C. Langhammer
Max Planck Society
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Featured researches published by C. Langhammer.
Physica B-condensed Matter | 2001
Bernd G. Schmidt; Hidekazu Aoki; T. Cichorek; J. Custers; P. Gegenwart; M. Kohgi; M. Lang; C. Langhammer; Akira Ochiai; S. Paschen; F. Steglich; T. Suzuki; Peter Thalmeier; B. Wand; A. N. Yaresko
We investigate a new route to quasi-one-dimensional spin-chain systems which originates from the charge-ordering transition out of a homogeneous mixed-valence state. We present evidence that Yb4As3 and P, Sb doped mixed crystals are well described by this mechanism. Many thermodynamic and low-temperature transport heavy-fermion-like properties can be explained by the existence of low-lying quasi-one-dimensional spin excitations in the Yb3+-chains. The observation of soliton excitations in a transverse external field gives further support to the existence of spin chains in Yb4As3. We also present recent results on spin-glass behavior at very low temperature caused by the interchain coupling and disorder. In addition, the yet unexplained magnetotransport effects are discussed.
Physica C-superconductivity and Its Applications | 2000
F. Steglich; N. Sato; T. Tayama; T. Lühmann; C. Langhammer; P. Gegenwart; P. Hinze; C. Geibel; M. Lang; G. Sparn; W. Assmus
Abstract We discuss (i) UPd 2 Al 3 , for which local-moment antiferromagnetism ( T N = 14.3 K) coexists with heavy-fermion (HF) superconductivity below T c ≈ 2 K, (ii) the HF superconductor CeCu 2 Si 2 ( T c ≈ 0.6 K) which, for B > B c2 , shows pronounced “non-Fermi-liquid” (NFL) effects near a quantum-critical point at which the spin-density-wave “phase A” disappears, and (iii) the NFL CeNi 2 Ge 2 , a “clean-limit” HF superconductor below T c ≈ 0.1 K.
Physica C-superconductivity and Its Applications | 2004
P. Gegenwart; C. Langhammer; R. Helfrich; N. Oeschler; M. Lang; J. S. Kim; G.R. Stewart; F. Steglich
Abstract Non-Fermi liquid (NFL) behavior in the normal state of the heavy-fermion superconductor UBe 13 is studied by means of low-temperature measurements of the specific heat, C , and electrical resistivity, ρ , on a high-quality single crystal in magnetic fields up to 15.5 T. At B =0, unconventional superconductivity forms at T c =0.9 K out of an incoherent state, characterized by a large and strongly temperature dependent ρ ( T ). In the magnetic field interval 4 T ⩽B⩽10 T , ρ ( T ) follows a T 3/2 behavior for T c ( B )⩽ T ⩽1 K, while ρ is proportional to T at higher temperatures. Corresponding non-Fermi liquid behavior is observed in C / T as well and hints at a nearby antiferromagnetic (AF) quantum critical point (QCP) covered by the superconducting state. We speculate that the suppression of short-range AF correlations observed by thermal expansion and specific heat measurements below T L ≈0.7 K ( B =0) yields a field-induced QCP, T L →0, at B =4.5 T.
Physica B-condensed Matter | 2000
F. Steglich; P. Gegenwart; C. Geibel; P. Hinze; M. Lang; C. Langhammer; G. Sparn; O. Trovarelli
We discuss strong deviations from the signatures of a heavy Landau–Fermi liquid already at p=0 that are observed for normal (n)-state CeCu2Si2, for CeNi2Ge2 and YbRh2Si2. No quantum critical point could yet be established in the case of CeNi2Ge2. For this compound, preliminary investigations of the chemical phase diagram have revealed certain similarities to CeCu2Si2. The non-Fermi-liquid (NFL) effects observed for YbRh2Si2 suggest a quasi-2D system of magnetic fluctuations. We also discuss briefly disparities between resistivity and specific-heat results in all three compounds as well as the relationship between a NFL n-state and the occurrence of heavy-fermion superconductivity.
Physica B-condensed Matter | 2000
O. Trovarelli; C. Geibel; C. Langhammer; S. Mederle; P. Gegenwart; F.M. Grosche; M. Lang; G. Sparn; F. Steglich
Abstract Strong deviations from the properties of a heavy Landau Fermi-liquid already at ambient pressure and zero field are reported for single-crystalline YbRh2Si2. The low-temperature specific-heat coefficient and the electrical resistivity show a logarithmic and a linear temperature dependence, respectively, in more than a decade in temperature. This anomalous metallic state is ascribed to the proximity of a very nearby magnetic instability. Application of hydrostatic pressure induces anomalies in the electrical resistivity, indicating the onset of long-range magnetic order at a critical pressure p c ≃0.5 GPa , which is so far the lowest required for the onset of magnetism in Yb compounds.
Journal of Magnetism and Magnetic Materials | 1998
C. Langhammer; R. Helfrich; A. Bach; F. Kromer; M. Lang; T. Michels; M. Deppe; F. Steglich; G.R. Stewart
Abstract Low-temperature specific-heat and thermal-expansion measurements on high-quality single crystals of the heavy-fermion superconductor UBe 13 combined with literature results give evidence for the existence of two variants of this compound. They can be classified according to their superconducting transition temperatures into H-type (high T c : 0.85 ⩽ T c ⩽ 0.95K) and L-type (low T c : T c ≈ 0.75K) UBe 13 . A comparative investigation of both types reveals striking differences both above and below T c .
Physica B-condensed Matter | 1999
M. Lang; R. Helfrich; F. Kromer; C. Langhammer; F. Steglich; G.R. Stewart; Jeong Soo Kim
Abstract We report on low-temperature thermal-expansion and specific-heat measurements on single crystals of the heavy-fermion superconductor UBe13. A new “line of anomalies”, B*(T), in the superconducting state is discovered. The evolution of this feature is studied as a function of Th doping on several U1−xThxBe13 polycrystals (x⩽0.03), giving rise to a new “line of anomalies”, TL(x), in the T–x phase diagram. TL(x) hits the multi-critical point at xcr≈0.019 and merges smoothly into the Tc2 line that marks the lower of the two phase transitions at x>xcr. Our data strongly suggest that the anomaly at TL(x) represents the precursor of the transition at Tc2(x) for 0.019
Physica B-condensed Matter | 1997
P. Gegenwart; M. Lohmann; M. Lang; R. Helfrich; C. Langhammer; M. Köppen; Christoph Geibel; F. Steglich; W. Assmus
Abstract We present transport and thermodynamic measurements on two high-quality single crystals of CeCu2Si2 with different ground-state properties: crystal #1 (“A/S”-type) shows a competition of superconductivity and a presumably magnetically “A-phase”. The latter develops out of a low-temperature state where the resistivity varies as ϱ − ϱ0 = aT2 indicative of a coherent heavy Fermi liquid. By contrast, non-Fermi-liquid behavior is found for crystal #2 (“S”-type) which lacks the A-phase for B
LECTURES ON THE PHYSICS OF HIGHLY CORRELATED ELECTRON SYSTEMS VI: Sixth Training Course in the Physics of Correlated Electron Systems and High-Tc Superconductors | 2002
H. Q. Yuan; F. M. Grosche; Wilder Carrillo-Cabrera; S. Paschen; C. Langhammer; G. Sparn; M. Baenitz; Yu. Grin; F. Steglich
The newly‐discovered clathrates Ba6Ge25 and its relatives consist of a rigidG ermanium framework, in which Barium or other metallic atoms are trapped. These ‘guest’ atoms can ‘rattle’ freely in the oversized cages at high temperature. In Ba6Ge25, some of Ba atoms aref rozen into the split positions randomly below 200 K. This structural phase transition results in a two‐step resistivity jump from metallic behavior to semiconductor‐like behavior characterized by a mean free path of 3A. Surprisingly, a BCS‐like superconducting transition occursa t Tc ≈ 0.24 K in such a ‘bad metal’. By applying hydrostatic pressure, the structural phase transition is suppressed but Tc increases drastically. Tc reaches a maximum value of 3.85K at a critical pressure pc ≈ 2.8 GPa, where the structural distortion is completely suppressed and the system exhibits metallic behavior over the whole temperature range. Higher pressures lead to a slight decrease of Tc. On replacing 1/3 of the Ba atoms with Na, no structural phase transi...
Archive | 1999
M. Lang; P. Gegenwart; R. Helfrich; M. Köppen; F. Kromer; C. Langhammer; C. Geibel; F. Steglich; J. S. Kim; G. R. Stewart
Tetragonal CeCu2Si2 [1] and cubic UBe13 [2] belong to the class of strongly correlated (electron materials where a periodic lattice of partially filled f shells is embedded in a metallic environment. Below a characteristic “Kondo temperature” Tk, typically of the order of 10K, quasiparticles composed of both local f degrees of freedom and itinerant conduction-electron degrees of freedom form. As has been inferred from the giant Sommerfeld coefficient r of about 1 J/K2mole, the huge effective quasiparticle masses m· (100–1000 mel) are governed by the f degrees of freedom. The discontinuity of the specific heat at Tc which was found to scale with the large γ [1, 2] proved that superconductivity is, indeed, formed by those heavy fermions (HF).