Raymond Osborn | Argonne National Laboratory (original) (raw)

Papers by Raymond Osborn

Research paper thumbnail of Unconventional response of acoustic phonons to the onset of charge order in a bilayer manganite

ABSTRACT The acoustic phonons in the 50% doped bilayer manganite LaSr2Mn2O7 exhibiting CE type ch... more 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.

Research paper thumbnail of Electron-Phonon Coupling in the Conventional Superconductor YNi_{2}B_{2}C at High Phonon Energies Studied by Time-of-Flight Neutron Spectroscopy

Physical Review Letters, 2012

We report an inelastic neutron scattering investigation of phonons with energies up to 159 meV in... more We report an inelastic neutron scattering investigation of phonons with energies up to 159 meV in the conventional superconductor YNi2B2C. Using the SWEEP mode, a newly developed timeof-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 A1g (≈ 102 meV) and Au (≈ 159 meV) type phonon modes for the whole Brillouin zone. Despite of having linewidths of Γ = 10 meV, A1g modes do not strongly contribute to the total electron-phonon coupling constant λ. 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.

Research paper thumbnail of Incommensurate spin-density wave and magnetic lock-in transition in CaFe_{4}As_{3}

Physical Review B, 2010

The magnetic structure for the newly discovered iron-arsenide compound CaFe4As3 has been studied ... more The magnetic structure for the newly discovered iron-arsenide compound CaFe4As3 has been studied by neutron powder diffraction. Long-range magnetic order is detected below 85K, with an incommensurate modulation described by the propagation vector k=(0,δ,0), δ ∼ 0.39. Below ∼ 25K, 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.

Research paper thumbnail of Magnetoelastic coupling in the phase diagram of Ba(1-x)K(x)Fe(2)As(2) as seen via neutron diffraction

Research paper thumbnail of Extended Phonon Collapse and the Origin of the Charge-Density Wave in 2H-NbSe_{2}

Physical Review Letters, 2011

We report inelastic x-ray scattering measurements of the temperature dependence of phonon dispers... more We report inelastic x-ray scattering measurements of the temperature dependence of phonon dispersion in the prototypical charge-density-wave (CDW) compound NbSe2. Surprisingly, acoustic phonons soften to zero frequency and become overdamped over an extended region around the CDW wavevector. This extended phonon collapse is dramatically different from the sharp cusp in the phonon dispersion expected from Fermi surface nesting. Instead, our experiments combined with ab initio calculations, show that it is the wavevector dependence of the electron-phonon coupling that drives the CDW formation in NbSe2 and determines its periodicity. This mechanism explains the so far enigmatic behavior of CDW in NbSe2 and may provide a new approach to other strongly correlated systems where electron-phonon coupling is important.

Research paper thumbnail of Optical phonons and the soft mode in 2H-NbSe_{2}

Physical Review B, 2013

We present an investigation of the lattice dynamics of the charge-density-wave compound 2H-NbSe 2... more We present an investigation of the lattice dynamics of the charge-density-wave compound 2H-NbSe 2 . 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 Γ -M line, i.e. q = (h, 0, 0) with 0 ≤ h ≤ 0.5, with the same longitudinal symmetry (Σ 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.

Research paper thumbnail of Phase diagram of Ba_{1−x}K_{x}Fe_{2}As_{2}

Physical Review B, 2012

We report the results of a systematic investigation of the phase diagram of the iron-based superc... more 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 ∆x 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 space group F mmm and a striped antiferromagnet (AF) with 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 falling 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. arXiv:1205.0489v1 [cond-mat.supr-con]

Research paper thumbnail of Effect of Fermi Surface Nesting on Resonant Spin Excitations in Ba_{1-x}K_{x}Fe_{2}As_{2}

Physical Review Letters, 2011

We report inelastic neutron scattering measurements of the resonant spin excitations in Ba1−xKxFe... more We report inelastic neutron scattering measurements of the resonant spin excitations in Ba1−xKxFe2As2 over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s±-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight caused by the weakening of electron-electron correlations.

Research paper thumbnail of Electron-Phonon Coupling and the Soft Phonon Mode in TiSe_{2}

Physical Review Letters, 2011

We report high-resolution inelastic x-ray measurements of the soft phonon mode in the chargedensi... more We report high-resolution inelastic x-ray measurements of the soft phonon mode in the chargedensity-wave compound TiSe2. We observe a complete softening of a transverse optic phonon at the L point, i.e. q = (0.5, 0, 0.5), at T ≈ TCDW . Renormalized phonon energies are observed over a large wavevector range (0.3, 0, 0.5) ≤ q ≤ (0.5, 0, 0.5). Detailed ab-initio calculations for the electronic and lattice dynamical properties of TiSe2 are in quantitative agreement with experimental frequencies for the phonon branch involving the soft mode. The observed broad range of renormalized phonon frequencies is directly related to a broad peak in the electronic susceptibility stabilizing the chargedensity-wave ordered state. Our analysis demonstrates that a conventional electron-phonon coupling mechanism can explain a structural instability and the charge-density-wave order in TiSe2 although other mechanisms might further boost the transition temperature.

Research paper thumbnail of Magnetically driven suppression of nematic order in an iron-based superconductor

Nature Communications, 2014

A theory of superconductivity in the iron-based materials requires an understanding of the phase ... more A theory of superconductivity in the iron-based materials requires an understanding of the phase diagram of the normal state. In these compounds, superconductivity emerges when stripe spin density wave (SDW) order is suppressed by doping, pressure or atomic disorder. This magnetic order is often pre-empted by nematic order, whose origin is yet to be resolved. One scenario is that nematic order is driven by orbital ordering of the iron 3d electrons that triggers stripe SDW order. Another is that magnetic interactions produce a spin-nematic phase, which then induces orbital order. Here we report the observation by neutron powder diffraction of an additional fourfold-symmetric phase in Ba 1 À x Na x Fe 2 As 2 close to the suppression of SDW order, which is consistent with the predictions of magnetically driven models of nematic order.

Research paper thumbnail of X-ray Diffraction measurements of the CDW phase in doped TiSe $ _ {2} $ and NbSe $ _ {2} $

Bulletin of the American …, 2009

... Abstract: H41.00001 : X-ray Diffraction measurements of the CDW phase in doped TiSe$_{2}$ and... more ... Abstract: H41.00001 : X-ray Diffraction measurements of the CDW phase in doped TiSe$_{2}$ and NbSe$_{2}$. 8:00 AM–8:12 AM. Preview Abstract. Authors: JP Castellan (Argonne Nat. Lab). CD Malliakas (Northwestern University). M. Lavarone (Argonne Nat. Lab). ...

Research paper thumbnail of D-D Excitations in Bilayer Manganites Probed by Resonant Inelastic X-Ray Scattering

Physical Review B, 2010

We report a high resolution resonant inelastic x-ray scattering investigation of the bilayer mang... more We report a high resolution resonant inelastic x-ray scattering investigation of the bilayer manganites La2−2xSr1+2xMn2O7 with x = 0.36 and 0.5. The momentum dependence along the crystallographic (110) direction for energy losses 1 eV ≤ ∆E ≤ 15 eV has been measured in detail with the data analysis focusing on the energy loss region 1 eV ≤ ∆E ≤ 5 eV, which includes a strong peak located at ∆E ≈ 2 eV. We observe a clear dispersion of up to 0.5 eV in the measured 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 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.

Research paper thumbnail of Emergence of coherence in the charge-density wave state of 2H-NbSe2

Nature communications, 2015

A charge-density wave (CDW) state has a broken symmetry described by a complex order parameter wi... more 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 T(cdw). 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 T(cdw) 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 signif...

Research paper thumbnail of Response of Acoustic Phonons to Charge and Orbital Order in the 50% Doped Bilayer Manganite LaSr_{2}Mn_{2}O_{7}

Physical Review Letters, 2011

Response of acoustic phonons to charge and orbital order in the 50% doped bilayer manganite LaSr ... more Response of acoustic phonons to charge and orbital order in the 50% doped bilayer manganite LaSr 2 Mn 2 O 7

Research paper thumbnail of Disorder Induced Melting of Charge Density Wave Order in doped 2H-NbSe2 systems

Bulletin of the American …, 2012

Using a combination of Angle Resolved Photoemission Spectroscopy (ARPES), X-ray diffraction, tran... more 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 ...

Research paper thumbnail of Crystal fields in Nd2−xSrxNiO4+y

Journal of the Less Common Metals, 1990

We have measured, by means of neutron inelastic scattering, the ground state crystalline electric... more We have measured, by means of neutron inelastic scattering, the ground state crystalline electric field excitations in the system Nd~_&r$IiO~, for x = 0 and 0.4. The excitations occur at 8.0, 23, 38 and 49 meV in the undoped compound, and at similar energies for the Sr-doped material.

Research paper thumbnail of Specific heat studies of the chiral phase transition in charge ordered 1T-TiSe2

ABSTRACT We use high-resolution steady-state ac-micro-calorimetry to investigate the transition o... more 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.

Research paper thumbnail of Crystal field excitations in Nd2−xCexCuO4

Physica C: Superconductivity, 1990

... PAUL and DS MISRA Department of Physics, University of Warwick, Coventry, CV4 7AL, UK R. OSBO... more ... PAUL and DS MISRA Department of Physics, University of Warwick, Coventry, CV4 7AL, UK R. OSBORN Rutherford Appleton Laboratory, Chilton, Didcot, Oxon., OXll OQX, UK Received 1 September 1989 Revised ... [ I 1 ] U. Walter, S. Fahy, A. Zettl, SG Louie, ML Cohen, P ...

Research paper thumbnail of The Anisotropic Kondo Model in Ce_xLa_1-xAl3 and Implications for Small-Moment Magnetism

Inelastic neutron scattering measurements of the spin dynamics in the heavy fermion compounds Ce_... more Inelastic neutron scattering measurements of the spin dynamics in the heavy fermion compounds Ce_1-xLa_xAl3 (0.05 >= x >= 0.5) provide evidence of a transition from a quasielastic to an inelastic magnetic response below a characteristic temperature T^* ~ 1--2 K E. A. Goremychkin, R. Osborn, B. D. Rainford, and A. P. Murani, Phys. Rev. Lett. 84, 2211 (2000). Such a

Research paper thumbnail of MAGNONS IN CeAl2

Anomalous Rare Earths and Actinides, 1987

Research paper thumbnail of Unconventional response of acoustic phonons to the onset of charge order in a bilayer manganite

ABSTRACT The acoustic phonons in the 50% doped bilayer manganite LaSr2Mn2O7 exhibiting CE type ch... more 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.

Research paper thumbnail of Electron-Phonon Coupling in the Conventional Superconductor YNi_{2}B_{2}C at High Phonon Energies Studied by Time-of-Flight Neutron Spectroscopy

Physical Review Letters, 2012

We report an inelastic neutron scattering investigation of phonons with energies up to 159 meV in... more We report an inelastic neutron scattering investigation of phonons with energies up to 159 meV in the conventional superconductor YNi2B2C. Using the SWEEP mode, a newly developed timeof-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 A1g (≈ 102 meV) and Au (≈ 159 meV) type phonon modes for the whole Brillouin zone. Despite of having linewidths of Γ = 10 meV, A1g modes do not strongly contribute to the total electron-phonon coupling constant λ. 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.

Research paper thumbnail of Incommensurate spin-density wave and magnetic lock-in transition in CaFe_{4}As_{3}

Physical Review B, 2010

The magnetic structure for the newly discovered iron-arsenide compound CaFe4As3 has been studied ... more The magnetic structure for the newly discovered iron-arsenide compound CaFe4As3 has been studied by neutron powder diffraction. Long-range magnetic order is detected below 85K, with an incommensurate modulation described by the propagation vector k=(0,δ,0), δ ∼ 0.39. Below ∼ 25K, 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.

Research paper thumbnail of Magnetoelastic coupling in the phase diagram of Ba(1-x)K(x)Fe(2)As(2) as seen via neutron diffraction

Research paper thumbnail of Extended Phonon Collapse and the Origin of the Charge-Density Wave in 2H-NbSe_{2}

Physical Review Letters, 2011

We report inelastic x-ray scattering measurements of the temperature dependence of phonon dispers... more We report inelastic x-ray scattering measurements of the temperature dependence of phonon dispersion in the prototypical charge-density-wave (CDW) compound NbSe2. Surprisingly, acoustic phonons soften to zero frequency and become overdamped over an extended region around the CDW wavevector. This extended phonon collapse is dramatically different from the sharp cusp in the phonon dispersion expected from Fermi surface nesting. Instead, our experiments combined with ab initio calculations, show that it is the wavevector dependence of the electron-phonon coupling that drives the CDW formation in NbSe2 and determines its periodicity. This mechanism explains the so far enigmatic behavior of CDW in NbSe2 and may provide a new approach to other strongly correlated systems where electron-phonon coupling is important.

Research paper thumbnail of Optical phonons and the soft mode in 2H-NbSe_{2}

Physical Review B, 2013

We present an investigation of the lattice dynamics of the charge-density-wave compound 2H-NbSe 2... more We present an investigation of the lattice dynamics of the charge-density-wave compound 2H-NbSe 2 . 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 Γ -M line, i.e. q = (h, 0, 0) with 0 ≤ h ≤ 0.5, with the same longitudinal symmetry (Σ 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.

Research paper thumbnail of Phase diagram of Ba_{1−x}K_{x}Fe_{2}As_{2}

Physical Review B, 2012

We report the results of a systematic investigation of the phase diagram of the iron-based superc... more 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 ∆x 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 space group F mmm and a striped antiferromagnet (AF) with 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 falling 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. arXiv:1205.0489v1 [cond-mat.supr-con]

Research paper thumbnail of Effect of Fermi Surface Nesting on Resonant Spin Excitations in Ba_{1-x}K_{x}Fe_{2}As_{2}

Physical Review Letters, 2011

We report inelastic neutron scattering measurements of the resonant spin excitations in Ba1−xKxFe... more We report inelastic neutron scattering measurements of the resonant spin excitations in Ba1−xKxFe2As2 over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s±-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight caused by the weakening of electron-electron correlations.

Research paper thumbnail of Electron-Phonon Coupling and the Soft Phonon Mode in TiSe_{2}

Physical Review Letters, 2011

We report high-resolution inelastic x-ray measurements of the soft phonon mode in the chargedensi... more We report high-resolution inelastic x-ray measurements of the soft phonon mode in the chargedensity-wave compound TiSe2. We observe a complete softening of a transverse optic phonon at the L point, i.e. q = (0.5, 0, 0.5), at T ≈ TCDW . Renormalized phonon energies are observed over a large wavevector range (0.3, 0, 0.5) ≤ q ≤ (0.5, 0, 0.5). Detailed ab-initio calculations for the electronic and lattice dynamical properties of TiSe2 are in quantitative agreement with experimental frequencies for the phonon branch involving the soft mode. The observed broad range of renormalized phonon frequencies is directly related to a broad peak in the electronic susceptibility stabilizing the chargedensity-wave ordered state. Our analysis demonstrates that a conventional electron-phonon coupling mechanism can explain a structural instability and the charge-density-wave order in TiSe2 although other mechanisms might further boost the transition temperature.

Research paper thumbnail of Magnetically driven suppression of nematic order in an iron-based superconductor

Nature Communications, 2014

A theory of superconductivity in the iron-based materials requires an understanding of the phase ... more A theory of superconductivity in the iron-based materials requires an understanding of the phase diagram of the normal state. In these compounds, superconductivity emerges when stripe spin density wave (SDW) order is suppressed by doping, pressure or atomic disorder. This magnetic order is often pre-empted by nematic order, whose origin is yet to be resolved. One scenario is that nematic order is driven by orbital ordering of the iron 3d electrons that triggers stripe SDW order. Another is that magnetic interactions produce a spin-nematic phase, which then induces orbital order. Here we report the observation by neutron powder diffraction of an additional fourfold-symmetric phase in Ba 1 À x Na x Fe 2 As 2 close to the suppression of SDW order, which is consistent with the predictions of magnetically driven models of nematic order.

Research paper thumbnail of X-ray Diffraction measurements of the CDW phase in doped TiSe $ _ {2} $ and NbSe $ _ {2} $

Bulletin of the American …, 2009

... Abstract: H41.00001 : X-ray Diffraction measurements of the CDW phase in doped TiSe$_{2}$ and... more ... Abstract: H41.00001 : X-ray Diffraction measurements of the CDW phase in doped TiSe$_{2}$ and NbSe$_{2}$. 8:00 AM–8:12 AM. Preview Abstract. Authors: JP Castellan (Argonne Nat. Lab). CD Malliakas (Northwestern University). M. Lavarone (Argonne Nat. Lab). ...

Research paper thumbnail of D-D Excitations in Bilayer Manganites Probed by Resonant Inelastic X-Ray Scattering

Physical Review B, 2010

We report a high resolution resonant inelastic x-ray scattering investigation of the bilayer mang... more We report a high resolution resonant inelastic x-ray scattering investigation of the bilayer manganites La2−2xSr1+2xMn2O7 with x = 0.36 and 0.5. The momentum dependence along the crystallographic (110) direction for energy losses 1 eV ≤ ∆E ≤ 15 eV has been measured in detail with the data analysis focusing on the energy loss region 1 eV ≤ ∆E ≤ 5 eV, which includes a strong peak located at ∆E ≈ 2 eV. We observe a clear dispersion of up to 0.5 eV in the measured 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 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.

Research paper thumbnail of Emergence of coherence in the charge-density wave state of 2H-NbSe2

Nature communications, 2015

A charge-density wave (CDW) state has a broken symmetry described by a complex order parameter wi... more 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 T(cdw). 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 T(cdw) 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 signif...

Research paper thumbnail of Response of Acoustic Phonons to Charge and Orbital Order in the 50% Doped Bilayer Manganite LaSr_{2}Mn_{2}O_{7}

Physical Review Letters, 2011

Response of acoustic phonons to charge and orbital order in the 50% doped bilayer manganite LaSr ... more Response of acoustic phonons to charge and orbital order in the 50% doped bilayer manganite LaSr 2 Mn 2 O 7

Research paper thumbnail of Disorder Induced Melting of Charge Density Wave Order in doped 2H-NbSe2 systems

Bulletin of the American …, 2012

Using a combination of Angle Resolved Photoemission Spectroscopy (ARPES), X-ray diffraction, tran... more 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 ...

Research paper thumbnail of Crystal fields in Nd2−xSrxNiO4+y

Journal of the Less Common Metals, 1990

We have measured, by means of neutron inelastic scattering, the ground state crystalline electric... more We have measured, by means of neutron inelastic scattering, the ground state crystalline electric field excitations in the system Nd~_&r$IiO~, for x = 0 and 0.4. The excitations occur at 8.0, 23, 38 and 49 meV in the undoped compound, and at similar energies for the Sr-doped material.

Research paper thumbnail of Specific heat studies of the chiral phase transition in charge ordered 1T-TiSe2

ABSTRACT We use high-resolution steady-state ac-micro-calorimetry to investigate the transition o... more 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.

Research paper thumbnail of Crystal field excitations in Nd2−xCexCuO4

Physica C: Superconductivity, 1990

... PAUL and DS MISRA Department of Physics, University of Warwick, Coventry, CV4 7AL, UK R. OSBO... more ... PAUL and DS MISRA Department of Physics, University of Warwick, Coventry, CV4 7AL, UK R. OSBORN Rutherford Appleton Laboratory, Chilton, Didcot, Oxon., OXll OQX, UK Received 1 September 1989 Revised ... [ I 1 ] U. Walter, S. Fahy, A. Zettl, SG Louie, ML Cohen, P ...

Research paper thumbnail of The Anisotropic Kondo Model in Ce_xLa_1-xAl3 and Implications for Small-Moment Magnetism

Inelastic neutron scattering measurements of the spin dynamics in the heavy fermion compounds Ce_... more Inelastic neutron scattering measurements of the spin dynamics in the heavy fermion compounds Ce_1-xLa_xAl3 (0.05 >= x >= 0.5) provide evidence of a transition from a quasielastic to an inelastic magnetic response below a characteristic temperature T^* ~ 1--2 K E. A. Goremychkin, R. Osborn, B. D. Rainford, and A. P. Murani, Phys. Rev. Lett. 84, 2211 (2000). Such a

Research paper thumbnail of MAGNONS IN CeAl2

Anomalous Rare Earths and Actinides, 1987