M. Lang

Goethe-Universität Frankfurt am Main, Frankfurt, Hesse, Germany

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Publications (211)463.77 Total impact

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    ABSTRACT: The effect of hydrostatic pressure and partial Na substitution on the normal-state properties and the superconducting transition temperature ($T_c$) of K$_{1-x}$Na$_x$Fe$_2$As$_2$ single crystals were investigated. It was found that a partial Na substitution leads to a deviation from the standard $T^2$ Fermi-liquid behavior in the temperature dependence of the normal-state resistivity. It was demonstrated that non-Fermi liquid like behavior of the resistivity for K$_{1-x}$Na$_{x}$Fe$_2$As$_2$ and some KFe$_2$As$_2$ samples can be explained by disorder effect in the multiband system with rather different quasiparticle effective masses. Concerning the superconducting state our data support the presence of a shallow minimum around 2 GPa in the pressure dependence of $T_c$ for stoichiometric KFe$_2$As$_2$. The analysis of $T_c$ in the K$_{1-x}$Na$_{x}$Fe$_2$As$_2$ at pressures below 1.5 GPa showed, that the reduction of $T_c$ with Na substitution follows the Abrikosov-Gor'kov law with the critical temperature $T_{c0}$ of the clean system (without pair-breaking) which linearly depends on the pressure. Our observations, also, suggest that $T_c$ of K$_{1-x}$Na$_x$Fe$_2$As$_2$ is nearly independent of the lattice compression produced by the Na substitution. Further, we theoretically analyzed the behavior of the band structure under pressure within the generalized gradient approximation (GGA). A qualitative agreement between the calculated and the recently in de Haas-van Alphen experiments [T. Terashima et al., Phys.Rev.B89, 134520(2014)] measured pressure dependencies of the Fermi-surface cross-sections has been found. These calculations, also, indicate that the observed minimum around 2~GPa in the pressure dependence of $T_c$ may occur without a change of the pairing symmetry.
    09/2014;
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    ABSTRACT: The spin excitations in the spin-liquid phase of the anisotropic triangular lattice quantum antiferromagnet Cs$_2$CuCl$_4$ have been shown to propagate dominantly along the crystallographic $b$-axis [L. Balents, Nature (London) 464, 199 (2010)]. To test this dimensional reduction scenario, we have performed ultrasound experiments in the spin-liquid phase of Cs$_2$CuCl$_4$ probing the elastic constant $c_{22}$ and the sound attenuation along the $b$-axis as a function of an external magnetic field along the $a$-axis. We show that our data can be quantitatively explained within the framework of a nearest neighbor spin-1/2 Heisenberg chain, where fermions are introduced via the Jordan-Wigner transformation and the spin-phonon interaction arises from the usual exchange-striction mechanism.
    09/2014;
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    ABSTRACT: In the present study we report the synthesis of some novel nitronyl nitroxide biradical systems 1–4c with various π-bridges between the radical centres. UV-Vis, IR, EPR and X-ray diffraction studies, along with MS and NMR data where appropriate, are described. Magnetic measurements revealed that the biradicals 1c, 3c and 4c exhibit a moderately strong antiferromagnetic intra-molecular exchange, whereas nitroxide 2c shows a significantly higher exchange coupling, which can only be explained by the presence of strong inter-molecular interactions. From DFT calculations performed on the basis of the X-ray crystal structure of compound 4c, a theoretical value of the intra-dimer coupling constant Jintra = −8.6 K is obtained. Direct proof also for inter-molecular arrangement with Jinter −2 K was provided by the low temperature AC studies of biradical 4c. According to the magnetic characterization, the nitronyl biradical 4c is a promising candidate for a purely organic-based low-dimensional quantum magnet.
    J. Mater. Chem. C. 07/2014;
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    ABSTRACT: A detailed investigation of the out-of-plane electrical properties of charge-ordered alpha-(BEDT-TTF)2I3 reveals clear indications for relaxor ferroelectricity. Similar to multiferroic kappa-(BEDT-TTF)2Cu[N(CN)2]Cl, the polar order in this material is ascribed to the occurrence of bond- and site-centered charge order. Cluster-like polar order is observed deep in the charge-ordered state, which is explained by the existence of polar and nonpolar stacks of the organic molecules in this material, preventing long-range ferroelectricity. The results are discussed in relation to the formation or absence of electronic polar order in related charge-transfer salts.
    07/2014;
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    ABSTRACT: The coupling of magnetic and electronic degrees of freedom to the crystal lattice in the ferromagnetic semimetal EuB$_6$, which exhibits a complex ferromagnetic order and a colossal magnetoresistance (CMR) effect, %, very likely involving magnetic polarons, is studied by high-resolution thermal expansion and magnetostriction experiments. EuB$_6$ may be viewed as a model system, where pure magnetism-tuned transport and the response of the crystal lattice can be studied in a comparatively simple environment,i.e., not influenced by strong crystal-electric field effects and Jahn-Teller distortions. We find a very large lattice response, quantified by (i) the magnetic Gr\"uneisen parameter, (ii) the spontaneous strain when entering the ferromagnetic region and (iii) the magnetostriction in the paramagnetic temperature regime. Our analysis reveals that a significant part of the lattice effects originates in the magnetically-driven delocalization of charge carriers, consistent with the scenario of percolating magnetic polarons. A strong effect of the formation and dynamics of local magnetic clusters on the lattice parameters is suggested to be a general feature of CMR materials.
    Physical review letters. 04/2014; 113(6).
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    ABSTRACT: We present results of ultrasonic measurements on a single crystal of the distorted diamond-chain compound azurite Cu$_3$(CO$_3$)$_2$(OH)$_2$. Pronounced elastic anomalies are observed in the temperature dependence of the longitudinal elastic mode $c_{22}$ which can be assigned to the relevant magnetic interactions in the system and their couplings to the lattice degrees of freedom. From a quantitative analysis of the magnetic contribution to $c_{22}$ the magneto-elastic coupling $G$ = $\partial J_2$/$\partial \epsilon_b$ can be determined, where $J_2$ is the intra-dimer coupling constant and $\epsilon_b$ the strain along the intra-chain $b$ axis. We find an exceptionally large coupling constant of $|G| \sim ($3650 $\pm$ 150) K highlighting an extraordinarily strong sensitivity of $J_2$ against changes of the $b$-axis lattice parameter. These results are complemented by measurements of the hydrostatic pressure dependence of $J_2$ by means of thermal expansion and magnetic susceptibility measurements performed both at ambient and finite hydrostatic pressure. We propose that a structural peculiarity of this compound, in which Cu$_2$O$_6$ dimer units are incorporated in an unusually stretched manner, is responsible for the anomalously large magneto-elastic coupling.
    02/2014;
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    ABSTRACT: Depending on the crystal growth conditions, an orthorhombic (O-type) or a tetragonal (T-type) structure can be found in the solid solution Cs2CuCl4-xBrx (0 < x < 4). Here we present measurements of the temperature-dependent magnetic susceptibility and isothermal magnetization on the T-type compounds x = 1.6 and 1.8 and compare these results with the magnetic properties recently derived for the O-type variant by Cong et al., Phys. Rev. B 83, 064425 (2011). The systems were found to exhibit quite dissimilar magnetic properties which can be assigned to differences in the Cu coordination in these two structural variants. Whereas the tetragonal compounds can be classified as quasi-2D ferromagnets, characterized by ferromagnetic layers with a weak antiferromagnetic inter-layer coupling, the orthorhombic materials, notably the border compounds x = 0 and 4, are model systems for frustrated 2D Heisenberg antiferromagnets
    11/2013;
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    ABSTRACT: The recently proposed multiferroic state of the charge-transfer salt {\kappa}-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl [P. Lunkenheimer et al., Nature Mater., vol. 11, pp. 755-758, Sept. 2012] has been studied by dc-conductivity, magnetic susceptibility and measurements of the dielectric constant on various, differently prepared single crystals. In the majority of crystals we confirm the existence of an order-disorder-type ferroelectric state which coincides with antiferromagnetic order. This phenomenology rules out scenarios which consider an inhomogeneous, short-range-ordered ferroelectric state. Measurements of the dielectric constant and the magnetic susceptibility on the same crystals reveal that both transitions lie very close to each other or even collapse, indicating that both types of order are intimately coupled to each other. We address issues of the frequency dependence of the dielectric constant {\epsilon}' and the dielectric loss {\epsilon}'' and discuss sample-to-sample variations.
    11/2013;
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    ABSTRACT: The organic charge-transfer salt EtMe$_3$P[Pd(dmit)$_2$]$_2$ is a quasi-two-dimensional Mott insulator with localized spins $S$ = 1/2 residing on a distorted triangular lattice. Here we report measurements of the uniaxial thermal expansion coefficients $\alpha_i$ along the in-plane i = $a$- and c-axis as well as along the out-of-plane b-axis for temperatures 1.4\,K $\leq T \leq$ 200\,K. Particular attention is paid to the lattice effects around the phase transition at $T_{VBS}$ = 25\,K into a low-temperature valence-bond-solid phase and the paramagnetic regime above where effects of short-range antiferromagnetic correlations can be expected. The salient results of our study include (i) the observation of strongly anisotropic lattice distortions accompanying the formation of the valence-bond-solid, and (ii) a distinct maximum in the thermal expansion coefficients in the paramagnetic regime around 40\,K. Our results demonstrate that upon cooling through $T_{VBS}$ the in-plane c-axis, along which the valence bonds form, contracts while the second in-plane a-axis elongates by the same relative amount. Surprisingly, the dominant effect is observed for the out-of-plane b-axis which shrinks significantly upon cooling through $T_{VBS}$. The pronounced anomaly in $\alpha_i$ around 40\,K is attributed to short-range magnetic correlations. It is argued that the position of this maximum, relative to that in the magnetic susceptibility around 70\,K, speaks in favor of a more anisotropic triangular-lattice scenario for this compound than previously thought.
    09/2013; 89(4).
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    ABSTRACT: We present a detailed low-temperature investigation of the statics and dynamics of the anions and methyl groups in the organic conductors (TMTSF)2PF6 and (TMTSF)2AsF6 (TMTSF: tetramethyl-tetraselenafulvalene). The 4 K neutron-scattering structure refinement of the fully deuterated (TMTSF)2PF6-D12 salt allows locating precisely the methyl groups at 4 K. This structure is compared to the one of the fully hydrogenated (TMTSF)2PF6-H12 salt previously determined at the same temperature. Surprisingly, it is found that deuteration corresponds to the application of a negative pressure of 5×102 MPa to the H12 salt. Accurate measurements of the Bragg intensity show anomalous thermal variations at low temperature both in the deuterated PF6 and AsF6 salts. Two different thermal behaviors have been distinguished. Small Bragg-angle measurements reflect the presence of low-frequency modes at characteristic energies θE = 8.3 K and θE = 6.7 K for the PF6-D12 and AsF6-D12 salts, respectively. These modes correspond to the low-temperature methyl group motion. Large Bragg-angle measurements evidence an unexpected structural change around 55 K, which probably corresponds to the linkage of the anions to the methyl groups via the formation of F…D-CD2 bonds observed in the 4 K structural refinement. Finally we show that the thermal expansion coefficient of (TMTSF)2PF6 is dominated by the librational motion of the PF6 units. We quantitatively analyze the low-temperature variation of the lattice expansion via the contribution of Einstein oscillators, which allows us to determine for the first time the characteristic frequency of the PF6 librations: θE ≈ 50 K and θE = 76 K for the PF6-D12 and PF6-H12 salts, respectively.
    Physical review. B, Condensed matter 07/2013; 88(2). · 3.77 Impact Factor
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    ABSTRACT: Materials close to a quantum-critical point – a zero-temperature phase transition – exhibit anomalous thermodynamic properties also at finite temperatures. Close to a magnetic field-induced quantum-critical point, for example, the finite-temperature entropy S_T shows strong variations upon varying the magnetic field. Here we discuss the possibility to use this accumulation of entropy around a field-induced quantum-critical point for realizing an efficient magnetic cooling. Our proof-of-principle demonstration is based on measurements and theoretical calculations of the magnetocaloric properties of low-dimensional spin-1/2 antiferromagnets close to their field-induced quantum-critical points. We present results of the magnetocaloric effect Γ_B = T−1(∂T/∂B)_S ≈ const as a function of both field and temperature in the vicinity of the quantum-critical point and discuss various performance characteristics, such as range of operation, efficiency and hold time. These figures are compared with those of a state-of-the-art paramagnetic coolant.
    physica status solidi (b) 03/2013; 250(3):457-463. · 1.49 Impact Factor
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    ABSTRACT: We report on measurements of the magnetic susceptibility and electrical resistance under He-gas pressure on single crystals of Ca(Fe1−xCox)2As2. We find that for properly heat-treated crystals with modest Co concentration, x = 0.028, the salient ground states associated with iron-arsenide superconductors, i.e., orthorhombic/antiferromagnetic (o/afm), superconducting, and nonmagnetic collapsed-tetragonal (cT) states can be accessed all in one sample with reasonably small and truly hydrostatic pressure. This is possible owing to the extreme sensitivity of the o/afm (for T≤ Ts,N) and superconducting (T≤Tc) states against variation of pressure, disclosing pressure coefficients of dTs,N/dP=−(1100±50) K/GPa and dTc/dP=−(60±3) K/GPa, respectively. Systematic investigations of the various phase transitions and ground states via pressure tuning revealed no coexistence of bulk superconductivity (sc) with the o/afm state which we link to the strongly first-order character of the corresponding structural/magnetic transition in this compound. Our results, together with literature results, indicate that preserving fluctuations associated with the o/afm transition to low enough temperatures is vital for sc to form.
    Physical review. B, Condensed matter 12/2012; 86(22). · 3.77 Impact Factor
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    ABSTRACT: We report measurements and theoretical calculations of the magnetocaloric properties of low-dimensional spin-1/2 antiferromagnets close to their magnetic field-induced quantum critical points. We demonstrate that the accumulation of entropy around the quantum critical point, giving rise to a critically enhanced magnetocaloric effect ΓB, can be used for realizing a very efficient low-temperature magnetic cooling.
    Journal of Physics Conference Series 12/2012; 400(3):032043.
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    ABSTRACT: We present a structural investigation of the organic charge-transfer salt superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br at a temperature of 90 K using high energy (90 keV) powder x-ray diffraction. With this approach we access the scattering profile over a wide range in Q-space (from ~ 0.3 to 19 Å-1). Overall, the observed scattering intensity is understood as a superposition of Bragg diffraction from a crystalline phase superimposed onto a large quasi-amorphous signal, indicative of short range order effects in our powder material. We discuss implications of these findings regarding the structural properties of this material.
    Journal of Physics Conference Series 12/2012; 391(1):2118-.
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    ABSTRACT: Measurements of the longitudinal elastic constants c11, c22 and c33 and the ultrasonic attenuation α of the quasi-two-dimensional triangular-lattice spin-1/2 Heisenberg antiferromagnet Cs2CuCl4 reveal distinct anomalies near the B-induced quantum-critical point (QCP). These anomalies are particularly strongly pronounced in the ultrasonic attenuation. In isothermal field sweeps performed at low temperatures 0.027 K <= T <= 0.3 K around the saturation field Bs, the ultrasonic attenuation of all three modes shows a pronounced double-peak structure, indicating two anomalies of different origin. Upon cooling, however, both features merge suggesting a coincidence at the QCP. While one peak, which can be attributed to the ordering temperature TN(B), becomes critically enhanced upon approaching the QCP, the other one reduces in size and narrows upon cooling. The latter effect has been tentatively assigned to the material's spin-liquid features which precede the long-range antiferromagnetic ordering.
    Journal of Physics Conference Series 12/2012; 400(3):2113-.
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    ABSTRACT: We report a Muon Spin Rotation (μSR) study on single crystalline azurite. The measurements revealed clear evidence for long range magnetic order below the Néel temperature at TN = 1.9 K. The spectra in the magnetically ordered regime were fitted with three different muon sites, which can be related to muons close to Cu2+ monomer and dimer ions, respectively. The spontaneous muon spin rotation frequencies revealed the development of a magnetic order parameter. Below 0.4 K, transverse damping data indicated a further change in the local magnetic environment of the Cu2+ monomer ions.
    Journal of Physics Conference Series 12/2012; 391(1):2145-.
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    ABSTRACT: Multiferroics, showing simultaneous ordering of electrical and magnetic degrees of freedom, are remarkable materials as seen from both the academic and technological points of view. A prominent mechanism of multiferroicity is the spin-driven ferroelectricity, often found in frustrated antiferromagnets with helical spin order. There, as for conventional ferroelectrics, the electrical dipoles arise from an off-centre displacement of ions. However, recently a different mechanism, namely purely electronic ferroelectricity, where charge order breaks inversion symmetry, has attracted considerable interest. Here we provide evidence for ferroelectricity, accompanied by antiferromagnetic spin order, in a two-dimensional organic charge-transfer salt, thus representing a new class of multiferroics. We propose a charge-order-driven mechanism leading to electronic ferroelectricity in this material. Quite unexpectedly for electronic ferroelectrics, dipolar and spin order arise nearly simultaneously. This can be ascribed to the loss of spin frustration induced by the ferroelectric ordering. Hence, here the spin order is driven by the ferroelectricity, in marked contrast to the spin-driven ferroelectricity in helical magnets.
    Nature Material 08/2012; 11(9):755-8. · 35.75 Impact Factor
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    ABSTRACT: We report on the realization of a capacitive dilatometer, designed for high-resolution measurements of length changes of a material for temperatures 1.4 K ≤ T ≤ 300 K and hydrostatic pressure P ≤ 250 MPa. Helium ((4)He) is used as a pressure-transmitting medium, ensuring hydrostatic-pressure conditions. Special emphasis has been given to guarantee, to a good approximation, constant-pressure conditions during temperature sweeps. The performance of the dilatometer is demonstrated by measurements of the coefficient of thermal expansion at pressures P ~/= 0.1 MPa (ambient pressure) and 104 MPa on a single crystal of azurite, Cu(3)(CO(3))(2)(OH)(2), a quasi-one-dimensional spin S = 1/2 Heisenberg antiferromagnet. The results indicate a strong effect of pressure on the magnetic interactions in this system.
    The Review of scientific instruments 08/2012; 83(8):085111. · 1.52 Impact Factor
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    ABSTRACT: Three new ternary copper phosphides, Sr(2)Cu(6)P(5), Eu(2)Cu(6)P(5), and EuCu(4)P(3), have been synthesized from the elements in evacuated silica capsules. Eu(2)Cu(6)P(5) and Sr(2)Cu(6)P(5) adopt the Ca(2)Cu(6)P(5)-type structure, while EuCu(4)P(3) is isostructural to BaMg(4)Si(3) and still remains the only representative of this structure type among the ternary Cu pnictides. All three materials show metallic conductivity in the temperature range 2 K ≤ T ≤ 290 K, with no indication for superconductivity. For Eu(2)Cu(6)P(5) and EuCu(4)P(3), long-range magnetic order was observed, governed by 4f local moments on the Eu atoms with predominant ferromagnetic interactions. While Eu(2)Cu(6)P(5) shows a single ferromagnetic transition at T(C) = 34 K, the magnetic behavior of EuCu(4)P(3) is more complex, giving rise to three consecutive magnetic phase transitions at 70, 43, and 18 K.
    Inorganic Chemistry 07/2012; 51(16):8948-55. · 4.59 Impact Factor
  • Physical review. B, Condensed matter 07/2012; 86(1). · 3.77 Impact Factor

Publication Stats

2k Citations
463.77 Total Impact Points

Institutions

  • 2002–2014
    • Goethe-Universität Frankfurt am Main
      • • Institute of Biophysics
      • • Institute of Theoretical Physics
      • • Institut für Anorganische und Analytische Chemie
      Frankfurt, Hesse, Germany
  • 2013
    • RIKEN
      • Condensed Molecular Materials Laboratory
      Вако, Saitama, Japan
  • 2012
    • Universität Augsburg
      Augsberg, Bavaria, Germany
  • 2001–2012
    • University Hospital Frankfurt
      Frankfurt, Hesse, Germany
  • 2011
    • University of Amsterdam
      • Institute for Theoretical Physics
      Amsterdamo, North Holland, Netherlands
  • 1987–2007
    • Darmstadt University of Applied Sciences
      Darmstadt, Hesse, Germany
  • 2004
    • University of Cologne
      • Institute for Theoretical Physics
      Köln, North Rhine-Westphalia, Germany
  • 1997–2003
    • Max Planck Institute for Chemical Physics of Solids
      Dresden, Saxony, Germany
  • 1992–2003
    • Tohoku University
      • Institute for Materials Research
      Sendai, Kagoshima-ken, Japan
  • 1999
    • Japan Advanced Institute of Science and Technology
      KMQ, Ishikawa, Japan
  • 1998
    • Niigata University
      • Department of Material Science and Technology
      Niahi-niigata, Niigata, Japan