the intermediate valence semiconductor YbB12 are studied at frequencies 8cm-1-10000cm-1 (quantum energies 1 meV to 1.2 eV) and temperatures 5 K-300 K. Microscopic parameters of charge carriers are determined: effective mass, mobility, relaxation rate, relaxation time, plasma frequency. A strong decrease of the relaxation rate is observed when cooled down to 70 K, indicating that scattering is mediated mainly by phonons and not by the Kondo effect. Below the coherence temperature of T*=70K a Drude-like response of renormalized quasiparticles is observed with a scattering rate gamma following the Fermi liquid behavior gamma∝T2 , an effective mass m*&ap;34m0 , and an enhanced relaxation time tau=4×10-13s , implying that coherent effects play a crucial role in formation of the ground state in YbB12 . At T=5K an absorption peak is found at 2.7 meV whose origin can be ascribed to an exciton-polaronic bound state.
    Physical Review B 01/2006; 73(4). DOI:10.1103/PhysRevB.73.045207 · 3.74 Impact Factor
  • Toomas Room · P. Haas · J. Akimitsu
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    ABSTRACT: Sr14Cu24O41 consists of two-leg ladders and chains of copper ions. Planes of ladders and chains alternate in b axis direction. Oxygen holes, present in stochiometric Sr14Cu24O41, and Cu^2+ spins in the chain are coupled into Zhang-Rice spin singlet. Chain Cu^2+ spins that are separated by Zhang-Rice singlets form spin dimers. It is known from inelastic neutron scattering studies that because of inter-chain coupling there are two weakly dispersing triplet modes in the chain layer. We found that these triplet modes are optically active. Light is absorbed at 77.80 and 87.54,m-1 in zero magnetic field and T=4,. When magnetic field is applied along the c axis both resonances split. The triplet state g-factor is 2.048±0.002 and 2.063±0.006 for the lower and upper triplet. The results on the absorption line intensities and frequencies as a function of T, B0, and light polarization are presented.
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    ABSTRACT: The charge response of the ladders in Sr14-xCaxCu24O41 is characterized by dc resistivity, low frequency dielectric, and optical spectroscopy in all three crystallographic directions. The collective charge-density wave screened mode is observed in the direction of the rungs for x=0 , 3, and 6, in addition to the mode along the legs. For x=8 and 9, the charge-density-wave response along the rungs fully vanishes, while the one along the legs persists. The transport perpendicular to the planes is always dominated by hopping.
    Physical review. B, Condensed matter 01/2005; 71(1). DOI:10.1103/PhysRevB.71.012508 · 3.66 Impact Factor
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    ABSTRACT: By angle-resolved photoemission spectroscopy and polarization-dependent infrared reflectivity measurements the electronic and vibrational properties of the low-dimensional perovskites Sr1-yLayNbO3.5-x were studied along different crystal directions. The electronic behavior strongly depends on the oxygen and La content, including quasi-one-dimensional metallic and ferroelectric insulating behavior. An extremely small energy gap at the Fermi level is found for SrNbO3.41 and SrNbO3.45 along the conducting direction at low temperature, suggestive for a Peierls-type instability. Despite the similar nominal carrier density, for Sr0.8La0.2NbO3.50 the quasi-one-dimensional metallic character is suppressed and no gap opening is observed, which can be explained by differences in the crystal structure. Electron-phonon interaction appears to play an important role in this series of compounds.
    Physical Review B 12/2004; 70(24). DOI:10.1103/PhysRevB.70.245123 · 3.74 Impact Factor
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    ABSTRACT: The charge response in the spin chain and/or ladder compound Sr14-xCaxCu24O41 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 interladder 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.
    Physical Review Letters 07/2003; 90(25 Pt 1):257002. DOI:10.1103/PhysRevLett.90.257002 · 7.51 Impact Factor
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    ABSTRACT: We show that the spin chain/ladder compound La3Sr3Ca8Cu24O41 is an insulator with hopping transport along the chains. In the temperature range 35 280 K, dc conductivity sigmadc(T) follows Mott's law of variable-range hopping conduction; the frequency dependence has the form sigma(nu,T)=sigmadc(T)+A(T)nus, where s&ap;1. The conduction mechanism changes from variable-range hopping to nearest-neighbor hopping around Tc=300 K. The chain array thus behaves as a one-dimensional disordered system. Disorder is due to random structural distortions of chains induced by irregular coordination of the La/Sr/Ca ions.
    Physical review. B, Condensed matter 05/2003; 67(18). DOI:10.1103/PhysRevB.67.184521 · 3.66 Impact Factor
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    ABSTRACT: The far-infrared in-plane reflectivity of the organic superconductor alpha(t)-(BEDT-TTF)(2)I-3 is measured down to frequencies of 7 cm(-1); the optical conductivity is obtained by a Kramers-Kronig procedure. In the superconducting state below T-c = 8 K an energy gap opens at 25 cm(-1) which corresponds to 2Delta/k(B)T(c) = 4.4. Even well below T-c, a considerable absorption remains for hw < 2Delta; different posibilities are discussed to explain this behaviour.
    Synthetic Metals 04/2003; 137(1). DOI:10.1016/S0379-6779(02)01008-1 · 2.22 Impact Factor
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    ABSTRACT: Optical properties of an S=1/2 antiferromagnetic spin-chain system β-CuSb2O6 are measured in the frequency range 50–10 000 cm-1 at temperatures of 5–300 K for the electric field polarized along the [010] and [001] crystallographic directions. The number of observed polar phonon modes at low temperature is in agreement with the factor-group analysis for the monoclinic form of β-CuSb2O6. The mode parameters are determined by using the dispersion analysis of the reflectivity spectra at different temperatures. The parameters of several phonon modes show anomalies around 60 K, at the temperature where the one-dimensional magnetic correlations in the copper chains are largest. This may indicate a finite interaction between the phonon and electron subsystems in β-CuSb2O6. The theoretical group analysis of the possible rutile low-symmetry phases is performed within the framework of Landau’s theory of phase transitions. The trirutile-rutile phase transformation can be uniquely described by an order parameter which transforms according to the two-dimensional irreducible representation Λ4 with k=1/3 b3 of the Brillouin zone of the proper rutile unit cell only. Optical modes assignment of β-CuSb2O6 performed with use of the aristotype rutile structure allows one to reproduce the tentative picture of the α-CuSb2O6 phonon dispersion curves.
    Physical review. B, Condensed matter 04/2003; 67(13). DOI:10.1103/PhysRevB.67.134433 · 3.66 Impact Factor
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    ABSTRACT: Resistivity, optical, and angle-resolved photoemission experiments reveal unusual one-dimensional electronic properties of highly anisotropic ${\mathrm{S}\mathrm{r}\mathrm{N}\mathrm{b}\mathrm{O}}_{3.41}$. 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 ${\mathrm{S}\mathrm{r}\mathrm{N}\mathrm{b}\mathrm{O}}_{3.41}$.
    Physical Review Letters 01/2003; 89(23):236403. DOI:10.1103/PhysRevLett.89.236403 · 7.51 Impact Factor
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    ABSTRACT: The European Physical Society (EPS) is a not for profit association whose members include 41 National Physical Societies in Europe, individuals from all fields of physics, and European research institutions. As a learned society, the EPS engages in activities that strengthen ties among the physicists in Europe. As a federation of National Physical Societies, the EPS studies issues of concern to all European countries relating to physics research, science policy and education. Visit www.eps.org
    EPL (Europhysics Letters) 09/2002; 59(5):774-778. DOI:10.1209/epl/i2002-00193-6 · 2.27 Impact Factor
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    ABSTRACT: The electrodynamic response of the spin-ladder compound Sr14-xCaxCu24O41(x=0,3,9) has been studied from radio frequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around 12 cm-1 in Sr14Cu24O41 for the radiation polarized along the chains/ladders (E||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 and produces a phason mode pinned at 12 cm-1. With increasing x 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.
    Physical review. B, Condensed matter 08/2002; 66(6). DOI:10.1103/PhysRevB.66.060508 · 3.66 Impact Factor
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    ABSTRACT: The infrared spectra of the one-dimensional antiferromagnet LiCuVO4 are measured in the frequency range from 10 cm-1 to 10 000 cm-1 and at temperatures from 2 K to 300 K, for the electric field vector E of the radiation polarized either along the a- or along the b-crystallographic directions. For each polarization six infrared active phonon modes are observed in accordance with factor group analysis of the crystal structure of LiCuVO4. The theoretical group analysis of the possible spinel low-symmetry phases is performed within the framework of Landau's theory of phase transitions. The parameters of several phonon lines show noticeable anomalies around 150 K where the magnetic correlations appear in the copper chains, which may indicate a finite interaction between the phonon and the magnon subsystems in LiCuVO4.
    Physics of Condensed Matter 09/2001; 23(4):427-435. DOI:10.1007/s100510170033 · 1.46 Impact Factor
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    ABSTRACT: Far-infrared reflectivity studies on the polycrystalline intermetallic compound MgB2 with a superconducting transition temperature T c = 39 K were performed at temperatures 20 K to 300 K. We observe a significant raise of the superconducting-to-normal state reflectivity ratio below 70 cm -1 , with a maximum at about 25-30 cm -1 , which gives a lower estimate of the superconducting gap of 2Δ(0) ≈ 3-4 meV.
    Physics of Condensed Matter 05/2001; 21(2):159-161. DOI:10.1007/s100510170191 · 1.46 Impact Factor