J. P. Castellan

Karlsruhe Institute of Technology, Carlsruhe, Baden-Württemberg, Germany

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Publications (37)109.34 Total impact

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    ABSTRACT: A charge-density wave (CDW) state has a broken symmetry described by a complex order parameter with an amplitude and a phase. The conventional view, based on clean, weak-coupling systems, is that a finite amplitude and long-range phase coherence set in simultaneously at the CDW transition temperature Tcdw. Here we investigate, using photoemission, X-ray scattering and scanning tunnelling microscopy, the canonical CDW compound 2H-NbSe2 intercalated with Mn and Co, and show that the conventional view is untenable. We find that, either at high temperature or at large intercalation, CDW order becomes short-ranged with a well-defined amplitude, which has impacts on the electronic dispersion, giving rise to an energy gap. The phase transition at Tcdw marks the onset of long-range order with global phase coherence, leading to sharp electronic excitations. Our observations emphasize the importance of phase fluctuations in strongly coupled CDW systems and provide insights into the significance of phase incoherence in 'pseudogap' states.
    Nature Communications 02/2015; 6:6313. DOI:10.1038/ncomms7313 · 10.74 Impact Factor
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    ABSTRACT: The magnetic excitations in the cuprate superconductors might be essential for an understanding of high-temperature superconductivity. In these cuprate superconductors the magnetic excitation spectrum resembles an hour-glass and certain resonant magnetic excitations within are believed to be connected to the pairing mechanism, which is corroborated by the observation of a universal linear scaling of superconducting gap and magnetic resonance energy. So far, charge stripes are widely believed to be involved in the physics of hour-glass spectra. Here we study an isostructural cobaltate that also exhibits an hour-glass magnetic spectrum. Instead of the expected charge stripe order we observe nano phase separation and unravel a microscopically split origin of hour-glass spectra on the nano scale pointing to a connection between the magnetic resonance peak and the spin gap originating in islands of the antiferromagnetic parent insulator. Our findings open new ways to theories of magnetic excitations and superconductivity in cuprate superconductors.
    Nature Communications 12/2014; 5:5731. DOI:10.1038/ncomms6731 · 10.74 Impact Factor
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    ABSTRACT: Parent compounds of Fe-based superconductors undergo a structural phase transition from a tetragonal to an orthorhombic structure. We investigated the temperature dependence of the frequencies of transverse acoustic (TA) phonons that extrapolate to the shear vibrational mode at the zone center, which corresponds to the orthorhombic deformation of the crystal structure at low temperatures in BaFe 2 As 2 and SrFe 2 As 2 . We found that acoustic phonons at small wavevectors soften gradually towards the transition from high temperatures, tracking the increase of the size of slowly fluctuating magnetic domains. On cooling below the transition to base temperature the phonons harden, following the square of the magnetic moment. Our results provide evidence for close correlation between magnetic and phonon properties in Fe-based superconductors.
    Physical Review B 10/2014; 91(13). DOI:10.1103/PhysRevB.91.134426 · 3.74 Impact Factor
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    ABSTRACT: We present an investigation of the lattice dynamics of the charge-density-wave compound 2H-NbSe2. We analyze the precise nature of the wave vector dependent electron-phonon coupling (EPC) and derive the bare dispersion of the charge-density-wave (CDW) soft phonon mode using inelastic x-ray scattering combined with ab-initio calculations. Experimentally, phonon modes along the {\Gamma} - M line, i.e. q = (h, 0, 0) with 0 <= h <= 0.5, with the same longitudinal symmetry ({\Sigma}1) as the CDW soft mode were investigated up to 32 meV. In agreement with our calculations we observe significant EPC in the optic modes at h <= 0.2. We analyze the EPC in the optic as well as acoustic modes and show that the q dependences stem from scattering processes between two bands at the Fermi surface both having Nb 4d character. Finally, we demonstrate that the soft mode dispersion at T = 33 K (= {T_{CDW}}) can be well described on the basis of a strongly q dependent EPC matrix element and an acoustic-like bare phonon dispersion in agreement with observations near room temperature.
    Physical Review B 06/2013; 87(24):245111. DOI:10.1103/PhysRevB.87.245111 · 3.74 Impact Factor
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    ABSTRACT: Single crystal diffuse scattering provides a powerful probe of the complex disorder associated with many emergent phenomena of great interest. It provides a determination not only of the local distortions around a point defect but also of the length scale and morphology of short-range order on the nanoscale. However, obtaining accurate models of the local structure usually demands measurements over large volumes of reciprocal space with sufficiently high momentum and energy resolution. In order to overcome limitations of current instrumentation, we propose to utilize the cross-correlation method at pulsed neutron sources. This concept that combines the high efficiency of white-beam Laue diffraction for measuring large volumes of reciprocal space with energy discrimination produced by the use of a statistical chopper is currently being implemented in a dedicated instrument, Corelli , under construction at the Spallation Neutron Source. Here, we present our detailed investigation of the effectiveness of this method for measuring weak diffuse signals, based on full experiment simulations as well as actual measurements of the diffuse scattering from powder and single crystal samples obtained utilizing the cross correlation method on a prototype instrument.
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    ABSTRACT: We use high-resolution steady-state ac-micro-calorimetry to investigate the transition of 1T-TiSe2 into the charge-ordered state. A mean-field like step of ˜ 0.4 J/molK in the specific heat C(T) near 193 K signals the transition into the commensurate CDW state. Upon further cooling, C(T) varies linearly in temperature until near 180 K a clear break in the slope of C(T) by 13mJ/molK^2 and possibly a small step indicate a second phase transition. Comparisons with theoretical predictions based on the Ginzburg-Landau free energy, with resistivity measurements, and with x-ray diffraction indicate that, at this transition, the commensurate CDW state changes into a helically ordered state along the crystal c-axis.
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    ABSTRACT: We report an inelastic neutron scattering investigation of phonons with energies up to 159 meV in the conventional superconductor YNi$_2$B$_2$C. Using the SWEEP mode, a newly developed time-of-flight technique involving the continuous rotation of a single crystal specimen, allowed us to measure a four dimensional volume in (Q,E) space and, thus, determine the dispersion surface and linewidths of the $A_{1g}$ (~ 102 meV) and $A_u$ (~ 159 meV) type phonon modes for the whole Brillouin zone. Despite of having linewidths of $\Gamma = 10 meV$, $A_{1g}$ modes do not strongly contribute to the total electron-phonon coupling constant $\lambda$. However, experimental linewidths show a remarkable agreement with ab-initio calculations over the complete phonon energy range demonstrating the accuracy of such calculations in a rare comparison to a comprehensive experimental data set.
    Physical Review Letters 07/2012; 109(Aug. 3, 2012). DOI:10.1103/PhysRevLett.109.057001 · 7.51 Impact Factor
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    ABSTRACT: The discovery of spin-Peierls transitions in inorganic materials such as CuGeO3 with Tsp of 14K allowed for the growth of large single crystals. With the availability of large single crystals the opportunity arises to introduce both magnetic and nonmagnetic impurities and study the resulting perturbations from the ground state. Recently a new class of unconventional spin-Peierls materials were discovered TiOBr and TiOCl. TiOBr and TiOCl have been shown to exhibit dimerized singlet ground states and undergo not one but two successive phase transitions. We have performed x-ray scattering measurements on single crystals of TiOBr. These measurements reveal both commensurate and incommensurate spin-Peierls phases; below Tc2 ˜48K incommensurate super-lattice reflections arise at Q=[H±δ,K+1/2±ɛ,L] which persists down to the lock in transition at Tc1˜27K. We will report on the details of these successive transitions and the destruction of the long-range ordered spin-Peierls state with introduction of magnetic vacancies by doping with Sc.
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    ABSTRACT: The acoustic phonons in the 50% doped bilayer manganite LaSr2Mn2O7 exhibiting CE type charge order were investigated using inelastic neutron scattering. At the onset of charge ordering, we observe an abrupt increase of the energies and a decrease of the linewidts of the transverse mode along (1,1,0), which crosses the CE ordering wavevector. This effect is however not localized to the CE ordering wavevector, but is observed over an extended range of momentum transfers,for which the phonon energy is lower than 15 meV. These observations indicate a reduced electron-phonon coupling due to a partial removal of the Fermi surface and provide direct evidence for a link between electron-phonon coupling and charge order in manganites. However, the observed response of the phonons is not consistent with a standard CDW mechanism, clearly showing that the transition is unconventional. Work supported by US DOE BES-DMS DE-AC02-06CH11357.
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    ABSTRACT: Using a combination of Angle Resolved Photoemission Spectroscopy (ARPES), X-ray diffraction, transport and Scanning Tunneling Microscopy (STM) measurements on pristine as well as disordered 2H-NbSe2 samples, we have found that the onset Temperature Tcdw for Long Ranged Charge Density Wave (CDW) order gets quickly suppressed with concentration of disorder ions (X) and at certain critical concentration (Xc) it undergoes a quantum melting. Our STM measurements provide the evidence for local CDW ordering in doped samples for temperatures way above Tcdw. On the other hand, our ARPES measurements have found evidences for the presence of energy gap for both T>Tcdw & X>Xc. We argue, all these experimental observations from completely different probes hint towards phase fluctuations of the order parameter as the mechanism behind the destruction of CDW order in quasi 2-d systems, such as 2H-NbSe2.
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    ABSTRACT: We report the results of a systematic investigation of the phase diagram of the iron-based superconductor Ba1−xKxFe2As2 from x = 0 to x = 1.0 using high-resolution neutron and x-ray diffraction and magnetization measurements. The polycrystalline samples were prepared with an estimated compositional variation of Δx ≲ 0.01, allowing a more precise estimate of the phase boundaries than reported so far. At room temperature, Ba1−xKxFe2As2 crystallizes in a tetragonal structure with the space group symmetry of I4/mmm, but at low doping, the samples undergo a coincident first-order structural and magnetic phase transition to an orthorhombic (O) structure with the space group Fmmm and a striped antiferromagnet (AF) with the space group Fcmm′m′. The transition temperature falls from a maximum of 139 K in the undoped compound to 0 K at x = 0.252, with a critical exponent as a function of doping of 0.25(2) and 0.12(1) for the structural and magnetic order parameters, respectively. The onset of superconductivity occurs at a critical concentration of x = 0.130(3), and the superconducting transition temperature grows linearly with x until it crosses the AF/O phase boundary. Below this concentration, there is microscopic phase coexistence of the AF/O and superconducting order parameters, although a slight suppression of the AF/O order is evidence that the phases are competing. At higher doping, superconductivity has a maximum Tc of 38 K at x = 0.4 that falls to 3 K at x = 1.0. We discuss reasons for the suppression of the spin density wave order and the electron–hole asymmetry in the phase diagram.
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    ABSTRACT: We report the results of a systematic investigation of the phase diagram of the iron-based superconductor Ba{sub 1-x}KâFeâAsâ from x = 0 to x = 1.0 using high-resolution neutron and x-ray diffraction and magnetization measurements. The polycrystalline samples were prepared with an estimated compositional variation of Îx ⤠0.01, allowing a more precise estimate of the phase boundaries than reported so far. At room temperature, Ba{sub 1-x}KâFeâAsâ crystallizes in a tetragonal structure with the space group symmetry of I4/mmm, but at low doping, the samples undergo a coincident first-order structural and magnetic phase transition to an orthorhombic (O) structure with the space group Fmmm and a striped antiferromagnet (AF) with the space group F{sub c}mm'm'. The transition temperature falls from a maximum of 139 K in the undoped compound to 0 K at x = 0.252, with a critical exponent as a function of doping of 0.25(2) and 0.12(1) for the structural and magnetic order parameters, respectively. The onset of superconductivity occurs at a critical concentration of x = 0.130(3), and the superconducting transition temperature grows linearly with x until it crosses the AF/O phase boundary. Below this concentration, there is microscopic phase coexistence of the AF/O and superconducting order parameters, although a slight suppression of the AF/O order is evidence that the phases are competing. At higher doping, superconductivity has a maximum T{sub c} of 38 K at x = 0.4 that falls to 3 K at x = 1.0. We discuss reasons for the suppression of the spin density wave order and the electron-hole asymmetry in the phase diagram.
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    ABSTRACT: We report a high resolution neutron diffraction investigation of the coupling of structural and magnetic transitions in Ba1-xKxFe2As2. The tetragonal-orthorhombic and antiferromagnetic transitions are suppressed with potassium doping, falling to zero at x≲0.24. However, unlike Ba(Fe1-xCox)2As2, the two transitions are first order and coincident over the entire phase diagram, with a biquadratic coupling of the two order parameters. The phase diagram is refined showing that the onset of superconductivity is at x=0.133 with all three phases coexisting until x⩽0.24.
    Physical review. B, Condensed matter 05/2011; 83(17-17). DOI:10.1103/PhysRevB.83.172503 · 3.66 Impact Factor
  • S. Rosenkranz · F. Weber · J. P. Castellan · R. Osborn · A. Said · R. Hott · R. Heid · D. Reznik
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    ABSTRACT: The soft-phonons in the charge density wave (CDW) compound NbSe2 were investigated using high-resolution inelastic X-ray scattering. As the CDW transition at TC=33K is approached from high temperature, we observe a breakdown of the dispersion with the phonons becoming overdapmed over an extended region around the CDW wavevector. This is in contrast to the cusp in the phonon dispersion expected from the commonly invoked electronic nesting scenario of the CDW transition. Instead, our results, combined with ab initio calculations, show that the wavevector of the CDW order is dictated by the momentum dependence of the intrinsic electron-phonon coupling. The strong influence of electron-phonon matrix-elements could also be of importance to other systems, where CDW-like correlations have been attributed to unusual physical properties. Work supported by US DOE BES-DMS DE-AC02-06CH11357.
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    F. Weber · S. Rosenkranz · J. -P. Castellan · R Osborn · J. F. Mitchell · H Zheng · D. Casa · J. H. Kim · T. Gog
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    ABSTRACT: We report a high resolution resonant inelastic x-ray scattering investigation of the bilayer manganites La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ with $x = 0.36$ and $0.5$. The momentum dependence along the crystallographic $(110)$ direction for energy losses $1\,\rm{eV} \leq \Delta E \leq 15\,\rm{eV}$ has been measured in detail with the data analysis focusing on the energy loss region $1\,\rm{eV} \leq \Delta E \leq 5\,\rm{eV}$, which includes a strong peak located at $\Delta E \approx 2\,\rm{eV}$. We observe a clear dispersion of up to $0.5\,\rm{eV}$ in the measured $\textbf{q}$ range, which is direct evidence of the non-local character of this excitation. Further, we found that the intensity in this low energy region strongly depends on both the reduced wave vector $\textbf{q} = (h, h, 0)$, $h = 0.1 - 0.5$, and temperature, i.e. different ordered phases. Results can be explained via an intersite $d-d$ charge transfer excitation, proposed for pseudo-cubic manganites, where the hopping rate is strongly increased (decreased) by ferromagnetic (antiferromagnetic) alignment of neighboring in-plane Mn ion core spins.
    Physical Review B 07/2010; 82(8):085105. DOI:10.1103/PhysRevB.82.085105 · 3.74 Impact Factor
  • S. Rosenkranz · J. P. Castellan · F. Weber · G. Karapetrov · R. Osborn
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    ABSTRACT: Charge density wave order in layered chalcogenides has been studied for several decades, but the nature of the instability driving the transition is still a subject of debate. For CuxTiSe2, it was reported that the suppression of CDW with Cu doping results in superconductivity, with a phase diagram strongly reminiscent of copper oxide and heavy fermion superconductors. It is however not understood whether or how the order parameters are coupled in this or any other chalcogenide compounds where CDW and superconductivity can coexist. Here, we present our synchrotron X-ray diffraction measurements of the CDW order parameter and the diffuse scattering associated with CDW fluctuations on either side of the alleged quantum phase transition of CuxTiSe2. We will discus the behavior of the critical exponents and the lengths of the CDW correlations as a function of temperature and doping. Our results indicate that CDW order exists at high doping levels, above the optimal doping for superconductivity, where bulk probes reported absence of CDW formation.
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    ABSTRACT: Iron arsenide superconductors have attracted a great deal of interest because of their similarities with high-Tc copper oxides. In both the copper oxides and iron arsenides, the parent compounds are antiferromagnetically ordered and superconductivity arises by suppressing this antiferromagnetic order by chemical doping. An important feature of the copper oxide superconductors is the existence of a resonant magnetic excitation. This resonance is localized both in energy and wavevector and is predicted to occur only when there is a change of sign of the superconducting energy gap on different parts of the Fermi surface. Following our observation of the resonance below Tc in a polycrystalline sample of Ba0.6K0.4Fe2As2 [Nature 456, 930 (2008)], we have now performed inelastic neutron scattering measurements in a composite single crystal confirming that the resonance is centered at the gamma-M point with modulations along the c-axis.
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    ABSTRACT: The magnetic structure for the recently synthesized iron-arsenide compound CaFe4As3 has been studied by neutron-powder diffraction. Long-range magnetic order is detected below 85 K, with an incommensurate modulation described by the propagation vector k=(0,δ,0), δ∼0.39. Below ∼25 K, our measurements detect a first-order phase transition where δ locks into the commensurate value 3/8. A model of the magnetic structure is proposed for both temperature regimes, based on Rietveld refinements of the powder data and symmetry considerations. The structures correspond to longitudinal spin-density waves with magnetic moments directed along the b axis. A Landau analysis captures the change in thermodynamic quantities observed at the two magnetic transitions, in particular, the drop in resistivity at the lock-in transition.
    Physical review. B, Condensed matter 02/2010; 81(18). DOI:10.1103/PhysRevB.81.184402 · 3.66 Impact Factor
  • F Weber · J P Castellan · S Rosenkranz · R Osborn · D Rosenmann · M Iavarone
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    ABSTRACT: We present a study of the local environment of the Co atom in single crystalline CoxNbSe2, x = 0.013, via Extended X-ray Absorption Fine Structure (EXAFS) measurements at the Co K-edge (7.7 keV) at various temperatures. Co intercalation quickly suppresses superconductivity and the charge-density wave (CDW) present in pure NbSe2. In order to study the effect of impurities on superconducting and CDW states one has to verify the random distribution of the intercalated atoms in contrast to possible clustering which could lead to additional, e.g. magnetic, interactions in the case of Co intercalation. Our measurements show that the Co atoms are indeed randomly distributed in Co0.013NbSe2.
    Journal of Physics Conference Series 01/2010; 200:012224. DOI:10.1088/1742-6596/200/1/012224
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    ABSTRACT: Layered transition metal chalcogenides display a phase diagram similar to that of high Tc superconducting materials. In high Tc materials the superconductivity coincides with the suppression of magnetic order. In the case of TiSe2 and NbSe2 the onset of superconductivity involves a charge density wave (CDW) rather than magnetic order. The phase diagram of doped TiSe2 and NbSe2 has so far been interpreted as a competition of the CDW order and superconductivity order parameters. Another interpretation is that the softening of the excitons responsible for the CDW order enables the superconductivity. We have measured diffuse scattering associated with the fluctuations in the order parameter of the CDW on either side of the quantum phase transition using X-ray scattering. We will present the behavior of the critical exponents as the quantum phase transition is approached and discuss whether there is a coupling of the CDW and superconducting order parameters. Work supported by US DOE BES-DMS DE-AC02-06CH11357

Publication Stats

266 Citations
109.34 Total Impact Points

Institutions

  • 2014–2015
    • Karlsruhe Institute of Technology
      • Institute for Solid-State Physics
      Carlsruhe, Baden-Württemberg, Germany
  • 2010–2013
    • Argonne National Laboratory
      • Division of Materials Science
      Lemont, Illinois, United States
  • 2000–2008
    • McMaster University
      • Department of Physics and Astronomy
      Hamilton, Ontario, Canada