Showing 1–50 of 66 results for author: Egami, T

Orbital-selective oxygen holes in cuprate ladders beyond the Zhang-Rice paradigm

Authors: Chengyun Hua, Tianran Chen, Isaac C. Ownby, Colin L. Sarkis, Garrett Granoth, Masaaki Matsuda, Jiaqiang Yan, Ho Nyung Lee, Jeongkeun Song, Yuya Shinohara, Masatomo Uehara, Jun Akimitsu, Oleksandr Prokhnenko, Eugen Weschke, Takeshi Egami, D. Alan Tennant

Abstract: The electronic structure of the spin-ladder cuprate Sr14Cu24O41 challenges the presumed universality of the Zhang-Rice singlet (ZRS) framework and models based exclusively on Cu-O hybridized orbitals. Combining polarization-dependent resonant soft X-ray scattering at the O K-edge with inelastic neutron scattering, we show that doped holes in the Cu2O3 ladders localize predominantly in planar non-b… ▽ More The electronic structure of the spin-ladder cuprate Sr14Cu24O41 challenges the presumed universality of the Zhang-Rice singlet (ZRS) framework and models based exclusively on Cu-O hybridized orbitals. Combining polarization-dependent resonant soft X-ray scattering at the O K-edge with inelastic neutron scattering, we show that doped holes in the Cu2O3 ladders localize predominantly in planar non-bonding O 2pz (pπ) orbitals of rung oxygen sites rather than forming conventional ZRS states. Polarization-resolved RSXS uniquely identifies this orbital assignment, while lattice and magnetic excitations reveal its coupled consequences, establishing a unified microscopic picture that excludes the conventional σ-bonded singlet. This oxygen-sublattice charge order produces an anomalous diagonal stretching phonon and explains the absence of incommensurate magnetic fluctuations and anomalous magnon splitting. These findings motivate a reassessment of hole pairing in ladder cuprates and the sufficiency of copper-centric models for cuprate superconductors. △ Less

Submitted 10 August, 2026; originally announced August 2026.

Real-Space Dynamic Electron Correlation in Beryllium

Authors: Rudra B. Bista, Yuya Shinohara, Wojciech Dmowski, Chae Woo Ryu, Jung Ho Kim, Mary Upton, Hlynur Gretarsson, Martin Sundermann, Takeshi Egami

Abstract: Electron correlation in solid has a major impact on material properties. However, it has been studied mainly by theory, with very limited direct experimental investigations. Here, we demonstrate that dynamic electron correlation function can be experimentally measured using inelastic X-ray scattering on polycrystalline beryllium. The data are expressed as the energy-resolved dynamic pair-distribut… ▽ More Electron correlation in solid has a major impact on material properties. However, it has been studied mainly by theory, with very limited direct experimental investigations. Here, we demonstrate that dynamic electron correlation function can be experimentally measured using inelastic X-ray scattering on polycrystalline beryllium. The data are expressed as the energy-resolved dynamic pair-distribution function. Our results confirm the size of the exchange-correlation hole as ~2 Å, consistent with theoretical expectations. However, at the plasmon energy of ~21 eV, the exchange-correlation hole is extended up to 4-5 Å, suggesting a unique influence of the dynamic plasmon state. △ Less

Submitted 30 June, 2026; v1 submitted 28 January, 2026; originally announced January 2026.

Comments: 7 pages, 4 figures, 1 supplementary Information

Journal ref: Physical Review Letters 136, 256402 (2026)

Real-space Atomic Dynamics in Liquid Gallium Studied by Inelastic Neutron Scattering

Authors: Chengyun Hua, Yadu K. Sarathchandran, Eva Zarkadoula, Wojciech Dmowski, Douglas L. Abernathy, Takeshi Egami, Yuya Shinohara

Abstract: Gallium is a prototypical liquid metal and has gained renewed attention due to its unique properties. Characterizing and elucidating its atomic dynamics remains elusive despite numerous studies, primarily due to the challenges of quantifying atomic-scale dynamics in liquids. Recent developments in inelastic neutron scattering enable us to measure the Van Hove correlation function that describes th… ▽ More Gallium is a prototypical liquid metal and has gained renewed attention due to its unique properties. Characterizing and elucidating its atomic dynamics remains elusive despite numerous studies, primarily due to the challenges of quantifying atomic-scale dynamics in liquids. Recent developments in inelastic neutron scattering enable us to measure the Van Hove correlation function that describes the real-space motion of liquid atoms. In this work, we use this approach to reveal the dynamics in gallium liquids and find the co-existence of two dynamical medium-range orders (MROs), which have a dynamical behavior distinct from that of the short-range order (SRO). We propose that these MROs are driven by global forces in the form of two density waves, as a direct consequence of the underlying competition between ionic core repulsion and valence electron cohesion. We suggest that the density wave approach is not only applicable to other metallic liquids exhibiting similar structural anomalies, but also offers a promising direction for elucidating the dynamics of complex liquids and glasses by linking electronic-state fluctuations to atomic dynamics. △ Less

Submitted 18 December, 2025; originally announced December 2025.

Complexity in the medium-range order of gallium as a polyvalent liquid metal

Authors: Chengyun Hua, Yadu K. Sarathchandran, Eva Zarkadoula, Wojciech Dmowski, Douglas L. Abernathy, Yuya Shinohara, Takeshi Egami

Abstract: Simplicity in chemical composition does not always translate into simplicity in the structures and dynamics of liquids and solids. Some elementary liquid metals, such as gallium, show unusual behaviors in thermodynamic and transport properties as a result of their complex atomic structure and dynamics. In this work, we study the real-space atomic correlation function of liquid gallium by neutron s… ▽ More Simplicity in chemical composition does not always translate into simplicity in the structures and dynamics of liquids and solids. Some elementary liquid metals, such as gallium, show unusual behaviors in thermodynamic and transport properties as a result of their complex atomic structure and dynamics. In this work, we study the real-space atomic correlation function of liquid gallium by neutron scattering. In the pair-distribution function, there exist two kinds of medium-range order (MRO), characterized by oscillations beyond the first nearest neighbors. On the other hand, the first neighbor shell shows only one kind of bond. The two types of MRO are strongly overlapping in space and fluctuating in time. We propose that they are the basis for anomalous behavior of liquid gallium. This view challenges the current view that liquid gallium consists of fluctuating metallic and insulating domains. These findings shed new light on the interpretation of similar microscopic anomalies observed in other semi-metallic liquids. △ Less

Submitted 18 December, 2025; originally announced December 2025.

Cooperative atomic motion during shear deformation in metallic glass

Authors: Yoshinori Shiihara, Takuya Iwashita, Nozomu Adachi, Yoshikazu Todaka, Takeshi Egami

Abstract: Elucidating mechanical deformation in glassy materials at the atomic level is challenging due to their disordered atomic structure. Using our novel "frozen atom analysis," we reveal that anelastic deformation in CuZr metallic glasses is fundamentally driven by cooperative atomic motions of approximately 40 elastically linked atoms, forming trigger groups. They initiate localized rearrangements, wh… ▽ More Elucidating mechanical deformation in glassy materials at the atomic level is challenging due to their disordered atomic structure. Using our novel "frozen atom analysis," we reveal that anelastic deformation in CuZr metallic glasses is fundamentally driven by cooperative atomic motions of approximately 40 elastically linked atoms, forming trigger groups. They initiate localized rearrangements, which can cascade into plastic flow. Notably, these groups show no distinct structural or physical features, challenging the idea that deformation occurs in defective regions. Instead, deformation events are highly stochastic and transient, driven by collective atomic motion. This finding not only reshapes our understanding of glassy material deformation mechanisms but also highlights cooperative motion as a key factor in avalanche-like phenomena governing the behavior of disordered systems across multiple scales. △ Less

Submitted 19 March, 2025; originally announced March 2025.

Atomic dynamics in fluids: Normal mode analysis revisited

Authors: Jaeyun Moon, Lucas Lindsay, Takeshi Egami

Abstract: Developing microscopic understanding of the thermal properties of liquids is challenging due to their strong dynamic disorder, which prevents characterization of the atomic degrees of freedom. There have been significant research interests in the past few decades to extend the normal mode analysis for solids to instantaneous structures of liquids. However, the nature of normal modes that arise fro… ▽ More Developing microscopic understanding of the thermal properties of liquids is challenging due to their strong dynamic disorder, which prevents characterization of the atomic degrees of freedom. There have been significant research interests in the past few decades to extend the normal mode analysis for solids to instantaneous structures of liquids. However, the nature of normal modes that arise from these unstable structures is still elusive. In this work, we explore the instantaneous eigenmodes of dynamical matrices of various Lennard-Jones argon liquid/gas systems at high temperatures and show that the normal modes can be interpreted as an interpolation of T \to \infty (gas) and T = 0 (solid) mode descriptions. We find that normal modes become increasingly collisional and translational, recovering atomistic gas-like behavior rather than vibrational with increase in temperature, suggesting that normal modes in liquids may be described by both solid-like and gas-like modes. △ Less

Submitted 4 May, 2023; v1 submitted 19 April, 2023; originally announced April 2023.

Medium-Range Atomic Correlation in Simple Liquids. III. Density Wave Theory

Authors: Takeshi Egami, Chae Woo Ryu

Abstract: Elucidating the atomic structure of liquid and glass is one of the important open questions in condensed matter physics. In the conventional bottom-up approach one starts with focusing on an atom and the short-range order of its neighboring atoms, and the global structure is described in terms of overlapping local clusters of atoms as building units. However, this local approach fails to explain t… ▽ More Elucidating the atomic structure of liquid and glass is one of the important open questions in condensed matter physics. In the conventional bottom-up approach one starts with focusing on an atom and the short-range order of its neighboring atoms, and the global structure is described in terms of overlapping local clusters of atoms as building units. However, this local approach fails to explain the strong drive to form the medium-range order which is distinct in nature from the short-range order. We propose an even-handed scheme with an additional top-down approach. In the top-down approach one starts with a high-density gas state and seeks to minimize the global potential energy through density waves. The local bottom-up and global top-down driving forces are not compatible, and the competition and compromise between them result in a final structure with the medium-range order. The density waves are pinned to atoms through the phase factors and amplitudes which reflect atomic dynamics. This even-handed approach provides a more intuitive explanation of the structure of simple liquid and glass and its relation to properties of liquid, such as viscosity and fragility. △ Less

Submitted 14 November, 2022; originally announced November 2022.

Comments: Continuation of C. W. Ryu and T. Egami, Phys. Rev. E 104, 064109 (2021) and T. Egami and C. W. Ryu, Phys. Rev. E 104, 064110 (2021)

Microscopic view of heat capacity of matter: solid, liquid, and gas

Authors: Jaeyun Moon, Simon Thébaud, Lucas Lindsay, Takeshi Egami

Abstract: Understanding thermodynamics in liquids at the atomic level is challenging because of strong atomic interactions and lack of symmetry. Recent prior theoretical works have focused on describing heat capacity of liquids in terms of phonon-like excitations but often rely on fitting parameters and ad hoc assumptions. In this work, we perform microscopic analysis on instantaneous normal modes and veloc… ▽ More Understanding thermodynamics in liquids at the atomic level is challenging because of strong atomic interactions and lack of symmetry. Recent prior theoretical works have focused on describing heat capacity of liquids in terms of phonon-like excitations but often rely on fitting parameters and ad hoc assumptions. In this work, we perform microscopic analysis on instantaneous normal modes and velocity autocorrelations on molecular dynamics simulations of single element systems over wide ranges of temperature (up to $10^8$ K) and pressure (up to 1 TPa). Our results demonstrate that heat capacity of liquids can be described by a combination of both solid-like and gas-like degrees of freedom, leading to a unified framework to describe heat capacity of all three phases of matter: solid, liquid, and gas. △ Less

Submitted 13 October, 2022; v1 submitted 12 October, 2022; originally announced October 2022.

Atomic-scale Deformation Process of Glasses Unveiled by Stress-induced Structural Anisotropy

Authors: Jie Dong, Hailong Peng, Hui Wang, Yang Tong, Yutian Wang, Wojciech Dmowski, Baoan Sun, Takeshi Egami, Weihua Wang, Haiyang Bai

Abstract: Experimentally resolving atomic-scale structural changes of a deformed glass remains challenging owing to the disordered nature of glass structure. Here, we show that the structural anisotropy emerges as a general hallmark for different types of glasses (metallic glasses, oxide glass, amorphous selenium, and polymer glass) after thermo-mechanical deformation, and it is highly correlates with local… ▽ More Experimentally resolving atomic-scale structural changes of a deformed glass remains challenging owing to the disordered nature of glass structure. Here, we show that the structural anisotropy emerges as a general hallmark for different types of glasses (metallic glasses, oxide glass, amorphous selenium, and polymer glass) after thermo-mechanical deformation, and it is highly correlates with local nonaffine atomic displacements detected by the high-energy X-ray diffraction technique. By analyzing the anisotropic pair density function, we unveil the atomic-level mechanism responsible for the plastic flow, which notably differs between metallic glasses and covalent glasses. The structural rearrangements in metallic glasses are mediated through cutting and formation of atomic bonds, which occurs in some localized inelastic regions embedded in elastic matrix, whereas that of covalent glasses is mediated through the rotation of atomic bonds or chains without bond length change, which occurs in a less localized manner. △ Less

Submitted 28 May, 2022; originally announced May 2022.

Comments: 19 pages, 4 figures

Dynamical heterogeneities in liquid and glass originate from medium-range order

Authors: Charles K. C. Lieou, Takeshi Egami

Abstract: Slow relaxation and plastic deformation in disordered materials such as metallic glasses and supercooled liquids occur at dynamical heterogeneities, or neighboring particles that rearrange in a correlated, cooperative manner. Dynamical heterogeneities have historically been described by a four-point, time-dependent density correlation function $χ_4 (r, t)$. In this paper, we posit that… ▽ More Slow relaxation and plastic deformation in disordered materials such as metallic glasses and supercooled liquids occur at dynamical heterogeneities, or neighboring particles that rearrange in a correlated, cooperative manner. Dynamical heterogeneities have historically been described by a four-point, time-dependent density correlation function $χ_4 (r, t)$. In this paper, we posit that $χ_4 (r, t)$ contains essentially the same information about medium-range order as the Van Hove correlation function $G(r, t)$. In other words, medium-range order is the origin of spatially correlated regions of cooperative particle motion. The Van Hove function is the preferred tool for describing dynamical heterogeneities than the four-point function, for which the physical meaning is less transparent. △ Less

Submitted 18 May, 2022; originally announced May 2022.

Mean-field model for the Curie-Weiss temperature dependence of coherence length in metallic liquids

Authors: Charles K. C. Lieou, Takeshi Egami

Abstract: The coherence length of the medium-range order (MRO) in metallic liquids is known to display a Curie-Weiss temperature dependence; its inverse is linearly related to temperature, and when extrapolated from temperatures above the glass transition, the coherence length diverges at a negative temperature with a critical exponent of unity. We propose a mean-field pseudospin model that explains this be… ▽ More The coherence length of the medium-range order (MRO) in metallic liquids is known to display a Curie-Weiss temperature dependence; its inverse is linearly related to temperature, and when extrapolated from temperatures above the glass transition, the coherence length diverges at a negative temperature with a critical exponent of unity. We propose a mean-field pseudospin model that explains this behavior. Specifically, we model the atoms and their local environment as Ising spins with antiferromagnetic exchange interactions. We further superimpose an exchange interaction between dynamical heterogeneities, or clusters of atoms undergoing cooperative motion. The coherence length in the metallic liquid is thus the correlation length between dynamical heterogeneities. Our results reaffirm the idea that the MRO coherence length is a measure of point-to-set correlations, and that local frustrations in the interatomic interactions are prominent in metallic liquids. △ Less

Submitted 26 January, 2022; originally announced January 2022.

Transient Nature of Fast Relaxation in Metallic Glass

Authors: Leo Zella, Jaeyun Moon, David Keffer, Takeshi Egami

Abstract: Metallic glasses exhibit fast mechanical relaxations at temperatures well below the glass transition, one of which shows little variation with temperature known as nearly constant loss (NCL). Despite the important implications of this phenomenon to in aging and deformation, the origin of the relaxation is unclear. Through molecular dynamics simulations of a model metallic glass, Cu_64.5Zr_35.5, we… ▽ More Metallic glasses exhibit fast mechanical relaxations at temperatures well below the glass transition, one of which shows little variation with temperature known as nearly constant loss (NCL). Despite the important implications of this phenomenon to in aging and deformation, the origin of the relaxation is unclear. Through molecular dynamics simulations of a model metallic glass, Cu_64.5Zr_35.5, we implement dynamic mechanical analysis with system stress decomposed into atomic-level stresses to identify the group of atoms responsible for NCL. This work demonstrates that NCL relaxation is due to fully transient groups of atoms that become normal over picosecond timescales. They are spatially distributed throughout the glass and have no outstanding features, rather than defect-like as previously reported. △ Less

Submitted 19 January, 2022; originally announced January 2022.

Structural effect on phonon attenuation in metallic liquids and glasses

Authors: Jaeyun Moon, Takeshi Egami

Abstract: The attenuation rate of vibrational excitations in various metallic liquids and glasses has been reported to change from the quadratic dependence on wavevector at low wavevectors to the linear dependence at high wavevectors. However, the origin of this behavior is not clear. Here, the analysis of this phenomenon through molecular dynamics is presented for prototypical metallic liquids, Cu56Zr44 an… ▽ More The attenuation rate of vibrational excitations in various metallic liquids and glasses has been reported to change from the quadratic dependence on wavevector at low wavevectors to the linear dependence at high wavevectors. However, the origin of this behavior is not clear. Here, the analysis of this phenomenon through molecular dynamics is presented for prototypical metallic liquids, Cu56Zr44 and Fe. It is shown that the crossover wavevector is strongly correlated with the structural coherence length characterizing coarse-grained density correlations. We suggest that the linear dependence is caused by scattering of vibrational excitations by structural activation processes with low activation energies which are distinctively observed in metallic systems. △ Less

Submitted 19 October, 2021; originally announced October 2021.

Structural magnetic glassiness in spin ice Dy$_2$Ti$_2$O$_7$

Authors: Anjana M. Samarakoon, Andre Sokolowski, Bastian Klemke, Ralf Feyerherm, Michael Meissner, R. A. Borzi, Feng Ye, Qiang Zhang, Zhiling Dun, Haidong Zhou, T. Egami, Jonathan N. Hallen, Ludovic Jaubert, Claudio Castelnovo, Roderich Moessner, S. A. Grigera, D. Alan Tennant

Abstract: The spin ice compound Dy$_2$Ti$_2$O$_7$ is well-known to realise a three-dimensional Coulomb spin liquid with magnetically charged monopole excitations. Its fate at low temperatures, however, remains an intriguing open question. Based on a low-temperature analysis of the magnetic noise and diffuse neutron scattering under different cooling protocols, combined with extensive numerical modelling, we… ▽ More The spin ice compound Dy$_2$Ti$_2$O$_7$ is well-known to realise a three-dimensional Coulomb spin liquid with magnetically charged monopole excitations. Its fate at low temperatures, however, remains an intriguing open question. Based on a low-temperature analysis of the magnetic noise and diffuse neutron scattering under different cooling protocols, combined with extensive numerical modelling, we argue that upon cooling, the spins freeze into what may be termed a `structural magnetic glass', without an a priori need for chemical or structural disorder. Specifically, our model indicates the presence of frustration on two levels, first producing a near-degenerate constrained manifold inside which phase ordering kinetics is in turn frustrated. Our results suggest that spin ice Dy$_2$Ti$_2$O$_7$ provides one prototype of magnetic glass formation specifically, and a setting for the study of kinetically constrained systems more generally. △ Less

Submitted 25 October, 2022; v1 submitted 26 July, 2021; originally announced July 2021.

Comments: Paper+Sup. Info

Journal ref: Phys. Rev. Research 4, 033159 (2022)

Split-pulse X-ray photon correlation spectroscopy with seeded X-rays from X-ray laser to study atomic-level dynamics

Authors: Yuya Shinohara, Taito Osaka, Ichiro Inoue, Takuya Iwashita, Wojciech Dmowski, Chae Woo Ryu, Yadu Sarathchandran, Takeshi Egami

Abstract: With their brilliance and temporal structure, X-ray free-electron laser can unveil atomic-scale details of ultrafast phenomena. Recent progress in split-and-delay optics (SDO), which produces two X-ray pulses with time-delays, offers bright prospects for observing dynamics at the atomic-scale. However, their insufficient pulse energy has limited its application either to phenomena with longer corr… ▽ More With their brilliance and temporal structure, X-ray free-electron laser can unveil atomic-scale details of ultrafast phenomena. Recent progress in split-and-delay optics (SDO), which produces two X-ray pulses with time-delays, offers bright prospects for observing dynamics at the atomic-scale. However, their insufficient pulse energy has limited its application either to phenomena with longer correlation length or to measurement with a fixed delay-time. Here we show that the combination of the SDO and self-seeding of X-rays increases the pulse energy and makes it possible to observe the atomic-scale dynamics in a timescale of picoseconds. We show that the speckle contrast in scattering from water depends on the delay-time as expected. Our results demonstrate the capability of measurement using the SDO with seeded X-rays for resolving the dynamics in temporal and spatial scales that are not accessible by other techniques, opening opportunities for studying the atomic-level dynamics. △ Less

Submitted 23 December, 2020; v1 submitted 22 May, 2020; originally announced May 2020.

Comments: 22 pages, 5 figures

Journal ref: Nature Communications 11, 6213 (2020)

Critical spin fluctuation mechanism for the spin Hall effect

Authors: Satoshi Okamoto, Takeshi Egami, Naoto Nagaosa

Abstract: We propose mechanisms for the spin Hall effect in metallic systems arising from the coupling between conduction electrons and local magnetic moments that are dynamically fluctuating. Both a side-jump-type mechanism and a skew-scattering-type mechanism are considered. In either case, dynamical spin fluctuation gives rise to a nontrivial temperature dependence in the spin Hall conductivity. This lea… ▽ More We propose mechanisms for the spin Hall effect in metallic systems arising from the coupling between conduction electrons and local magnetic moments that are dynamically fluctuating. Both a side-jump-type mechanism and a skew-scattering-type mechanism are considered. In either case, dynamical spin fluctuation gives rise to a nontrivial temperature dependence in the spin Hall conductivity. This leads to the enhancement in the spin Hall conductivity at nonzero temperatures near the ferromagnetic instability. The proposed mechanisms could be observed in $4d$ or $5d$ metallic compounds. △ Less

Submitted 7 November, 2019; v1 submitted 26 May, 2019; originally announced May 2019.

Comments: 5 pages for the main text with 4 figures, and 5 pages with 1 figure for the supplementary material

Journal ref: Phys. Rev. Lett. 123, 196603 (2019)

Anomalous local distortion in BCC refractory high-entropy alloys

Authors: Yang Tong, Shijun Zhao, Hongbin Bei, Takeshi Egami, Yanwen Zhang, Fuxiang Zhang

Abstract: Whereas exceptional mechanical and radiation performances have been found in the emergent medium- and high-entropy alloys (MEAs and HEAs), the importance of their complex atomic environment, reflecting diversity in atomic size and chemistry, to defect transport has been largely unexplored at the atomic level. Here we adopt a local structure approach based on the atomic pair distribution function m… ▽ More Whereas exceptional mechanical and radiation performances have been found in the emergent medium- and high-entropy alloys (MEAs and HEAs), the importance of their complex atomic environment, reflecting diversity in atomic size and chemistry, to defect transport has been largely unexplored at the atomic level. Here we adopt a local structure approach based on the atomic pair distribution function measurements in combination with density functional theory calculations to investigate a series of body-centered cubic (BCC) MEAs and HEAs. Our results demonstrate that all alloys exhibit local lattice distortions (LLD) to some extent, but an anomalous LLD, merging of the first and second atomic shells, occurs only in the Zr- and/or Hf-containing MEAs and HEAs. In addition, through the ab-initio simulations we show that charge transfer among the elements profoundly reduce the size mismatch effect. The observed competitive coexistence between LLD and charge transfer not only demonstrates the importance of the electronic effects on the local environments in MEAs and HEAs, but also provides new perspectives to in-depth understanding of the complicated defect transport in these alloys. △ Less

Submitted 25 February, 2019; originally announced February 2019.

Comments: 5 figures, 2 tables

Experimental measurements of the temperature-dependent Van Hove function in a $\text{Zr}_{80} \text{Pt}_{20}$ liquid

Authors: R. Ashcraft, Z. Wang, D. L. Abernathy, D. G. Quirinale, T. Egami, K. F. Kelton

Abstract: Even though the viscosity is one of the most fundamental properties of liquids, the connection with the atomic structure of the liquid has proven elusive. By combining inelastic neutron scattering with the electrostatic levitation technique the time-dependent pair-distribution function (i.e. the Van Hove function) has been determined for liquid Zr80Pt20. We show that the decay-time of the first pe… ▽ More Even though the viscosity is one of the most fundamental properties of liquids, the connection with the atomic structure of the liquid has proven elusive. By combining inelastic neutron scattering with the electrostatic levitation technique the time-dependent pair-distribution function (i.e. the Van Hove function) has been determined for liquid Zr80Pt20. We show that the decay-time of the first peak of the Van Hove function is directly related to the Maxwell relaxation time of the liquid, which is proportional to the shear viscosity. This result demonstrates that the local dynamics for increasing or decreasing the coordination number of local clusters by one determines the viscosity at high temperature, supporting earlier predictions from molecular dynamics simulations. △ Less

Submitted 5 July, 2019; v1 submitted 4 October, 2018; originally announced October 2018.

Comments: Main article: 7 pages, 7 figures

Metallic glasses for spintronics: anomalous temperature dependence and giant enhancement of inverse spin Hall effect

Authors: W. Jiao, D. Z. Hou, C. Chen, H. Wang, Y. Z. Zhang, Y. Tian, Z. Y. Qiu, S. Okamoto, K. Watanabe, A. Hirata, T. Egami, E. Saitoh, M. W. Chen

Abstract: Spin-charge conversion via inverse spin Hall effect (ISHE) is essential for enabling various applications of spintronics. The spin Hall response usually follows a universal scaling relation with longitudinal electric resistivity and has mild temperature dependence because elementary excitations play only a minor role in resistivity and hence ISHE. Here we report that the ISHE of metallic glasses s… ▽ More Spin-charge conversion via inverse spin Hall effect (ISHE) is essential for enabling various applications of spintronics. The spin Hall response usually follows a universal scaling relation with longitudinal electric resistivity and has mild temperature dependence because elementary excitations play only a minor role in resistivity and hence ISHE. Here we report that the ISHE of metallic glasses shows nearly two orders of magnitude enhancements with temperature increase from a threshold of 80-100 K to glass transition points. As electric resistivity changes only marginally in the temperature range, the anomalous temperature dependence is in defiance of the prevailing scaling law. Such a giant temperature enhancement can be well described by a two-level thermal excitation model of glasses and disappears after crystallization, suggesting a new mechanism which involves unique thermal excitations of glasses. This finding may pave new ways to achieve high spin-charge conversion efficiency at room and higher temperatures for spintronic devices and to detect structure and dynamics of glasses using spin currents. △ Less

Submitted 30 August, 2018; originally announced August 2018.

Stabilization of Polar Nano Regions in Pb-free ferroelectrics

Authors: A. Pramanick, W. Dmowski, T. Egami, A. Setiadi Budisuharto, F. Weyland, N. Novak, A. D. Christianson, J. M. Borreguero, D. L. Abernathy, MRV Jørgensen

Abstract: Formation of polar nano regions through solid-solution additions are known to enhance significantly the functional properties of ferroelectric materials. Despite considerable progress in characterizing the microscopic behavior of polar nano regions, understanding their real-space atomic structure and dynamics of formation remains a considerable challenge. Here, using the method of dynamic pair dis… ▽ More Formation of polar nano regions through solid-solution additions are known to enhance significantly the functional properties of ferroelectric materials. Despite considerable progress in characterizing the microscopic behavior of polar nano regions, understanding their real-space atomic structure and dynamics of formation remains a considerable challenge. Here, using the method of dynamic pair distribution function, we provide direct insights into the role of solid-solution additions towards the stabilization of polar nano regions in the Pb-free ferroelectric of Ba(Zr,Ti)O3. It is shown that for an optimum level of substitution of Ti by larger Zr ions, the dynamics of atomic displacements for ferroelectric polarization are slowed sufficiently, which leads to increased local correlation among dipoles below THz frequencies. The dynamic pair distribution function technique demonstrates unique capability to obtain insights into locally correlated atomic dynamics in disordered materials, including new Pb-free ferroelectrics, which is necessary to understand and control their functional properties. △ Less

Submitted 24 October, 2017; originally announced October 2017.

Journal ref: Phys. Rev. Lett. 120, 207603 (2018)

Dynamically Correlated Region in Sheared Colloidal Glasses Revealed by Neutron Scattering

Authors: Zhe Wang, Takuya Iwashita, Lionel Porcar, Yangyang Wang, Yun Liu, Luis E. Sanchez-Diaz, Bin Wu, Takeshi Egami, Wei-Ren Chen

Abstract: The microscopic deformation mechanism of charged colloidal glasses with extended-range interactions under shear is investigated by in-situ small-angle neutron scattering, and a dynamically correlated region (DCR) is identified. This short-lived region provides the resistance to the configurational rearrangement imposed by the external deformation, as evidenced by the evolution of the size of DCR i… ▽ More The microscopic deformation mechanism of charged colloidal glasses with extended-range interactions under shear is investigated by in-situ small-angle neutron scattering, and a dynamically correlated region (DCR) is identified. This short-lived region provides the resistance to the configurational rearrangement imposed by the external deformation, as evidenced by the evolution of the size of DCR in the shear thinning regime and the quantitative agreement between the local stress sustained by DCR and the macroscopic stress from rheological measurements at low and mediate shear rates. This finding suggests that DCR is an important quantity for microscopically addressing the flow and deformation behavior of strongly interacting colloids. △ Less

Submitted 21 September, 2017; originally announced September 2017.

Comments: arXiv admin note: substantial text overlap with arXiv:1611.03135

Local Elasticity in Nonlinear Rheology of Interacting Colloidal Glasses Revealed by Neutron Scattering and Rheometry

Authors: Zhe Wang, Takuya Iwashita, Lionel Porcar, Yangyang Wang, Yun Liu, Luis E. Sanchez-Diaz, Bin Wu, Guan-Rong Huang, Takeshi Egami, Wei-Ren Chen

Abstract: The flow of colloidal suspensions is ubiquitous in nature and industry. Colloidal suspensions exhibit a wide range of rheological behavior, which should be closely related to the microscopic structure of the systems. With in-situ small-angle neutron scattering complemented by rheological measurements, we investigated the deformation behavior of a charge-stabilized colloidal glass at particle level… ▽ More The flow of colloidal suspensions is ubiquitous in nature and industry. Colloidal suspensions exhibit a wide range of rheological behavior, which should be closely related to the microscopic structure of the systems. With in-situ small-angle neutron scattering complemented by rheological measurements, we investigated the deformation behavior of a charge-stabilized colloidal glass at particle level undergoing steady shear. A short-lived, localized elastic response at particle level, termed as transient elasticity zone (TEZ), was identified from the neutron spectra. The existence of the TEZ is a signature of the dynamical heterogeneity: The body of fluids under shear behaves like an elastic solid within the spatial range of TEZ but like fluid outside the TEZ. The size of TEZ shrinks as the shear rate increases in the shear thinning region, which shows that the shear thinning is accompanied by a diminishing dynamical heterogeneity. More interestingly, the TEZ is found to be the structural unit that provides the resistance to the imposed shear, as evidenced by the quantitative agreement between the local elastic stress sustained by TEZ and the macroscopic stress from rheological measurements at low and moderate shear rates. Besides the charged-stabilized colloidal suspension, a hard-sphere colloidal suspension at the same volume fraction and shear rates was also measured. The result highlights the key role of the electrostatic interparticle repulsion in promoting the local elasticity. Our findings provide an understanding on the nonlinear rheology of interacting colloidal glasses from a micro-mechanical view. △ Less

Submitted 12 December, 2018; v1 submitted 9 November, 2016; originally announced November 2016.

Comments: 18 pages,5 figures

Correlation between Fragility and the Arrhenius Crossover Phenomenon in Metallic, Molecular, and Network Liquids

Authors: Abhishek Jaiswal, Takeshi Egami, K. F. Kelton, Kenneth S. Schweizer, Yang Zhang

Abstract: We report the observation of a distinct correlation between the kinetic fragility index $m$ and the reduced Arrhenius crossover temperature $θ_A = T_A/T_g$ in various glass-forming liquids, identifying three distinguishable groups. In particular, for 11 glass-forming metallic liquids, we universally observe a crossover in the mean diffusion coefficient from high-temperature Arrhenius to low-temper… ▽ More We report the observation of a distinct correlation between the kinetic fragility index $m$ and the reduced Arrhenius crossover temperature $θ_A = T_A/T_g$ in various glass-forming liquids, identifying three distinguishable groups. In particular, for 11 glass-forming metallic liquids, we universally observe a crossover in the mean diffusion coefficient from high-temperature Arrhenius to low-temperature super-Arrhenius behavior at approximately $θ_A \approx 2$ which is in the stable liquid phases. In contrast, for fragile molecular liquids, this crossover occurs at much lower $θ_A \approx 1.4$ and usually in their supercooled states. The $θ_A$ values for strong network liquids spans a wide range higher than 2. Intriguingly, the high-temperature activation barrier $E_\infty$ is universally found to be $\sim 11\ k_B T_g$ and uncorrelated with the fragility or the reduced crossover temperature $θ_A$ for metallic and molecular liquids. These observations provide a way to estimate the low-temperature glassy characteristics ($T_g$ and $m$) from the high-temperature liquid quantities ($E_\infty$ and $θ_A$). △ Less

Submitted 13 December, 2016; v1 submitted 29 April, 2016; originally announced April 2016.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 117, 205701 (2016)

Selective control of oxygen sublattice stability by epitaxial strain in Ruddlesden-Popper films

Authors: Tricia L. Meyer, Lu Jiang, Jaekwang Lee, Mina Yoon, John W. Freeland, Jae Hyuck Jang, Dilpuneet S. Aidhy, Albina Borisevich, Matthew Chisholm, Takeshi Egami, Ho Nyung Lee

Abstract: Oxygen-defect control has long been considered an influential tuning knob for producing various property responses in complex oxide films. In addition to physical property changes, modification to the lattice structure, specifically lattice expansion, with increasing oxygen vacancy concentrations has been reported often and has become the convention for oxide materials. However, the current unders… ▽ More Oxygen-defect control has long been considered an influential tuning knob for producing various property responses in complex oxide films. In addition to physical property changes, modification to the lattice structure, specifically lattice expansion, with increasing oxygen vacancy concentrations has been reported often and has become the convention for oxide materials. However, the current understanding of the lattice behavior in oxygen-deficient films becomes disputable when considering compounds containing different bonding environments or atomic layering. Moreover, tensile strain has recently been discovered to stabilize oxygen vacancies in epitaxial films, which further complicates the interpretation of lattice behavior resulting from their appearance. Here, we report on the selective strain control of oxygen vacancy formation and resulting lattice responses in the layered, Ruddlesden-Popper phases, La1.85Sr0.15CuO4. We found that a drastically reduced Gibbs free energy for oxygen vacancy formation near the typical growth temperature for tensile-strained epitaxial LSCO accounts for the large oxygen non-stoichiometry. Additionally, oxygen vacancies form preferentially in the equatorial position of the CuO2 plane, leading to a lattice contraction, rather than the expected expansion, observed with apical oxygen vacancies. Since oxygen stoichiometry plays a key role in determining the physical properties of many complex oxides, the strong strain coupling of oxygen nonstoichiometry and the unusual structural response reported here can provide new perspectives and understanding to the structure and property relationships of many other functional oxide materials. △ Less

Submitted 27 August, 2015; originally announced August 2015.

Journal ref: APL Mater. 3, 126102 (2015)

Growth control of the oxidation state in vanadium oxide thin films

Authors: Shinbuhm Lee, Tricia L. Meyer, Sungkyun Park, Takeshi Egami, Ho Nyung Lee

Abstract: Precise control of the chemical valence or oxidation state of vanadium in vanadium oxide thin films is highly desirable for not only fundamental research, but also technological applications that utilize the subtle change in the physical properties originating from the metal- insulator transition (MIT) near room temperature. However, due to the multivalent nature of vanadium and the lack of a good… ▽ More Precise control of the chemical valence or oxidation state of vanadium in vanadium oxide thin films is highly desirable for not only fundamental research, but also technological applications that utilize the subtle change in the physical properties originating from the metal- insulator transition (MIT) near room temperature. However, due to the multivalent nature of vanadium and the lack of a good understanding on growth control of the oxidation state, stabilization of phase pure vanadium oxides with a single oxidation state is extremely challenging. Here, we systematically varied the growth conditions to clearly map out the growth window for preparing phase pure epitaxial vanadium oxides by pulsed laser deposition for providing a guideline to grow high quality thin films with well-defined oxidation states of V2(+3)O3, V(+4)O2, and V2(+5)O5. A well pronounced MIT was only observed in VO2 films grown in a very narrow range of oxygen partial pressure P(O2). The films grown either in lower (< 10 mTorr) or higher P(O2) (> 25 mTorr) result in V2O3 and V2O5 phases, respectively, thereby suppressing the MIT for both cases. We have also found that the resistivity ratio before and after the MIT of VO2 thin films can be further enhanced by one order of magnitude when the films are further oxidized by post-annealing at a well-controlled oxidizing ambient. This result indicates that stabilizing vanadium into a single valence state has to compromise with insufficient oxidation of an as grown thin film and, thereby, a subsequent oxidation is required for an improved MIT behavior. △ Less

Submitted 28 November, 2014; originally announced November 2014.

Comments: 16 pages, 4 figures

Local Energy Landscape in a Simple Liquid

Authors: Takuya Iwashita, Takeshi Egami

Abstract: It is difficult to relate the properties of liquids and glasses directly to their structure because of complexity in the structure which defies precise definition. The potential energy landscape (PEL) approach is a very insightful way to conceptualize the structure-property relationship in liquids and glasses, particularly on the effect of temperature and history. However, because of the highly mu… ▽ More It is difficult to relate the properties of liquids and glasses directly to their structure because of complexity in the structure which defies precise definition. The potential energy landscape (PEL) approach is a very insightful way to conceptualize the structure-property relationship in liquids and glasses, particularly on the effect of temperature and history. However, because of the highly multi-dimensional nature of the PEL it is hard to determine, or even visualize, the actual details of the energy landscape. In this article we introduce a modified concept of the local energy landscape (LEL) which is limited in phase space, and demonstrate its usefulness using molecular dynamics simulation on a simple liquid at high temperatures. The local energy landscape is given as a function of the local coordination number, the number of the nearest neighbor atoms. The excitations in the LEL corresponds to the so-called beta-relaxation process. The LEL offers a simple but useful starting point to discuss complex phenomena in liquids and glasses. △ Less

Submitted 31 October, 2014; originally announced October 2014.

Journal ref: Phys. Rev. E 90, 052307 (2014)

Anisotropic neutron spin resonance in underdoped superconducting NaFe1-xCoxAs

Authors: Chenglin Zhang, Yu Song, L. P. Regnault, Yixi Su, M. Enderle, J. Kulda, Guotai Tan, Zachary C. Sims, Takeshi Egami, Qimiao Si, Pengcheng Dai

Abstract: We use polarized inelastic neutron scattering (INS) to study spin excitations in superconducting NaFe0.985Co0.015As (C15) with static antiferromagnetic (AF) order along the a-axis of the orthorhombic structure and NaFe0.935Co0.045As (C45) without AF order. In previous unpolarized INS work, spin excitations in C15 were found to have a dispersive sharp resonance near Er1=3.25 meV and a broad dispers… ▽ More We use polarized inelastic neutron scattering (INS) to study spin excitations in superconducting NaFe0.985Co0.015As (C15) with static antiferromagnetic (AF) order along the a-axis of the orthorhombic structure and NaFe0.935Co0.045As (C45) without AF order. In previous unpolarized INS work, spin excitations in C15 were found to have a dispersive sharp resonance near Er1=3.25 meV and a broad dispersionless mode at Er2=6 meV. Our neutron polarization analysis reveals that the dispersive resonance in C15 is highly anisotropic and polarized along the a- and c-axis, while the dispersionless mode is isotropic similar to that of C45. Since the a-axis polarized spin excitations of the anisotropic resonance appear below Tc, our data suggests that the itinerant electrons contributing to the magnetism are also coupled to the superconductivity. △ Less

Submitted 23 September, 2014; originally announced September 2014.

Comments: PRB(R), 2014

Unexpected Universality in the Viscosity of Metallic Liquids

Authors: M. Blodgett, T. Egami, Z. Nussinov, K. F. Kelton

Abstract: The range of the magnitude of the liquid viscosity as a function of the temperature (T) is one of the most impressive of any physical property, changing by approximately 17 orders of magnitude from its extrapolated value at infinite temperature to that at the glass transition. We present experimental measurements of containerlessly processed metallic liquids that reveal that the ratio of the visco… ▽ More The range of the magnitude of the liquid viscosity as a function of the temperature (T) is one of the most impressive of any physical property, changing by approximately 17 orders of magnitude from its extrapolated value at infinite temperature to that at the glass transition. We present experimental measurements of containerlessly processed metallic liquids that reveal that the ratio of the viscosity to its extrapolated infinite temperature value follows a universal function of Tcoop/T. The temperature Tcoop corresponds to the onset of cooperative motion and is strongly correlated with the glass transition temperature. On average the extrapolated infinite temperature viscosity is found to be nh, where h is Planck's constant and n is the particle number density. A surprising universality in the viscosity of metallic liquids and its relation to the glass transition is demonstrated. △ Less

Submitted 28 July, 2014; originally announced July 2014.

Comments: 28 pages, 6 figures, 3 tables

Fractional Einstein relation for strongly disordered semiconductors

Authors: Takeshi Egami, Koshiro Suzuki, Katsuhiro Watanabe

Abstract: A novel Einstein relation (fractional Einstein relation, FER) for the electric conduction in non-crystalline semiconductors is presented. FER and the generalized Einstein relation (GER) [Phys. Rev. E 8, 1296 (1998)] are compared to the result of the Monte Carlo (MC) simulation, and is confirmed that FER exhibits better agreement than GER. The cruial feature of FER is that it reflects the violation… ▽ More A novel Einstein relation (fractional Einstein relation, FER) for the electric conduction in non-crystalline semiconductors is presented. FER and the generalized Einstein relation (GER) [Phys. Rev. E 8, 1296 (1998)] are compared to the result of the Monte Carlo (MC) simulation, and is confirmed that FER exhibits better agreement than GER. The cruial feature of FER is that it reflects the violation of the detailed balance in the coarse-grained hopping process, while it is preserved in the original Einstein relation or GER. △ Less

Submitted 1 June, 2014; originally announced June 2014.

Comments: 5 pages, 3 figures

Dynamical Crossover in Soft Colloids below the Overlap Concentration

Authors: Xin Li, Luis E. Sánchez-Diáz, Bin Wu, William A. Hamilton, Lionel Porcar, Péter Falus, Yun Liu, Changwoo Do, Gregory S. Smith, Takeshi Egami, Wei-Ren Chen

Abstract: The dynamics of soft colloids in solutions is characterized by internal collective motion as well as center-of-mass diffusion. Using neutron scattering we demonstrate that the competition between the relaxation processes associated with these two degrees of freedom results in strong dependence of dynamics and structure on colloid concentration, c, even well below the overlap concentration c*. We s… ▽ More The dynamics of soft colloids in solutions is characterized by internal collective motion as well as center-of-mass diffusion. Using neutron scattering we demonstrate that the competition between the relaxation processes associated with these two degrees of freedom results in strong dependence of dynamics and structure on colloid concentration, c, even well below the overlap concentration c*. We show that concurrent with increasing inter-particle collisions, substantial structural dehydration and slowing-down of internal dynamics occur before geometrically defined colloidal overlap develops. While previous experiments have shown that the average size of soft colloids changes very little below c*, we find a marked change in both the internal structure and internal dynamics with concentration. The competition between these two relaxation processes gives rise to a new dynamically-defined dilute threshold concentration well below c*. △ Less

Submitted 24 April, 2014; v1 submitted 7 March, 2014; originally announced March 2014.

Comments: 5 pages, 3 figures in the manuscript; 17 pages, 8 figures and 3 tables in the supplementary material

Tunneling Electroresistance Induced by Interfacial Phase Transitions in Ultrathin Oxide Heterostructures

Authors: Lu Jiang, Woo Seok Choi, Hyoungjeen Jeen, Shuai Dong, Yunseok Kim, Myung-Geun Han, Yimei Zhu, Sergei V. Kalinin, Elbio Dagotto, Takeshi Egami, Ho Nyung Lee

Abstract: The ferroelectric (FE) control of electronic transport is one of the emerging technologies in oxide heterostructures. Many previous studies in FE tunnel junctions (FTJs) exploited solely the differences in the electrostatic potential across the FTJs that are induced by changes in the FE polarization direction. Here, we show that in practice the junction current ratios between the two polarization… ▽ More The ferroelectric (FE) control of electronic transport is one of the emerging technologies in oxide heterostructures. Many previous studies in FE tunnel junctions (FTJs) exploited solely the differences in the electrostatic potential across the FTJs that are induced by changes in the FE polarization direction. Here, we show that in practice the junction current ratios between the two polarization states can be further enhanced by the electrostatic modification in the correlated electron oxide electrodes, and that FTJs with nanometer thin layers can effectively produce a considerably large electroresistance ratio at room temperature. To understand these surprising results, we employed an additional control parameter, which is related to the crossing of electronic and magnetic phase boundaries of the correlated electron oxide. The FE-induced phase modulation at the heterointerface ultimately results in an enhanced electroresistance effect. Our study highlights that the strong coupling between degrees of freedom across heterointerfaces could yield versatile and novel applications in oxide electronics. △ Less

Submitted 19 November, 2013; originally announced November 2013.

Comments: 23 pages, 5 figures, 1 supporting information

Double neutron spin resonances and gap anisotropy in underdoped superconducting NaFe0.985Co0.015As

Authors: Chenglin Zhang, Rong Yu, Yixi Su, Yu Song, Miaoyin Wang, Guotai Tan, Takeshi Egami, J. A. Fernandez-Baca, Enrico Faulhaber, Qimiao Si, Pengcheng Dai

Abstract: We use inelastic neutron scattering to show that superconductivity in electron-underdoped NaFe0.985Co0.015As induces a dispersive sharp resonance near Er1 = 3:25 meV and a broad dis- persionless mode at Er2 = 6 meV. However, similar measurements on overdoped superconducting NaFe0:955Co0:045As find only a single sharp resonance at Er = 7 meV. We connect these results with the observations of angle-… ▽ More We use inelastic neutron scattering to show that superconductivity in electron-underdoped NaFe0.985Co0.015As induces a dispersive sharp resonance near Er1 = 3:25 meV and a broad dis- persionless mode at Er2 = 6 meV. However, similar measurements on overdoped superconducting NaFe0:955Co0:045As find only a single sharp resonance at Er = 7 meV. We connect these results with the observations of angle-resolved photoemission spectroscopy that the superconducting gaps in the electron Fermi pockets are anisotropic in the underdoped material but become isotropic in the overdoped case. Our analysis indicates that both the double neutron spin resonances and gap anisotropy originate from the orbital dependence of the superconducting pairing in the iron pnic- tides. Our discovery also shows the importance of the inelastic neutron scattering in detecting the multiorbital superconducting gap structures of iron pnictides. △ Less

Submitted 17 October, 2013; originally announced October 2013.

Journal ref: PRL111,207002 (2013)

DC electric field effect on the anomalous exponent of the hopping conduction in the one-dimensional disorder model

Authors: Takeshi Egami, Koshiro Suzuki, Katsuhiro Watanabe

Abstract: DC electric field effect on the anomalous exponent of the hopping conduction in the disorder model is investigated. First, we explain the model and derive an analytical expression of the effective waiting time for the general case. We show that the exponent depends on the external field. Then we focus on a one-dimensional system in order to illustrate the features of the anomalous exponent. We der… ▽ More DC electric field effect on the anomalous exponent of the hopping conduction in the disorder model is investigated. First, we explain the model and derive an analytical expression of the effective waiting time for the general case. We show that the exponent depends on the external field. Then we focus on a one-dimensional system in order to illustrate the features of the anomalous exponent. We derive approximate expressions of the anomalous exponent of the system analytically. For the case of a weak field, the anomalous exponent is consistent with that of diffusive systems. This is consistent with the treatments of Barkai et al. [Phys. Rev. E {\bf 63}, 046118 (2001)] and our result supports their theory. On the other hand, for the case of a strong field and a strong disorder, the time evolution of the exponent clearly differs from that in the weak field. The exponent is consistent with the well-known expression of the anomalous exponent in the Multiple Trapping Model at mesoscopic time scales. In the long time limit, a transition of the anomalous exponent to the same value of the weak field occurs. For the case of a strong field and a weak disorder, the exponent is equal to 1 and thus the diffusion is normal. These findings are verified by the Monte Carlo simulation. △ Less

Submitted 9 September, 2013; originally announced September 2013.

Comments: 14 pages, 7 figures

Journal ref: Phys.Rev.E 88, 052123 (2013)

Elementary excitations and crossover phenomenon in liquids

Authors: T. Iwashita, D. M. Nicholson, T. Egami

Abstract: The elementary excitations of vibration in solids are phonons. But in liquids phonons are extremely short-lived and marginalized. In this letter through classical and ab-initio molecular dynamics simulations of the liquid state of various metallic systems we show that different excitations, the local configurational excitations in the atomic connectivity network, are the elementary excitations in… ▽ More The elementary excitations of vibration in solids are phonons. But in liquids phonons are extremely short-lived and marginalized. In this letter through classical and ab-initio molecular dynamics simulations of the liquid state of various metallic systems we show that different excitations, the local configurational excitations in the atomic connectivity network, are the elementary excitations in high temperature metallic liquids. We also demonstrate that the competition between the configurational excitations and phonons determines the so-called crossover phenomenon in liquids. These discoveries open the way to the explanation of various complex phenomena in liquids, such as fragility and the rapid increase in viscosity toward the glass transition, in terms of these excitations. △ Less

Submitted 24 April, 2013; originally announced April 2013.

Journal ref: Phys. Rev. Lett. 110, 205504 (2013)

Magnetic anisotropy in hole-doped superconducting Ba 0.67K 0.33Fe 2As2 probed by polarized inelastic neutron scattering

Authors: Chenglin Zhang, mengshu liu, Yixi Su, Louis-Pierre Regnault, Meng Wang, Guotai Tan, Th. Bruckel, Takeshi Egami, Pengcheng Dai

Abstract: We use polarized inelastic neutron scattering (INS) to study spin excitations of optimally hole-doped superconductor Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_{2}$ ($T_c=38$ K). In the normal state, the imaginary part of the dynamic susceptibility, $χ^{\prime\prime}(Q,ω)$, shows magnetic anisotropy for energies below $\sim$7 meV with c-axis polarized spin excitations larger than that of the in-plane compon… ▽ More We use polarized inelastic neutron scattering (INS) to study spin excitations of optimally hole-doped superconductor Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_{2}$ ($T_c=38$ K). In the normal state, the imaginary part of the dynamic susceptibility, $χ^{\prime\prime}(Q,ω)$, shows magnetic anisotropy for energies below $\sim$7 meV with c-axis polarized spin excitations larger than that of the in-plane component. Upon entering into the superconducting state, previous unpolarized INS experiments have shown that spin gaps at $\sim$5 and 0.75 meV open at wave vectors $Q=(0.5,0.5,0)$ and $(0.5,0.5,1)$, respectively, with a broad neutron spin resonance at $E_r=15$ meV. Our neutron polarization analysis reveals that the large difference in spin gaps is purely due to different spin gaps in the c-axis and in-plane polarized spin excitations, resulting resonance with different energy widths for the c-axis and in-plane spin excitations. The observation of spin anisotropy in both opitmally electron and hole-doped BaFe$_2$As$_2$ is due to their proximity to the AF ordered BaFe$_2$As$_2$ where spin anisotropy exists below $T_N$. △ Less

Submitted 7 January, 2013; originally announced January 2013.

Comments: 5 pages, 4 figures

Journal ref: Physical Review B 87, 081101(R) (2013)

Local structural variation as source of magnetic moment reduction in BaFe2As2

Authors: Jennifer L. Niedziela, M. A. McGuire, T. Egami

Abstract: We report time-of-flight neutron powder diffraction results on stoichiometric BaFe2As2. Pair distribution function analysis shows that the orthorhombic distortion in the a-b plane at short distances are significantly different from the average lattice distortion, indicating local variations in the lattice at the short-range. We propose that this local variation reflects a high density of nano-twin… ▽ More We report time-of-flight neutron powder diffraction results on stoichiometric BaFe2As2. Pair distribution function analysis shows that the orthorhombic distortion in the a-b plane at short distances are significantly different from the average lattice distortion, indicating local variations in the lattice at the short-range. We propose that this local variation reflects a high density of nano-twins, short-ranged structures which locally affect the magnetic alignment. This results suggests that the discrepancies between the observed and calculated magnetic moments in BaFe2As2 arise partly from short-ranged variation of the lattice in the a-b plane. △ Less

Submitted 21 November, 2012; originally announced November 2012.

Comments: Accepted in Phys. Rev. B

Journal ref: Phys. Rev. B 86, 174113 (2012)

Strongly coupled phase transition in ferroelectric/correlated electron oxide heterostructures

Authors: Lu Jiang, Woo Seok Choi, Hyoungjeen Jeen, Takeshi Egami, Ho Nyung Lee

Abstract: We fabricated ultrathin ferroelectric/correlated electron oxide heterostructures composed of the ferroelectric Pb(Zr0.2Ti0.8)O3 and the correlated electron oxide (CEO) La0.8Sr0.2MnO3 on SrTiO3 substrates by pulsed laser epitaxy. The hole accumulation in the ultrathin CEO layer was substantially modified by heterostructuring with the ferroelectric layer, resulting in an insulator-metal transition.… ▽ More We fabricated ultrathin ferroelectric/correlated electron oxide heterostructures composed of the ferroelectric Pb(Zr0.2Ti0.8)O3 and the correlated electron oxide (CEO) La0.8Sr0.2MnO3 on SrTiO3 substrates by pulsed laser epitaxy. The hole accumulation in the ultrathin CEO layer was substantially modified by heterostructuring with the ferroelectric layer, resulting in an insulator-metal transition. In particular, our thickness dependent study showed that drastic changes in transport and magnetic properties were strongly coupled to the modulation of charge carriers by ferroelectric field effect, which was confined to the vicinity of the interface. Thus, our results provide crucial evidence that strong ferroelectric field effect control can be achieved in ultrathin (10 nm) heterostructures, yielding at least a 100,000-fold change in resistivity. △ Less

Submitted 19 July, 2012; originally announced July 2012.

Journal ref: Appl. Phys. Lett. 101, 042902 (2012)

Atomic Mechanism of Flow in Simple Liquids under Shear

Authors: Takuya Iwashita, Takeshi Egami

Abstract: Atomic correlations in a simple liquid in steady-state flow under shear stress were studied by molecular dynamics simulation. The local atomic level strain was determined through the anisotropic pair-density function (PDF). The atomic level strain has a limited spatial extension whose range is dependent on the strain rate and extrapolates to zero at the critical strain rate. A failure event is ide… ▽ More Atomic correlations in a simple liquid in steady-state flow under shear stress were studied by molecular dynamics simulation. The local atomic level strain was determined through the anisotropic pair-density function (PDF). The atomic level strain has a limited spatial extension whose range is dependent on the strain rate and extrapolates to zero at the critical strain rate. A failure event is identified with altering the local topology of atomic connectivity by exchanging bonds among neighboring atoms. △ Less

Submitted 10 April, 2012; v1 submitted 10 April, 2012; originally announced April 2012.

Journal ref: Phys. Rev. Lett. 108, 196001 (2012)

Glass Dynamics at High Strain Rates

Authors: J. S. Langer, Takeshi Egami

Abstract: We present a shear-transformation-zone (STZ) theoretical analysis of molecular-dynamics simulations of a rapidly sheared metallic glass. These simulations are especially revealing because, although they are limited to high strain rates, they span temperatures ranging from well below to well above the glass transition. With one important discrepancy, the STZ theory reproduces the simulation data, i… ▽ More We present a shear-transformation-zone (STZ) theoretical analysis of molecular-dynamics simulations of a rapidly sheared metallic glass. These simulations are especially revealing because, although they are limited to high strain rates, they span temperatures ranging from well below to well above the glass transition. With one important discrepancy, the STZ theory reproduces the simulation data, including the way in which those data can be made to collapse onto simple curves by a scaling transformation. The STZ analysis implies that the system's behavior at high strain rates is controlled primarily by effective-temperature thermodynamics, as opposed to system-specific details of the molecular interactions. The discrepancy between theory and simulations occurs at the lower strain rates for temperatures near the glass transition. We argue that this discrepancy can be resolved by the same multi-species generalization of STZ theory that has been proposed recently for understanding frequency-dependent viscoelastic responses, Stokes-Einstein violations, and stretched-exponential relaxation in equilibrated glassy materials. △ Less

Submitted 11 May, 2012; v1 submitted 8 March, 2012; originally announced March 2012.

Comments: 9 pages, 6 figures

Itinerant electrons, local moments, and magnetic correlations in pnictides high temperature superconductors

Authors: P. Vilmercati, A. Fedorov, F. Bondino, F. Offi, G. Panaccione, P. Lacovig, L. Simonelli, M. A. McGuire, A. S. M. Sefat, D. Mandrus, B. C. Sales, T. Egami, W. Ku, N. Mannella

Abstract: A direct and element-specific measurement of the local Fe spin moment has been provided by analyzing the Fe 3s core level photoemission spectra in the parent and optimally doped CeFeAsO1-xFx (x = 0, 0.11) and Sr(Fe1 xCox)2As2 (x = 0, 0.10) pnictides. The rapid time scales of the photoemission process allowed the detection of large local spin moments fluctuating on a 10-15 s time scale in the param… ▽ More A direct and element-specific measurement of the local Fe spin moment has been provided by analyzing the Fe 3s core level photoemission spectra in the parent and optimally doped CeFeAsO1-xFx (x = 0, 0.11) and Sr(Fe1 xCox)2As2 (x = 0, 0.10) pnictides. The rapid time scales of the photoemission process allowed the detection of large local spin moments fluctuating on a 10-15 s time scale in the paramagnetic, anti-ferromagnetic and superconducting phases, indicative of the occurrence of ubiquitous strong Hund's magnetic correlations. The magnitude of the spin moment is found to vary significantly among different families, 1.3 \muB in CeFeAsO and 2.1 \muB in SrFe2As2. Surprisingly, the spin moment is found to decrease considerably in the optimally doped samples, 0.9 \muB in CeFeAsO0.89F0.11 and 1.3 \muB in Sr(Fe0.9Co0.1)2As2. The strong variation of the spin moment against doping and material type indicates that the spin moments and the motion of itinerant electrons are influenced reciprocally in a self-consistent fashion, reflecting the strong competition between the antiferromagnetic super-exchange interaction among the spin moments and the kinetic energy gain of the itinerant electrons in the presence of a strong Hund's coupling. By describing the evolution of the magnetic correlations concomitant with the appearance of superconductivity, these results constitute a fundamental step toward attaining a correct description of the microscopic mechanisms shaping the electronic properties in the pnictides, including magnetism and high temperature superconductivity. △ Less

Submitted 12 March, 2012; v1 submitted 8 March, 2012; originally announced March 2012.

Journal ref: Phys. Rev. B 85, 220503(R), (2012)

Phonon softening near structural transition in BaFe2As2 observed by inelastic x-ray scattering

Authors: Jennifer L. Niedziela, D. Parshall, K. A. Lokshin, A. S. Sefat, A. Alatas, T. Egami

Abstract: In this work we present the results of an inelastic x-ray scattering experiment detailing the behavior of the transverse acoustic [110] phonon in BaFe2As2 as a function of temperature. When cooling through the structural transition temperature, the transverse acoustic phonon energy is reduced from the value at room temperature, reaching a maximum shift near inelastic momentum transfer q=0.1. This… ▽ More In this work we present the results of an inelastic x-ray scattering experiment detailing the behavior of the transverse acoustic [110] phonon in BaFe2As2 as a function of temperature. When cooling through the structural transition temperature, the transverse acoustic phonon energy is reduced from the value at room temperature, reaching a maximum shift near inelastic momentum transfer q=0.1. This softening of the lattice results in a change of the symmetry from tetragonal to orthorhombic at the same temperature as the transition to long-range antiferromagnetic order. While the lattice distortion is minor, the anisotropy in the magnetic exchange constants in pnictide parent compounds is large. We suggest mechanisms of electron-phonon coupling to describe the interaction between the lattice softening and onset of magnetic ordering. △ Less

Submitted 1 December, 2011; originally announced December 2011.

Comments: As accepted for publication in Physical Review B

Journal ref: Physical Review B 84, 224305 (2011)

Space-time dependence of the anomalous exponent of electric transport in the disorder model

Authors: Takeshi Egami, Koshiro Suzuki, Katsuhiro Watanabe

Abstract: Space-time dependence of the anomalous exponent of electric transport in the disorder model is presented. We show that the anomalous exponent depends on time, according to the time-evolution of the number of the effective neighbouring sites. Transition from subdiffusive to normal transport at long-enough time is recovered. The above result indicates that the spatial structure, specifically the net… ▽ More Space-time dependence of the anomalous exponent of electric transport in the disorder model is presented. We show that the anomalous exponent depends on time, according to the time-evolution of the number of the effective neighbouring sites. Transition from subdiffusive to normal transport at long-enough time is recovered. The above result indicates that the spatial structure, specifically the network structure, of the hopping sites might be a novel element which determines the anomalous exponent. This leads to the feature that the scaling property of the electric transport to time is insensitive to other elements, such as the distance of the sites or the spatial dimension of the system. These findings are verified by means of Monte Carlo simulation. The relation of the result to the conventional knowledge of the Multiple Trapping Model is shown by deriving it as a special case of the disorder model. △ Less

Submitted 27 September, 2012; v1 submitted 28 September, 2011; originally announced September 2011.

Comments: 8 pages, 3 figures, accepted for publication in J. Phys. A: Math. Theor

Journal ref: J. Phys. A: Math. Theor. 45 (2012) 465002

Extended Phonon Collapse and the Origin of the Charge-Density-Wave in NbSe$_2$

Authors: F. Weber, S. Rosenkranz, J. -P. Castellan, R. Osborn, R. Hott, R. Heid, K. -P. Bohnen, T. Egami, A. H. Said, D. Reznik

Abstract: 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 expecte… ▽ 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 NbSe$_2$ and determines its periodicity. This mechanism explains the so far enigmatic behavior of CDW in NbSe$_2$ and may provide a new approach to other strongly correlated systems where electron-phonon coupling is important. △ Less

Submitted 29 March, 2011; originally announced March 2011.

Journal ref: Phys. Rev. Lett. 107, 107403 (2011)

Spin waves in the $(π,0)$ magnetically ordered iron chalcogenide Fe$_{1.05}$Te

Authors: O. J. Lipscombe, G. F. Chen, Chen Fang, T. G. Perring, D. L. Abernathy, A. D. Christianson, Takeshi Egami, Nanlin Wang, Jiangping Hu, Pengcheng Dai

Abstract: We use inelastic neutron scattering to show that the spin waves in the iron chalcogenide Fe$_{1.05}$Te display novel dispersion clearly different from those in the related iron pnictide systems. By fitting the spin waves to a Heisenberg Hamiltonian, we extract magnetic exchange couplings that are dramatically different from both predictions by density functional calculations and measurements on th… ▽ More We use inelastic neutron scattering to show that the spin waves in the iron chalcogenide Fe$_{1.05}$Te display novel dispersion clearly different from those in the related iron pnictide systems. By fitting the spin waves to a Heisenberg Hamiltonian, we extract magnetic exchange couplings that are dramatically different from both predictions by density functional calculations and measurements on the iron pnictide CaFe$_2$As$_2$. While the nearest-neighbor exchange couplings in CaFe$_2$As$_2$ and Fe$_{1.05}$Te are quite different, their next-nearest-neighbor exchange couplings are similar. These results suggest that superconductivity in the pnictides and chalcogenides share a common magnetic origin that is intimately associated with the next-nearest-neighbor magnetic coupling between the irons. △ Less

Submitted 14 February, 2011; v1 submitted 12 November, 2010; originally announced November 2010.

Comments: Includes supplementary information as appendices

Journal ref: Phys. Rev. Lett. 106, 057004 (2011)

Bulk magnetic order in a two dimensional Ni1+/Ni2+ (d9/d8) nickelate, isoelectronic with superconducting cuprates

Authors: Viktor V. Poltavets, Konstantin A. Lokshin, Takeshi Egami, Andriy H. Nevidomskyy, Mark Croft, Gabriel Kotliar, Trevor A. Tyson, Joke Hadermann, Gustaaf Van Tendeloo, Nicholas ApRoberts-Warren, Adam P. Dioguardi, Nicholas J. Curro, Martha Greenblatt

Abstract: Bulk magnetic order in two dimensional La4Ni3O8 nickelate with Ni1+/Ni2+ (d9/d8), isoelectronic with superconducting cuprates is demonstrated experimentally and theoretically. Magnetization, specific heat and 139La NMR evidence a transition at 105 K to an antiferromagnetic state. Theoretical calculations by DFT relate the transition to a nesting instability of the Fermi surface with ordering wave-… ▽ More Bulk magnetic order in two dimensional La4Ni3O8 nickelate with Ni1+/Ni2+ (d9/d8), isoelectronic with superconducting cuprates is demonstrated experimentally and theoretically. Magnetization, specific heat and 139La NMR evidence a transition at 105 K to an antiferromagnetic state. Theoretical calculations by DFT relate the transition to a nesting instability of the Fermi surface with ordering wave-vector Q = [1/3, 1/3, 0]. △ Less

Submitted 16 March, 2010; originally announced March 2010.

Journal ref: Phys. Rev. Lett. 104, 206403 (2010)

Anisotropic Neutron Spin Resonance in Superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_2$

Authors: O. J. Lipscombe, Leland W. Harriger, P. G. Freeman, M. Enderle, Chenglin Zhang, Miaoying Wang, Takeshi Egami, Jiangping Hu, Tao Xiang, M. R. Norman, Pengcheng Dai

Abstract: We use polarized inelastic neutron scattering to show that the neutron spin resonance below $T_c$ in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_2$ ($T_c=20$ K) is purely magnetic in origin. Our analysis further reveals that the resonance peak near 7~meV only occurs for the planar response. This challenges the common perception that the spin resonance in the pnictides is an isotropic triplet excited… ▽ More We use polarized inelastic neutron scattering to show that the neutron spin resonance below $T_c$ in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_2$ ($T_c=20$ K) is purely magnetic in origin. Our analysis further reveals that the resonance peak near 7~meV only occurs for the planar response. This challenges the common perception that the spin resonance in the pnictides is an isotropic triplet excited state of the singlet Cooper pairs, as our results imply that only the $S_{001}=\pm1$ components of the triplet are involved. △ Less

Submitted 23 August, 2010; v1 submitted 9 March, 2010; originally announced March 2010.

Journal ref: Physical Review B, 82, 064515 (2010)

Temperature and Pressure Dependence of the Fe-specific Phonon Density of States in Ba(Fe(1-x)Co(x))2As2

Authors: O. Delaire, M. S. Lucas, A. M. dos Santos, A. Subedi, A. S. Sefat, M. A. McGuire, L. Mauger, J. A. Munoz, C. A. Tulk, Y. Xiao, M. Somayazulu, J. Y. Zhao, W. Sturhahn, E. E. Alp, D. J. Singh, B. C. Sales, D. Mandrus, T. Egami

Abstract: The {57}Fe-specific phonon density of states of Ba(Fe(1-x)Co(x))2As2 single crystals (x=0.0, 0.08) was measured at cryogenic temperatures and at high pressures with nuclear-resonant inelastic x-ray scattering. Measurements were conducted for two different orientations of the single crystals, yielding the orientation-projected {57}Fe-phonon density of states (DOS) for phonon polarizations in-plan… ▽ More The {57}Fe-specific phonon density of states of Ba(Fe(1-x)Co(x))2As2 single crystals (x=0.0, 0.08) was measured at cryogenic temperatures and at high pressures with nuclear-resonant inelastic x-ray scattering. Measurements were conducted for two different orientations of the single crystals, yielding the orientation-projected {57}Fe-phonon density of states (DOS) for phonon polarizations in-plane and out-of-plane with respect to the basal plane of the crystal structure. In the tetragonal phase at 300 K, a clear stiffening was observed upon doping with Co. Increasing pressure to 4 GPa caused a marked increase of phonon frequencies, with the doped material still stiffer than the parent compound. Upon cooling, both the doped and undoped samples showed a stiffening, and the parent compound exhibited a discontinuity across the magnetic and structural phase transition. These findings are generally compatible with the changes in volume of the system upon doping, increasing pressure, or increasing temperature, but an extra softening of high-energy modes occurs with increasing temperature. First-principles computations of the phonon DOS were performed and showed an overall agreement with the experimental results, but underestimate the Grueneisen parameter. This discrepancy is explained in terms of a magnetic Grueneisen parameter, causing an extra phonon stiffening as magnetism is suppressed under pressure. △ Less

Submitted 15 December, 2009; originally announced December 2009.

Journal ref: Phys. Rev. B 81, 094504 (2010)

Unusual Relationship between Magnetism and Superconductivity in FeTe$_{0.5}$Se$_{0.5}$

Authors: H. A. Mook, M. D. Lumsden, A. D. Christianson, S. E. Nagler, Brian C. Sales, Rongying Jin, Michael A. McGuire, Athena Sefat, D. Mandrus, T. Egami, Clarina dela Cruz

Abstract: We use neutron scattering, to study magnetic excitations in crystals near the ideal superconducting composition of FeTe$_{0.5}$Se$_{0.5}$. Two types of excitations are found, a resonance at (0.5, 0.5, 0) and incommensurate fluctuations on either side of this position. We show that the two sets of magnetic excitations behave differently with doping, with the resonance being fixed in position whil… ▽ More We use neutron scattering, to study magnetic excitations in crystals near the ideal superconducting composition of FeTe$_{0.5}$Se$_{0.5}$. Two types of excitations are found, a resonance at (0.5, 0.5, 0) and incommensurate fluctuations on either side of this position. We show that the two sets of magnetic excitations behave differently with doping, with the resonance being fixed in position while the incommensurate excitations move as the doping is changed. These unusual results show that a common behavior of the low energy magnetic excitations is not necessary for pairing in these materials. △ Less

Submitted 28 December, 2009; v1 submitted 29 November, 2009; originally announced November 2009.

Comments: 8 pages, 3 figures

Journal ref: Phys. Rev. Lett. 104, 187002 (2010)

Spin-Phonon Coupling in Iron Pnictide Superconductors

Authors: T. Egami, B. V. Fine, D. J. Singh, D. Parshall, C. de la Cruz, P. Dai

Abstract: The magnetic moment in the parent phase of the iron-pnictide superconductors varies with composition even when the nominal charge of iron is unchanged. We propose the spin-lattice coupling due to the magneto-volume effect as the primary origin of this effect, and formulate a Landau theory to describe the dependence of the moment to the Fe-As layer separation. We then compare the superconductive… ▽ More The magnetic moment in the parent phase of the iron-pnictide superconductors varies with composition even when the nominal charge of iron is unchanged. We propose the spin-lattice coupling due to the magneto-volume effect as the primary origin of this effect, and formulate a Landau theory to describe the dependence of the moment to the Fe-As layer separation. We then compare the superconductive critical temperature of doped iron pnictides to the local moment predicted by the theory, and suggest that the spin-phonon coupling may play a role in the superconductivity of this compound. △ Less

Submitted 29 August, 2009; originally announced August 2009.

Phonons as a probe of the magnetic state in doped and undoped BaFe2As2

Authors: D. Reznik, K. Lokshin, D. C. Mitchell, D. Parshall, W. Dmowski, D. Lamago, R. Heid, K. -P. Bohnen, A. S. Sefat, M. A. McGuire, B. C. Sales, D. G. Mandrus, A. Subedi, D. J. Singh, A. Alatas, M. H. Upton, A. H. Said, A. Cunsolo, Yu. Shvydko, T. Egami

Abstract: We measured phonon frequencies and linewidths in doped and undoped BaFe2As2 single crystals by inelastic x-ray scattering and compared our results with density functional theory (DFT) calculations. In agreement with previous work, the calculated frequencies of some phonons depended on whether the ground state was magnetic or not and, in the former case, whether phonon wavevector was parallel or… ▽ More We measured phonon frequencies and linewidths in doped and undoped BaFe2As2 single crystals by inelastic x-ray scattering and compared our results with density functional theory (DFT) calculations. In agreement with previous work, the calculated frequencies of some phonons depended on whether the ground state was magnetic or not and, in the former case, whether phonon wavevector was parallel or perpendicular to the magnetic ordering wavevector. The experimental results agreed better with the magnetic calculation than with zero Fe moment calculations, except the peak splitting expected due to magnetic domain twinning was not observed. Furthermore, phonon frequencies were unaffected by the breakdown of the magnetic ground state due to either doping or increased temperature. Based on these results we propose that phonons strongly couple not to the static order, but to high frequency magnetic fluctuations. △ Less

Submitted 25 January, 2010; v1 submitted 29 August, 2009; originally announced August 2009.

Comments: Minor revision of the text

Journal ref: Phys. Rev. B 80, 214534 (2009)