Showing 1–50 of 95 results for author: Mydosh, J

A comparison of the hidden order transition in URu$_2$Si$_2$ to the $λ$-transition in $^4$He

Authors: W. Montfrooij, J. A. Mydosh

Abstract: The low-temperature states of ambient URu$_2$Si$_2$ and superfluid $^4$He are both characterized by momentum-dependent energy gaps between the ground and excited states. This behavior weakly persists even above the transition temperatures but becomes over-damped (ungapped) because of the number of excitations present at elevated temperature. We show that akin to the normal fluid to superfluid tran… ▽ More The low-temperature states of ambient URu$_2$Si$_2$ and superfluid $^4$He are both characterized by momentum-dependent energy gaps between the ground and excited states. This behavior weakly persists even above the transition temperatures but becomes over-damped (ungapped) because of the number of excitations present at elevated temperature. We show that akin to the normal fluid to superfluid transition in $^4$He, the hidden-order (HO) transition in URu$_2$Si$_2$ can be understood by a change of the ungapped excitations to the gapped, elementary excitations (EE) of the unknown ordered state. These under-damped EEs reflect the basic character and order parameters of the different phase transitions. This view accounts for the full amount of entropy released in these transitions, the jumps in the resistivity and thermal conductivity directly below the transition, as well as the reduction of the Fermi surface. We argue that the behavior in the HO phase is that of a gas of weakly interacting excitations from charge density wave or crystal field states in a similar manner to that of the phonon-roton excitations of the superfluid $^4$He phase. We discuss the influence of applying pressure and magnetic fields within this scenario and the role of the small moment antiferromagnetic clustering in the hidden order phase. △ Less

Submitted 19 March, 2024; v1 submitted 19 September, 2023; originally announced September 2023.

Comments: To appear in PRB, 14 pages - 8 figures

Increased muon field at surface and substrate interface of palladium thin films

Authors: Gesa Welker, Martin de Wit, Tjerk H. Oosterkamp, John A. Mydosh, Andreas Suter, Thomas Prokscha, Lucia Bossoni

Abstract: We performed depth-dependent low-energy muon spin spectroscopy ($μ$SR) studies on three palladium 100 nm thin films, both undoped and doped with 170 ppm of iron. Muons implanted in the surface and substrate interface region probe an increased local magnetic field compared to the inner part of the sample. The field increase extends over a few nanometers, it is temperature-independent (in the range… ▽ More We performed depth-dependent low-energy muon spin spectroscopy ($μ$SR) studies on three palladium 100 nm thin films, both undoped and doped with 170 ppm of iron. Muons implanted in the surface and substrate interface region probe an increased local magnetic field compared to the inner part of the sample. The field increase extends over a few nanometers, it is temperature-independent (in the range of 3.7 - 100 K), stronger for the iron-doped samples and accompanied by an increase in local field inhomogeneity. We consider various potential origins for this magnetic surface state, such as adsorbents and supressed d-states. Our conclusion is that orbital moments induced at the surface / interface by localized spins and charges are the most likely explanation, potentially accompanied by magnetic moments due to crystal irregularities. △ Less

Submitted 5 September, 2022; originally announced September 2022.

High temperature tetragonal crystal structure of UPt$_2$Si$_2$

Authors: K. Prokes, O. Fabelo, S. Sullow, J. Lee, J. Mydosh

Abstract: High temperature crystal structure of UPt$_2$Si$_2$ determined using single-crystal neutron diffraction at 400 K is reported. It is found that the crystal structure remains of the primitive tetragonal CaBe$_2$Ge$_2$ type with the space group P4/$n m m. Anisotropic displacement factors of the Pt atoms at the 2a (3/4 1/4 0) and Si atoms at the 2c (1/4 1/4 z) Wyckoff sites are found to be anomalously… ▽ More High temperature crystal structure of UPt$_2$Si$_2$ determined using single-crystal neutron diffraction at 400 K is reported. It is found that the crystal structure remains of the primitive tetragonal CaBe$_2$Ge$_2$ type with the space group P4/$n m m. Anisotropic displacement factors of the Pt atoms at the 2a (3/4 1/4 0) and Si atoms at the 2c (1/4 1/4 z) Wyckoff sites are found to be anomalously large. △ Less

Submitted 27 February, 2020; originally announced February 2020.

Comments: 5 Figures, 3 Tables

Hidden order and beyond: an experimental-theoretical overview of the multifaceted behavior of URu$_2$Si$_2$

Authors: J. A. Mydosh, P. M. Oppeneer, P. S. Riseborough

Abstract: This Topical Review describes the multitude of unconventional behaviors in the hidden order, heavy fermion, antiferromagnetic and superconducting phases of the intermetallic compound URu$_2$Si$_2$ when tuned with pressure, magnetic field, and substitutions for all three elements. Such `perturbations' result in a variety of new phases beyond the mysterious hidden order that are only now being slowl… ▽ More This Topical Review describes the multitude of unconventional behaviors in the hidden order, heavy fermion, antiferromagnetic and superconducting phases of the intermetallic compound URu$_2$Si$_2$ when tuned with pressure, magnetic field, and substitutions for all three elements. Such `perturbations' result in a variety of new phases beyond the mysterious hidden order that are only now being slowly understood through a series of state-of-the-science experimentation, along with an array of novel theoretical approaches. Despite all these efforts spanning more than 30 years, hidden order (HO) remains puzzling and non-clarified, and the search continues in 2019 into a fourth decade for its final resolution. Here we attempt to update the present situation of URu$_2$Si$_2$ importing the latest experimental results and theoretical proposals. First, let us consider the pristine compound as a function of temperature and report the recent measurements and models relating to its heavy Fermi liquid crossover, its HO and superconductivity (SC). Recent experiments and theories are surmized that address four-fold symmetry breaking (or nematicity), Isingness and unconventional excitation modes. Second, we review the pressure dependence of URu$_2$Si$_2$ and its transformation to antiferromagnetic long-range order. Next we confront the dramatic high magnetic-field phases requiring fields above 40 T. And finally, we attempt to answer how does random substitutions of other $5f$ elements for U, and $3d$, $4d$, and $5d$ elements for Ru, and even P for Si affect and transform the HO. Commensurately, recent theoretical models are summarized and then related to the intriguing experimental behavior. △ Less

Submitted 19 December, 2019; originally announced December 2019.

Comments: 75 pages, 27 figures. J. Phys.: Condensed Matter, in press

Charge density wave with anomalous temperature dependence in UPt2Si2

Authors: Jooseop Lee, Karel Prokes, Sohee Park, Igor Zaliznyak, Sachith Dissanayake, Masaaki Matsuda, Matthias Frontzek, Stanislav Stoupin, Greta L. Chappell, Ryan E. Baumbach, Changwon Park, John A. Mydosh, Garrett E. Granroth, Jacob P. C. Ruff

Abstract: Using single crystal neutron and x-ray diffraction, we discovered a charge density wave (CDW) below 320 K, which accounts for the long-sought origin of the heat capacity and resistivity anomalies in UPt2Si2. The modulation wavevector, Qmod, is intriguingly similar to the Fermi surface nesting wavevector of URu2Si2. Qmod shows an unusual temperature dependence, shifting from commensurate to incomme… ▽ More Using single crystal neutron and x-ray diffraction, we discovered a charge density wave (CDW) below 320 K, which accounts for the long-sought origin of the heat capacity and resistivity anomalies in UPt2Si2. The modulation wavevector, Qmod, is intriguingly similar to the Fermi surface nesting wavevector of URu2Si2. Qmod shows an unusual temperature dependence, shifting from commensurate to incommensurate position upon cooling and becoming locked at ~ (0.42 0 0) near 180 K. Bulk measurements indicate a cross-over toward a correlated coherent state around the same temperature, suggesting an interplay between the CDW and Kondo-lattice-like coherence before coexisting antiferromagnetic order sets in at TN = 35 K. △ Less

Submitted 29 June, 2020; v1 submitted 8 August, 2019; originally announced August 2019.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 102, 041112 (2020)

One-Component Order Parameter in URu$_2$Si$_2$ Uncovered by Resonant Ultrasound Spectroscopy and Machine Learning

Authors: Sayak Ghosh, Michael Matty, Ryan Baumbach, Eric D. Bauer, K. A. Modic, Arkady Shekhter, J. A. Mydosh, Eun-Ah Kim, B. J. Ramshaw

Abstract: The unusual correlated state that emerges in URu$_2$Si$_2$ below T$_{HO}$ = 17.5 K is known as "hidden order" because even basic characteristics of the order parameter, such as its dimensionality (whether it has one component or two), are "hidden". We use resonant ultrasound spectroscopy to measure the symmetry-resolved elastic anomalies across T$_{HO}$. We observe no anomalies in the shear elasti… ▽ More The unusual correlated state that emerges in URu$_2$Si$_2$ below T$_{HO}$ = 17.5 K is known as "hidden order" because even basic characteristics of the order parameter, such as its dimensionality (whether it has one component or two), are "hidden". We use resonant ultrasound spectroscopy to measure the symmetry-resolved elastic anomalies across T$_{HO}$. We observe no anomalies in the shear elastic moduli, providing strong thermodynamic evidence for a one-component order parameter. We develop a machine learning framework that reaches this conclusion directly from the raw data, even in a crystal that is too small for traditional resonant ultrasound. Our result rules out a broad class of theories of hidden order based on two-component order parameters, and constrains the nature of the fluctuations from which unconventional superconductivity emerges at lower temperature. Our machine learning framework is a powerful new tool for classifying the ubiquitous competing orders in correlated electron systems. △ Less

Submitted 11 March, 2020; v1 submitted 1 March, 2019; originally announced March 2019.

Journal ref: Sci. Adv. 6, eaaz4074 (2020)

Field-induced phases in a heavy-fermion U(Ru$_{0.92}$Rh$_{0.08}$)$_{2}$Si$_{2}$ single crystal

Authors: K. Prokes, T. Förster, Y. -K. Huang, J. A. Mydosh

Abstract: We report the high-field induced magnetic phases and phase diagram of a high quality \urxrs~single crystal prepared using a modified Czochralski method. Our study, that combines high-field magnetization and electrical resistivity measurements, shows for fields applied along the $c$-axis direction three field-induced magnetic phase transitions at $μ_{0} H_{c1}$ = 21.60 T, $μ_{0} H_{c2}$ = 37.90 T a… ▽ More We report the high-field induced magnetic phases and phase diagram of a high quality \urxrs~single crystal prepared using a modified Czochralski method. Our study, that combines high-field magnetization and electrical resistivity measurements, shows for fields applied along the $c$-axis direction three field-induced magnetic phase transitions at $μ_{0} H_{c1}$ = 21.60 T, $μ_{0} H_{c2}$ = 37.90 T and $μ_{0} H_{c3}$ = 38.25 T, respectively. In agreement with a microscopic up-up-down arrangement of the U magnetic moments the phase above $H_{c1}$ has a magnetization of about one third of the saturated value. In contrast the phase between $H_{c2}$ and $H_{c3}$ has a magnetization that is a factor of two lower than above the $H_{c3}$, where a polarized Fermi-liquid state with a saturated moment $M_{s}$ $\approx$ 2.1 $μ_{B}$/U is realized. Most of the respective transitions are reflected in the electrical resistivity as sudden drastic changes. Most notably, the phase between $H_{c1}$ and $H_{c2}$ exhibits substantially larger values. As the temperature increases, transitions smear out and disappear above $\approx$ 15 K. However, a substantial magnetoresistance is observed even at temperatures as high as 80 K. Due to a strong uniaxial magnetocrystalline anisotropy a very small field effect is observed for fields apllied perpendicular to the $c$-axis direction. △ Less

Submitted 7 January, 2019; originally announced January 2019.

Comments: 7 figures

Journal ref: Physical Rev. B 99 (2019) 045104

The dual nature of magnetism in a uranium heavy fermion system

Authors: Jooseop Lee, Masaaki Matsuda, John A. Mydosh, Igor Zaliznyak, Alexander I. Kolesnikov, Stefan Sullow, Jacob P. C. Ruff, Garrett E. Granroth

Abstract: The duality between localized and itinerant nature of magnetism in $5\textit{f}$ electron systems has been a longstanding puzzle. Here, we report inelastic neutron scattering measurements, which reveal both local and itinerant aspects of magnetism in a single crystalline system of UPt$_{2}$Si$_{2}$. In the antiferromagnetic state, we observe broad continuum of diffuse magnetic scattering with a re… ▽ More The duality between localized and itinerant nature of magnetism in $5\textit{f}$ electron systems has been a longstanding puzzle. Here, we report inelastic neutron scattering measurements, which reveal both local and itinerant aspects of magnetism in a single crystalline system of UPt$_{2}$Si$_{2}$. In the antiferromagnetic state, we observe broad continuum of diffuse magnetic scattering with a resonance-like gap of $\approx$ 7 meV, and surprising absence of coherent spin-waves, suggestive of itinerant magnetism. While the gap closes above the Neel temperature, strong dynamic spin correlations persist to high temperature. Nevertheless, the size and temperature dependence of the total magnetic spectral weight can be well described by local moment with $J=4$. Furthermore, polarized neutron measurements reveal that the magnetic fluctuations are mostly transverse, with little or none of the longitudinal component expected for itinerant moments. These results suggest that a dual description of local and itinerant magnetism is required to understand UPt$_{2}$Si$_{2}$, and by extension, other 5$f$ systems in general. △ Less

Submitted 10 July, 2018; originally announced July 2018.

Comments: see supplementary material for more details

Magnetic structure in U(Ru0.92Rh0.08)2Si2 single crystal studied by neutron diffraction in static magnetic fields up to 24 T

Authors: K. Prokes, M. Bartkowiak, O. Rivin, O. Prokhnenko, T. Foerster, S. Gerischer, R. Wahle, Y. -K. Huang, J. A. Mydosh

Abstract: We report the high-field induced magnetic phase in single crystal of U(Ru0.92Rh0.08)2Si2. Our neutron study combined with high-field magnetization, shows that the magnetic phase above the first metamagnetic transition at Hc1 = 21.6 T has an uncompensated commensurate antiferromagnetic structure with propagation vector Q2 = ( 2/3 0 0) possessing two single-Q domains. U moments of 1.45 (9) muB direc… ▽ More We report the high-field induced magnetic phase in single crystal of U(Ru0.92Rh0.08)2Si2. Our neutron study combined with high-field magnetization, shows that the magnetic phase above the first metamagnetic transition at Hc1 = 21.6 T has an uncompensated commensurate antiferromagnetic structure with propagation vector Q2 = ( 2/3 0 0) possessing two single-Q domains. U moments of 1.45 (9) muB directed along the c axis are arranged in an up-up-down sequence propagating along the a axis, in agreement with bulk measurements. The U magnetic form factor at high fields is consistent with both the U3+ and U4+ type. The low field short-range order that emerges from the pure URu2Si2 due to Rh-doping is initially strengthened by the field but disappears in the field-induced phase. The tetragonal symmetry is preserved across the transition but the a axis lattice parameter increases already at low fields. Our results are in agreement with itinerant electron model with 5f states forming bands pinned in the vicinity of the Fermi surface that is significantly reconstructed by the applied magnetic field. △ Less

Submitted 21 September, 2017; originally announced September 2017.

Comments: 5 pages, 4 figures, accepted as Rapid Communication, Physical Review B (2017)

Size-effect of Kondo scattering in point contacts (revisited)

Authors: I. K. Yanson, V. V. Fisun, N. L. Bobrov, J. A. Mydosh, J. M. van Ruitenbeek

Abstract: The size-effect of Kondo-scattering in nanometer-sized metallic point contacts is measured with the simplified, mechanically-controlled break-junction technique for CuMn alloy of different Mn concentrations: 0.017; 0.035; and 0.18 ($\pm$0.017) at.\%. The results are compared with our previous publication on nominally 0.1 at.\% CuMn alloy \cite{1,2}. The increase of width of the Kondo resonance and… ▽ More The size-effect of Kondo-scattering in nanometer-sized metallic point contacts is measured with the simplified, mechanically-controlled break-junction technique for CuMn alloy of different Mn concentrations: 0.017; 0.035; and 0.18 ($\pm$0.017) at.\%. The results are compared with our previous publication on nominally 0.1 at.\% CuMn alloy \cite{1,2}. The increase of width of the Kondo resonance and enhanced ratio of Kondo-peak intensity to electron-phonon scattering intensity is observed for contacts with sizes smaller than 10 $nm$. From the comparison of electron-phonon scattering intensity for the pressure-type contacts, which correspond to the clean orifice model, we conclude that the size effect is observed in \emph{clean} contacts with the shape of a \emph{channel} (nanowire). △ Less

Submitted 13 February, 2017; originally announced February 2017.

Comments: 8 pages, 7 figures

Journal ref: Fiz. Nizk. Temp., 24, 654 (1998); Low Temp. Phys., 24, 495(1998)

Electronic properties of a heavy-fermion U(Ru0.92Rh0.08)2Si2 single crystal

Authors: K. Prokes, Y. -K. Huang, M. Reehuis, B. Klemke, J. -U. Hoffmann, A. Sokolowski, J. A. Mydosh

Abstract: We report the crystal structure and highly-anisotropic magnetic, transport and thermal properties of an exceptionally good single crystal of U(Ru0.92Rh0.08)2Si2 prepared using a modified Czochralski method. Our study, that also includes neutron diffraction results, shows all the heavy-fermion signatures of pristine URu2Si2 , however, the superconductivity, hidden order and remanent weak antiferrom… ▽ More We report the crystal structure and highly-anisotropic magnetic, transport and thermal properties of an exceptionally good single crystal of U(Ru0.92Rh0.08)2Si2 prepared using a modified Czochralski method. Our study, that also includes neutron diffraction results, shows all the heavy-fermion signatures of pristine URu2Si2 , however, the superconductivity, hidden order and remanent weak antiferromagnetic orders are absent. Instead, the ground state of the doped system can be classified as a spin liquid that preserves the heavy-fermion character. U(Ru0.92Rh0.08)2Si2 exhibits a short-range magnetic order distinguished by reflections of a Lorentzian profile at qIII = (1/2 1/2 1/2) positions that disappear above approx. 15 K. The short-range order seems to be a precursor of a long-range magnetic order that occurs with higher Rh concentration. We indicate that these short-range fluctuations involve, at least partially, inelastic scattering processes. △ Less

Submitted 26 October, 2016; originally announced October 2016.

Comments: 12 Figures

Detailed optical spectroscopy of the hybridization gap and the hidden order transition in high quality URu$_{2}$Si$_{2}$ single crystals

Authors: N. Bachar, D. Stricker, S. Muleady, K. Wang, J. A. Mydosh, Y. K. Huang, D. van der Marel

Abstract: We present a detailed temperature and frequency dependence of the optical conductivity measured on clean high quality single crystals of URu$_{2}$Si$_{2}$ of $ac$- and $ab$-plane surfaces. Our data demonstrate the itinerant character of the narrow 5f bands, becoming progressively coherent as temperature is lowered below a cross-over temperature $T^*{\sim}75~K$. $T^*$ is higher than in previous rep… ▽ More We present a detailed temperature and frequency dependence of the optical conductivity measured on clean high quality single crystals of URu$_{2}$Si$_{2}$ of $ac$- and $ab$-plane surfaces. Our data demonstrate the itinerant character of the narrow 5f bands, becoming progressively coherent as temperature is lowered below a cross-over temperature $T^*{\sim}75~K$. $T^*$ is higher than in previous reports as a result of a different sample preparation, which minimizes residual strain. We furthermore present the density-response (energy-loss) function of this compound, and determine the energies of the heavy fermion plasmons with $a$-and $c$-axis polarization. Our observation of a suppression of optical conductivity below 50~meV both along $a$ and $c$-axis, along with a heavy fermion plasmon at 18~meV, points toward the emergence of a band of coherent charge carriers crossing the Fermi energy and the emergence of a hybridization gap on part of the Fermi surface. The evolution towards coherent itinerant states is accelerated below the hidden order temperature $T_{HO}=17.5$~K. In the hidden order phase the low frequency optical conductivity shows a single gap at $\sim 6.5$meV, which closes at $T_{HO}$. △ Less

Submitted 16 December, 2016; v1 submitted 14 October, 2016; originally announced October 2016.

Comments: 13 figures

Journal ref: Phys. Rev. B 94, 235101 (2016)

Field induced Lifshitz transition in UPt$_2$Si$_2$: Fermi surface under extreme conditions

Authors: D. Schulze Grachtrup, N. Steinki, S. Süllow, Z. Cakir, G. Zwicknagl, Y. Krupko, I. Sheikin, M. Jaime, J. A. Mydosh

Abstract: We have measured Hall effect, magnetotransport and magnetostriction on the field induced phases of single crystalline UPt$_2$Si$_2$ in magnetic fields up to 60\,T at temperatures down to 50\,mK. For the magnetic field applied along the $c$ axis we observe strong changes in the Hall effect at the phase boundaries. From a comparison to band structure calculations utilizing the concept of a dual natu… ▽ More We have measured Hall effect, magnetotransport and magnetostriction on the field induced phases of single crystalline UPt$_2$Si$_2$ in magnetic fields up to 60\,T at temperatures down to 50\,mK. For the magnetic field applied along the $c$ axis we observe strong changes in the Hall effect at the phase boundaries. From a comparison to band structure calculations utilizing the concept of a dual nature of the uranium 5$f$ electrons, we find evidence for field induced topological changes of the Fermi surface due to at least one Lifshitz transition. Furthermore, we find a unique history dependence of the magnetotransport and magnetostriction data, indicating that the Lifshitz type transition is of a discontinuous nature, as predicted for interacting electron systems. △ Less

Submitted 6 March, 2017; v1 submitted 4 October, 2016; originally announced October 2016.

Journal ref: Phys. Rev. B 95, 134422 (2017)

Analogy between the "Hidden Order" and the Orbital Antiferromagnetism in URu$_{2-x}$Fe$_x$Si$_2$

Authors: H. -H. Kung, S. Ran, N. Kanchanavatee, V. Krapivin, A. Lee, J. A. Mydosh, K. Haule, M. B. Maple, G. Blumberg

Abstract: We study URu$_{2-x}$Fe$_x$Si$_2$, in which two types of staggered phases compete at low temperature as the iron concentration $x$ is varied: the nonmagnetic "hidden order" (HO) phase below the critical concentration $x_c$, and unconventional antiferromagnetic (AF) phase above $x_c$. By using polarization resolved Raman spectroscopy, we detect a collective mode of pseudovector-like $A_{2g}$ symmetr… ▽ More We study URu$_{2-x}$Fe$_x$Si$_2$, in which two types of staggered phases compete at low temperature as the iron concentration $x$ is varied: the nonmagnetic "hidden order" (HO) phase below the critical concentration $x_c$, and unconventional antiferromagnetic (AF) phase above $x_c$. By using polarization resolved Raman spectroscopy, we detect a collective mode of pseudovector-like $A_{2g}$ symmetry whose energy continuously evolves with increasing $x$; it monotonically decreases in the HO phase until it vanishes at $x=x_c$, and then reappears with increasing energy in the AF phase. The mode's evolution provides direct evidence for unified order parameter for both nonmagnetic and magnetic phases arising from the orbital degrees-of-freedom of the uranium-5$f$ electrons. △ Less

Submitted 28 November, 2016; v1 submitted 4 August, 2016; originally announced August 2016.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 117, 227601 (2016)

Coexistence of different magnetic moments in CeRuSn probed by polarized neutrons

Authors: K. Prokes, S. Hartwig, A. Gukasov, J. A. Mydosh, Y. -K. Huang, O. Niehaus, R. Pottgen

Abstract: We report on the spin densities in CeRuSn determined at elevated and at low temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn crystal structure, commensurate with the CeCoAl type, contains two different crystallographic Ce sites, we observe that one Ce site is clearly more susceptible to the applied magnetic field whereas the other is hardly polarizable. This finding cle… ▽ More We report on the spin densities in CeRuSn determined at elevated and at low temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn crystal structure, commensurate with the CeCoAl type, contains two different crystallographic Ce sites, we observe that one Ce site is clearly more susceptible to the applied magnetic field whereas the other is hardly polarizable. This finding clearly documents that distnictly different local environment of the two Ce sites causes the Ce ions to split into magnetic Ce3+ and non-magnetic Ce(4-delta)+ valence states. With lowering the temperature, the crystal structure transforms to a structure incommensurately modulated along the c axis. This leads to new inequivalent crystallographic Ce sites resulting in a re-distribution of spin densities. Our analysis using the simplest structural approximant shows that in this metallic system Ce ions co-exist in different valence states. Localized 4f states that fulfill the third Hund's rule are found to be close to the ideal Ce3+ state (at sites with the largest Ce-Ru interatomic distances) whereas Ce(4-delta)+ valence states are found to be itinerant and situated at Ce sites with much shorter Ce-Ru distances. The similarity to the famous alpha-gamma transition in elemental cerium is discussed. △ Less

Submitted 3 November, 2014; originally announced November 2014.

Comments: 3 figures, 1 table

Chirality density wave of the 'hidden order' phase in URu$_2$Si$_2$

Authors: H. -H. Kung, R. E. Baumbach, E. D. Bauer, V. K. Thorsmølle, W. -L. Zhang, K. Haule, J. A. Mydosh, G. Blumberg

Abstract: A second-order phase transition is associated with emergence of an "order parameter" and a spontaneous symmetry breaking. For the heavy fermion superconductor URu$_2$Si$_2$, the symmetry of the order parameter associated with its ordered phase below 17.5 K has remained ambiguous despite 30 years of research, and hence is called "hidden order" (HO). Here we use polarization resolved Raman spectrosc… ▽ More A second-order phase transition is associated with emergence of an "order parameter" and a spontaneous symmetry breaking. For the heavy fermion superconductor URu$_2$Si$_2$, the symmetry of the order parameter associated with its ordered phase below 17.5 K has remained ambiguous despite 30 years of research, and hence is called "hidden order" (HO). Here we use polarization resolved Raman spectroscopy to specify the symmetry of the low energy excitations above and below the HO transition. These excitations involve transitions between interacting heavy uranium 5f orbitals, responsible for the broken symmetry in the HO phase. From the symmetry analysis of the collective mode, we determine that the HO parameter breaks local vertical and diagonal reflection symmetries at the uranium sites, resulting in crystal field states with distinct chiral properties, which order to a commensurate chirality density wave ground state. △ Less

Submitted 23 October, 2014; originally announced October 2014.

Journal ref: Science, Vol. 347 no. 6228 pp. 1339-1342 (20 March 2015)

Valence modulations in CeRuSn

Authors: R. Feyerherm, E. Dudzik, K. Prokes, J. A. Mydosh, Y. -K. Huang, R. Pöttgen

Abstract: CeRuSn exhibits an extraordinary room temperature structure at 300~K with coexistence of two types of Ce ions, namely trivalent Ce$^{3+}$ and intermediate valent Ce$^{(4-δ)+}$, in a metallic environment. The ordered arrangement of these two Ce types on specific crystallographic sites results in a doubling of the unit cell along the $c$-axis with respect to the basic monoclinic CeCoAl-type structur… ▽ More CeRuSn exhibits an extraordinary room temperature structure at 300~K with coexistence of two types of Ce ions, namely trivalent Ce$^{3+}$ and intermediate valent Ce$^{(4-δ)+}$, in a metallic environment. The ordered arrangement of these two Ce types on specific crystallographic sites results in a doubling of the unit cell along the $c$-axis with respect to the basic monoclinic CeCoAl-type structure. Below room temperature, structural modulation transitions with very broad hysteresis have been reported from measurements of various bulk properties. X-ray diffraction revealed that at low temperatures the doubling of the CeCoAl type structure is replaced by a different modulated ground state, approximating a near tripling of the basic CeCoAl cell. The transition is accompanied by a significant contraction of the $c$ axis. We present new x-ray absorption near-edge spectroscopy data at the Ce L$_{3}$ absorption edge, measured on a freshly cleaved surface of a CeRuSn single crystal. In contrast to a previous report, the new data exhibit small but significant variations as function of temperature that are consistent with a transition of a fraction of Ce$^{3+}$ ions to the intermediate valence state, analogous to the $γ\rightarrow α$ transition in elemental cerium, when cooling through the structural transitions of CeRuSn. Such results in a valence-modulated state. △ Less

Submitted 14 May, 2014; originally announced May 2014.

Journal ref: Phys. Rev. B 90, 041104 (2014)

Exceptional Ising Magnetic Behavior of Itinerant Spin-polarized Carriers in URu2Si2

Authors: Miroslaw Werwinski, Jan Rusz, John A. Mydosh, Peter M. Oppeneer

Abstract: We show on the basis of electronic structure calculations that the uranium 5$f$ magnetic moment in URu$_2$Si$_2$ exhibits a unique Ising behavior, which surprisingly, arises from itinerant electronic states. The origin of the unusual Ising behavior is analyzed as due to the peculiar near-Fermi edge nested electronic structure of URu$_2$Si$_2$ involving its strong spin-orbit interaction. The Ising… ▽ More We show on the basis of electronic structure calculations that the uranium 5$f$ magnetic moment in URu$_2$Si$_2$ exhibits a unique Ising behavior, which surprisingly, arises from itinerant electronic states. The origin of the unusual Ising behavior is analyzed as due to the peculiar near-Fermi edge nested electronic structure of URu$_2$Si$_2$ involving its strong spin-orbit interaction. The Ising anisotropy has pertinent implications for theories applicable to explaining the Hidden Order phase in URu$_2$Si$_2$. △ Less

Submitted 7 May, 2014; originally announced May 2014.

Comments: 5 pages, 3 figures; to be published

Hidden Order Behaviour in URu2Si2 (A Critical Review of the Status of Hidden Order in 2014)

Authors: John A. Mydosh, Peter M. Oppeneer

Abstract: Throughout the past three decades the hidden order (HO) problem in URu$_2$Si$_2$ has remained a "hot topic" in the physics of strongly correlated electron systems with well over 600 publications related to this subject. Presently in 2014 there has been significant progress in combining various experimental results embedded within electronic structure calculations using density functional theory (D… ▽ More Throughout the past three decades the hidden order (HO) problem in URu$_2$Si$_2$ has remained a "hot topic" in the physics of strongly correlated electron systems with well over 600 publications related to this subject. Presently in 2014 there has been significant progress in combining various experimental results embedded within electronic structure calculations using density functional theory (DFT) to give a consistent description of the itinerant behaviour of the HO transition and its low temperature state. Here we review six different experiments: ARPES, quantum oscillations, neutron scattering, RXD, optical spectroscopy and STM/STS. We then establish the consistencies among these experiments when viewed through the Fermi-surface nesting, folding and gapping framework as predicted by DFT. We also discuss a group of other experiments (torque, cyclotron resonance, NMR and XRD) that are more controversial and are presently in a "transition" state regarding their interpretation as rotational symmetry breaking and dotriacontapole formation. There are also a series of recent "exotic" experiments (Raman scattering, polar Kerr effect and ultrasonics) that require verification, yet they offer new insights into the HO symmetry breaking and order parameter. We conclude with some constraining comments on the microscopic models that rely on localised $5f$-U states and strong Ising anisotropy {for explaining} the HO transition, and with an examination of different models in the light of recent experiments. △ Less

Submitted 7 May, 2014; originally announced May 2014.

Comments: 21 pages, 12 figures; to appear in Phil. Mag

Strict limit on in-plane ordered magnetic dipole moment in URu2Si2

Authors: K. A. Ross, L. Harriger, Z. Yamani, W. J. L. Buyers, J. D. Garrett, A. A. Menovsky, J. A. Mydosh, C. L. Broholm

Abstract: Neutron diffraction is used to examine the polarization of weak static antiferromagnetism in high quality single crystalline URu2Si2. As previously documented, elastic Bragg-like diffraction develops for temperature T<T_{HO}= 17.5 K at q=(100) but not at wave vector transfer q=(001). The peak width indicates correlation lengths ξ_c=230(12) Å\ and ξ_a=240(15) Å. The integrated intensity of the T-de… ▽ More Neutron diffraction is used to examine the polarization of weak static antiferromagnetism in high quality single crystalline URu2Si2. As previously documented, elastic Bragg-like diffraction develops for temperature T<T_{HO}= 17.5 K at q=(100) but not at wave vector transfer q=(001). The peak width indicates correlation lengths ξ_c=230(12) Å\ and ξ_a=240(15) Å. The integrated intensity of the T-dependent peaks corresponds to a sample averaged c-oriented staggered moment of μ_{c}=0.022(1) μ_B at T=1.7 K. The absence of T-dependent diffraction at q=(001) places a limit μ_{\perp}<0.0011 μ_B on an f- or d-orbital based in-plane staggered magnetic dipole moment, which is associated with multipolar orders proposed for URu_2Si_2. △ Less

Submitted 11 February, 2014; originally announced February 2014.

Comments: 9 pages, 7 figures

Detection of electronic nematicity using scanning tunneling microscopy

Authors: Eduardo H. da Silva Neto, Pegor Aynajian, Ryan E. Baumbach, Eric D. Bauer, John Mydosh, Shimpei Ono, Ali Yazdani

Abstract: Electronic nematic phases have been proposed to occur in various correlated electron systems and were recently claimed to have been detected in scanning tunneling microscopy (STM) conductance maps of the pseudogap states of the cuprate high-temperature superconductor Bi2Sr2CaCu2O8+x (Bi-2212). We investigate the influence of anisotropic STM tip structures on such measurements and establish, with a… ▽ More Electronic nematic phases have been proposed to occur in various correlated electron systems and were recently claimed to have been detected in scanning tunneling microscopy (STM) conductance maps of the pseudogap states of the cuprate high-temperature superconductor Bi2Sr2CaCu2O8+x (Bi-2212). We investigate the influence of anisotropic STM tip structures on such measurements and establish, with a model calculation, the presence of a tunneling interference effect within an STM junction that induces energy-dependent symmetry-breaking features in the conductance maps. We experimentally confirm this phenomenon on different correlated electron systems, including measurements in the pseudogap state of Bi-2212, showing that the apparent nematic behavior of the imaged crystal lattice is likely not due to nematic order but is related to how a realistic STM tip probes the band structure of a material. We further establish that this interference effect can be used as a sensitive probe of changes in the momentum structure of the sample's quasiparticles as a function of energy. △ Less

Submitted 16 April, 2013; originally announced April 2013.

Comments: Accepted for publication (PRB - Rapid Communications). Main text (5 pages, 4 figures) + Supplemental Material (4 pages, 4 figures)

Journal ref: Phys. Rev. B 87, 161117(R) (2013)

Photoemission Imaging of 3D Fermi Surface Pairing at the Hidden Order Transition in URu2Si2

Authors: Jian-Qiao Meng, Peter M. Oppeneer, John A. Mydosh, Peter S. Riseborough, Krzysztof Gofryk, John J. Joyce, Eric D. Bauer, Yinwan Li, Tomasz Durakiewicz

Abstract: We report angle-resolved photoemission spectroscopy (ARPES) experiments probing deep into the hidden order (HO) state of URu2Si2, utilizing tunable photon energies with sufficient energy and momentum resolution to detect the near Fermi surface (FS) behavior. Our results reveal: (i) the full itinerancy of the 5f electrons; (ii) the crucial three-dimensional (3D) k-space nature of the FS and its cri… ▽ More We report angle-resolved photoemission spectroscopy (ARPES) experiments probing deep into the hidden order (HO) state of URu2Si2, utilizing tunable photon energies with sufficient energy and momentum resolution to detect the near Fermi surface (FS) behavior. Our results reveal: (i) the full itinerancy of the 5f electrons; (ii) the crucial three-dimensional (3D) k-space nature of the FS and its critical nesting vectors, in good comparison with density-functional theory calculations, and (iii) the existence of hot-spot lines and pairing of states at the FS, leading to FS gapping in the HO phase. △ Less

Submitted 18 February, 2013; originally announced February 2013.

High-magnetic field lattice length changes in URu2Si2

Authors: V. F. Correa, S. Francoual, M. Jaime, N. Harrison, T. P. Murphy, E. C. Palm, S. W. Tozer, A. H. Lacerda, P. A. Sharma, J. A. Mydosh

Abstract: We report high magnetic field (up to 45 T) c-axis thermal expansion and magnetostriction experiments on URu2Si2 single crystals. The sample length change associated with the transition to the hidden order phase becomes increasingly discontinous as the magnetic field is raised above 25 T. The re-entrant ordered phase III is clearly observed in both the thermal expansion and magnetostriction above 3… ▽ More We report high magnetic field (up to 45 T) c-axis thermal expansion and magnetostriction experiments on URu2Si2 single crystals. The sample length change associated with the transition to the hidden order phase becomes increasingly discontinous as the magnetic field is raised above 25 T. The re-entrant ordered phase III is clearly observed in both the thermal expansion and magnetostriction above 36 T, in good agreement with previous results. The sample length is also discontinuous at the boundaries of this phase, mainly at the upper boundary. A change in the sign of the coefficient of thermal-expansion is observed at the metamagnetic transition (B_M = 38 T) which is likely related to the existence of a quantum critical end point. △ Less

Submitted 13 November, 2012; originally announced November 2012.

Comments: 5 pages, 4 figures, to be published in PRL

Disorder-driven electronic localization and phase separation in superconducting Fe1+yTe0.5Se0.5 single crystals

Authors: S. Rößler, Dona Cherian, S. Harikrishnan, H. L. Bhat, Suja Elizabeth, J. A. Mydosh, L. H. Tjeng, F. Steglich, S. Wirth

Abstract: We have investigated the influence of Fe-excess on the electrical transport and magnetism of Fe1+yTe0.5Se0.5 (y=0.04 and 0.09) single crystals. Both compositions exhibit resistively determined superconducting transitions (Tc) with an onset temperature of about 15 K. From the width of the superconducting transition and the magnitude of the lower critical field Hc1, it is inferred that excess of Fe… ▽ More We have investigated the influence of Fe-excess on the electrical transport and magnetism of Fe1+yTe0.5Se0.5 (y=0.04 and 0.09) single crystals. Both compositions exhibit resistively determined superconducting transitions (Tc) with an onset temperature of about 15 K. From the width of the superconducting transition and the magnitude of the lower critical field Hc1, it is inferred that excess of Fe suppresses superconductivity. The linear and non-linear responses of the ac-susceptibility show that the superconducting state for these compositions is inhomogeneous. A possible origin of this phase separation is a magnetic coupling between Fe-excess occupying interstitial sites in the chalcogen planes and those in the Fe-square lattice. The temperature derivative of the resistivity drho/dT in the temperature range Tc < T < Ta with Ta being the temperature of a magnetic anomaly, changes from positive to negative with increasing Fe. A log 1/T divergence of the resistivity above Tc in the sample with higher amount of Fe suggests a disorder driven electronic localization. △ Less

Submitted 6 September, 2012; originally announced September 2012.

Comments: 7 pages

Journal ref: Phys. Rev. B, 82, 144523 (2010)

119Sn solid state NMR and Mössbauer spectroscopic studies of the intermediate-valent stannide CeRuSn

Authors: F. M. Schappacher, P. Khuntia, A. K. Rajarajan, M. Baenitz, J. A. Mydosh, B. Chevalier, S. F. Matar, R. Pöttgen

Abstract: The ternary stannide CeRuSn is a static mixed-valent cerium compound with an or-dering of trivalent and intermediate-valent cerium on two distinct crystallographic sites. 119Sn Mössbauer spectra showed two electronically almost identical tin atoms at 323 K, while at 298 K and below (77 and 4.2 K) two tin sites can clearly be distinguished. 119Sn solid state NMR experiments are performed to probe t… ▽ More The ternary stannide CeRuSn is a static mixed-valent cerium compound with an or-dering of trivalent and intermediate-valent cerium on two distinct crystallographic sites. 119Sn Mössbauer spectra showed two electronically almost identical tin atoms at 323 K, while at 298 K and below (77 and 4.2 K) two tin sites can clearly be distinguished. 119Sn solid state NMR experiments are performed to probe the local hyperfine fields at the two different Sn sites. 119Sn NMR powder spectra are nicely fitted with two Sn sites with nearly the same magnetic anisotropy, but with different absolute shift values. Both Sn sites are strongly affected by crossover-like transitions between 100 and 280 K. This local-site study confirms the superstructure modulations found in previous investiga-tions. Towards lower temperatures the powder spectra are broadened giving strong evidence for the antiferromagnetically ordered ground state. △ Less

Submitted 15 June, 2012; originally announced June 2012.

Journal ref: Z. Naturforsch. 67b, 473(2012)

Field induced phases in UPt2Si2

Authors: Dirk Schulze Grachtrup, Matthias Bleckmann, Britta Willenberg, Stefan Suellow, Marek Bartkowiak, Yurii Skourski, Harison Rakoto, Ilya Sheikin, John A. Mydosh

Abstract: The tetragonal compound UPt2Si2 has been characterised as a moderately mass enhanced system with an antiferromagnetic ground state below T_N = 32 K. Here, we present an extensive study of the behavior in high magnetic fields. We have performed pulsed field magnetization and static field resistivity measurements on single crystalline samples UPt2Si2. Along the crystallographic a axis, at low temper… ▽ More The tetragonal compound UPt2Si2 has been characterised as a moderately mass enhanced system with an antiferromagnetic ground state below T_N = 32 K. Here, we present an extensive study of the behavior in high magnetic fields. We have performed pulsed field magnetization and static field resistivity measurements on single crystalline samples UPt2Si2. Along the crystallographic a axis, at low temperatures, we find a metamagnetic-like transition in fields of the order 40 T, possibly indicating a first order transition. Along the crystallographic c axis, in magnetic fields of B>= ~24 T, we find distinct anomalies in both properties. From our analysis of the data we can distinguish new high field phases above the AFM ground state. We discuss the emergence of these new phases in the context of Fermi surface effects and the possible occurrence of a Lifshitz or electronic topological transition, this in contrast to previous modellings of UPt2Si2 based on crystal electric field effects. △ Less

Submitted 19 December, 2011; v1 submitted 14 December, 2011; originally announced December 2011.

Comments: 10 pages, submitted to PRB

Complex charge ordering in CeRuSn

Authors: R. Feyerherm, E. Dudzik, S. Valencia, J. A. Mydosh, Y. -K. Huang, W. Hermes, R. Pöttgen

Abstract: At ambient temperatures, CeRuSn exhibits an extraordinary structure with a coexistence of two types of Ce ions in a metallic environment, namely trivalent Ce3+ and intermediate valent Ce(4-x)+. Charge ordering produces a doubling of the unit cell along the c-axis with respect to the basic monoclinic CeCoAl type structure. Below room temperature, a phase transition with very broad hysteresis has be… ▽ More At ambient temperatures, CeRuSn exhibits an extraordinary structure with a coexistence of two types of Ce ions in a metallic environment, namely trivalent Ce3+ and intermediate valent Ce(4-x)+. Charge ordering produces a doubling of the unit cell along the c-axis with respect to the basic monoclinic CeCoAl type structure. Below room temperature, a phase transition with very broad hysteresis has been observed in various bulk properties like electrical resistivity, magnetic susceptibility, and specific heat. The present x-ray diffraction results show that at low temperatures the doubling of the CeCoAl type structure is replaced by an ill-defined modulated ground state. In this state, at least three different modulation periods compete, with the dominant mode close to a tripling of the basic cell. The transition is accompanied by a significant contraction of the c axis. XANES data suggest that the average Ce valence remains constant, thus the observed c axis contraction is not due to any valence transition. We propose a qualitative structure model with modified stacking sequences of Ce3+ and Ce(4-x)+ layers in the various modulated phases. Surprisingly, far below 100 K the modulated state is sensitive to x-ray irradiation at photon fluxes available at a synchrotron. With photon fluxes of order 10E12/s, the modulated ground state can be destroyed on a timescale of minutes and the doubling of the CeCoAl cell observed at room temperature is recovered. The final state is metastable at 10 K. Heating the sample above 60 K again leads to a recovery of the modulated state. Thus, CeRuSn exhibits both thermally and x-ray induced reversible transformations of the Ce3+/Ce(4-x)+ charge ordering pattern. Such a behavior is unique among any know intermetallic compound. △ Less

Submitted 24 November, 2011; originally announced November 2011.

Comments: submitted to Phys. Rev. B

Journal ref: Phys. Rev. B 85, 085120 (2012)

Role of commensurate and incommensurate low-energy excitations in the paramagnetic to hidden-order transition of URu$_2$Si$_2$

Authors: P. G. Niklowitz, S. R. Dunsiger, C. Pfleiderer, P. Link, A. Schneidewind, E. Faulhaber, M. Vojta, Y. -K. Huang, J. A. Mydosh

Abstract: We report low-energy inelastic neutron scattering data of the paramagnetic (PM) to hidden-order (HO) phase transition at $T_0=17.5\,{\rm K}$ in URu$_2$Si$_2$. While confirming previous results for the HO and PM phases, our data reveal a pronounced wavevector dependence of low-energy excitations across the phase transition. To analyze the energy scans we employ a damped harmonic oscillator model co… ▽ More We report low-energy inelastic neutron scattering data of the paramagnetic (PM) to hidden-order (HO) phase transition at $T_0=17.5\,{\rm K}$ in URu$_2$Si$_2$. While confirming previous results for the HO and PM phases, our data reveal a pronounced wavevector dependence of low-energy excitations across the phase transition. To analyze the energy scans we employ a damped harmonic oscillator model containing a fit parameter $1/Γ$ which is expected to diverge at a second-order phase transition. Counter to expectations the excitations at $\vec{Q}_1=(1.44,0,0)$ show an abrupt step-like suppression of $1/Γ$ below $T_0$, whereas excitations at $\vec{Q}_0=(1,0,0)$, associated with large-moment antiferromagnetism (LMAF) under pressure, show an enhancement and a pronounced peak of $1/Γ$ at $T_0$. Therefore, at the critical HO temperature $T_0$, LMAF fluctuations become nearly critical as well. This is the behavior expected of a super-vector order parameter with nearly degenerate components for the HO and LMAF leading to nearly isotropic fluctuations in the combined order-parameter space. △ Less

Submitted 29 October, 2015; v1 submitted 25 October, 2011; originally announced October 2011.

Comments: 6 pages; v3 accepted journal version; minor modifications compared to v2

Journal ref: Phys. Rev. B 92, 115116 (2015)

Spin and orbital hybridization at specifically nested Fermi surfaces in URu$_2$Si$_2$

Authors: Peter M. Oppeneer, Saad Elgazzar, Jan Rusz, Qingguo Feng, Tomasz Durakiewicz, John A. Mydosh

Abstract: The Fermi surface (FS) nesting properties of URu$_2$Si$_2$ are analyzed with particular focus on their implication for the mysterious hidden order phase. We show that there exist two Fermi surfaces that exhibit a strong nesting at the antiferromagnetic wavevector, $\boldsymbol{Q}_0$=(0,\,0,\,1). The corresponding energy dispersions fulfill the relation $ε_{1}(\boldsymbol{k})$=… ▽ More The Fermi surface (FS) nesting properties of URu$_2$Si$_2$ are analyzed with particular focus on their implication for the mysterious hidden order phase. We show that there exist two Fermi surfaces that exhibit a strong nesting at the antiferromagnetic wavevector, $\boldsymbol{Q}_0$=(0,\,0,\,1). The corresponding energy dispersions fulfill the relation $ε_{1}(\boldsymbol{k})$=$- ε_{2} (\boldsymbol{k}\pm \boldsymbol{Q}_0)$ at eight FS hotspot lines. The spin-orbital characters of the involved $5f$ states are {\it distinct} ($j_z$=$\pm$5/2 {\it vs.} $\pm$3/2) and hence the degenerate Dirac crossings are symmetry protected in the nonmagnetic normal state. Dynamical symmetry breaking through an Ising-like spin and orbital excitation mode with $Δj_z$=$\pm$1 induces a hybridization of the two states, causing substantial FS gapping. Concomitant spin and orbital currents in the uranium planes give rise to a rotational symmetry breaking. △ Less

Submitted 5 October, 2011; originally announced October 2011.

Comments: 5 pages, 3 figures

Colloquium: Hidden Order, Superconductivity, and Magnetism -- The Unsolved Case of URu2Si2

Authors: J. A. Mydosh, P. M. Oppeneer

Abstract: This Colloquium reviews the 25 year quest for understanding the continuous (second-order) mean-field-like phase transition occurring at 17.5 K in URu2Si2. About ten years ago, the term hidden order (HO) was coined and has since been utilized to describe the unknown ordered state, whose origin cannot be disclosed by conventional solid-state probes, such as x rays, neutrons, or muons. HO is able to… ▽ More This Colloquium reviews the 25 year quest for understanding the continuous (second-order) mean-field-like phase transition occurring at 17.5 K in URu2Si2. About ten years ago, the term hidden order (HO) was coined and has since been utilized to describe the unknown ordered state, whose origin cannot be disclosed by conventional solid-state probes, such as x rays, neutrons, or muons. HO is able to support superconductivity at lower temperatures (Tc ~ 1.5 K), and when magnetism is developed with increasing pressure both the HO and the superconductivity are destroyed. Other ways of probing the HO are via Rh-doping and very large magnetic fields. During the last few years a variety of advanced techniques have been tested to probe the HO state and their attempts will be summarized. A digest of recent theoretical developments is also included. It is the objective of this Colloquium to shed additional light on the HO state and its associated phases in other materials. △ Less

Submitted 16 November, 2011; v1 submitted 1 July, 2011; originally announced July 2011.

Comments: 25 pages, 16 figures, published in Reviews of Modern Physics

Journal ref: Reviews of Modern Physics, 83 (2011) 1301-1322

Observing the origin of superconductivity in quantum critical metals

Authors: Jian-Huang She, Bas J. Overbosch, Ya-Wen Sun, Yan Liu, Koenraad Schalm, John A. Mydosh, Jan Zaanen

Abstract: Despite intense efforts during the last 25 years, the physics of unconventional superconductors, including the cuprates with a very high transition temperature, is still a controversial subject. It is believed that superconductivity in many of these strongly correlated metallic systems originates in the physics of quantum phase transitions, but quite diverse perspectives have emerged on the fundam… ▽ More Despite intense efforts during the last 25 years, the physics of unconventional superconductors, including the cuprates with a very high transition temperature, is still a controversial subject. It is believed that superconductivity in many of these strongly correlated metallic systems originates in the physics of quantum phase transitions, but quite diverse perspectives have emerged on the fundamentals of the electron-pairing physics, ranging from Hertz style critical spin fluctuation glue to the holographic superconductivity of string theory. Here we demonstrate that the gross energy scaling differences that are behind these various pairing mechanisms are directly encoded in the frequency and temperature dependence of the dynamical pair susceptibility. This quantity can be measured directly via the second order Josephson effect and it should be possible employing modern experimental techniques to build a `pairing telescope' that gives a direct view on the origin of quantum critical superconductivity. △ Less

Submitted 17 August, 2011; v1 submitted 26 May, 2011; originally announced May 2011.

Comments: 19 pages, 9 figures; minor changes in the experimental part; added a new appendix section calculating the pair susceptibility of marginal Fermi liquid

Journal ref: Phys. Rev. B 84, 144527 (2011)

Anomalous Femtosecond Quasiparticle Dynamics of Hidden Order State in URu$_2$Si$_2$

Authors: Georgi L. Dakovski, Yinwan Li, Steve M. Gilbertson, George Rodriguez, Alexander V. Balatsky, Jian-Xin Zhu, Krzysztof Gofryk, Eric D. Bauer, Paul H. Tobash, Antoinette Taylor, John L. Sarrao, Peter M. Oppeneer, Peter S. Riseborough, John A. Mydosh, Tomasz Durakiewicz

Abstract: At T$_0$ = 17.5 K an exotic phase emerges from a heavy fermion state in {\ur}. The nature of this hidden order (HO) phase has so far evaded explanation. Formation of an unknown quasiparticle (QP) structure is believed to be responsible for the massive removal of entropy at HO transition, however, experiments and ab-initio calculations have been unable to reveal the essential character of the QP. H… ▽ More At T$_0$ = 17.5 K an exotic phase emerges from a heavy fermion state in {\ur}. The nature of this hidden order (HO) phase has so far evaded explanation. Formation of an unknown quasiparticle (QP) structure is believed to be responsible for the massive removal of entropy at HO transition, however, experiments and ab-initio calculations have been unable to reveal the essential character of the QP. Here we use femtosecond pump-probe time- and angle-resolved photoemission spectroscopy (tr-ARPES) to elucidate the ultrafast dynamics of the QP. We show how the Fermi surface is renormalized by shifting states away from the Fermi level at specific locations, characterized by vector $q_{<110>} = 0.56 \pm 0.08$ {\an}. Measurements of the temperature-time response reveal that upon entering the HO the QP lifetime in those locations increases from 42 fs to few hundred fs. The formation of the long-lived QPs is identified here as a principal actor of the HO. △ Less

Submitted 19 April, 2011; originally announced April 2011.

Journal ref: Phys. Rev. B 84, 161103(R) (2011)

Quantum fluctuations and strong mass renormalization in NiCl2-4SC(NH2)2

Authors: Y. Kohama, A. Sologubenko, N. R. Dilley, V. S. Zapf, M. Jaime, J. Mydosh, A. Paduan-Filho, K. Al-Hassanieh, P. Sengupta, S. Gangadharaiah, A. L. Chernyshev, C. D. Batista

Abstract: In a number of quantum paramagnets, magnetic field can induce a quantum phase transition to an antiferromagnetic state which exists for a range of fields Hc1 < H < Hc2. Generally, these compounds exhibit a significant asymmetry in their properties at low- and high-field transitions. Here we present detailed specific heat and thermal conductivity measurements in NiCl2-4SC(NH2)2 together with analyt… ▽ More In a number of quantum paramagnets, magnetic field can induce a quantum phase transition to an antiferromagnetic state which exists for a range of fields Hc1 < H < Hc2. Generally, these compounds exhibit a significant asymmetry in their properties at low- and high-field transitions. Here we present detailed specific heat and thermal conductivity measurements in NiCl2-4SC(NH2)2 together with analytical and numerical results. We show that the asymmetry is caused by a strong renormalization of the effective mass of spin excitations due to quantum fluctuations for H<Hc1 that are absent for H<Hc2. △ Less

Submitted 3 December, 2010; v1 submitted 31 August, 2010; originally announced September 2010.

Comments: 4 pages, 3 figures. Acepted for publication in Phy. Rev. Lett

Journal ref: Phys. Rev. Lett. 106, 037203 (2011)

Electronic structure theory of the hidden order material URu$_2$Si$_2$

Authors: P. M. Oppeneer, J. Rusz, S. Elgazzar, M. -T. Suzuki, T. Durakiewicz, J. A. Mydosh

Abstract: We report a comprehensive electronic structure investigation of the paramagnetic (PM), the large moment antiferromagnetic (LMAF), and the hidden order (HO) phases of URu$_2$Si$_2$. We have performed relativistic full-potential calculations on the basis of the density functional theory (DFT), employing different exchange-correlation functionals to treat electron correlations within the open $5f$-sh… ▽ More We report a comprehensive electronic structure investigation of the paramagnetic (PM), the large moment antiferromagnetic (LMAF), and the hidden order (HO) phases of URu$_2$Si$_2$. We have performed relativistic full-potential calculations on the basis of the density functional theory (DFT), employing different exchange-correlation functionals to treat electron correlations within the open $5f$-shell of uranium. Specifically, we investigate---through a comparison between calculated and low-temperature experimental properties---whether the $5f$ electrons are localized or delocalized in URu$_2$Si$_2$. We also performed dynamical mean field theory calculations (LDA+DMFT) to investigate the temperature evolution of the quasi-particle states at 100~K and above, unveiling a progressive opening of a quasi-particle gap at the chemical potential when temperature is reduced. A detailed comparison of calculated properties with known experimental data demonstrates that the LSDA and GGA approaches, in which the uranium $5f$ electrons are treated as itinerant, provide an excellent explanation of the available low-temperature experimental data of the PM and LMAF phases. We show furthermore that due to a materials-specific Fermi surface instability a large, but partial, Fermi surface gapping of up to 750 K occurs upon antiferromagnetic symmetry breaking. The occurrence of the HO phase is explained through dynamical symmetry breaking induced by a mode of long-lived antiferromagnetic spin-fluctuations. This dynamical symmetry breaking model explains why the Fermi surface gapping in the HO phase is similar but smaller than that in the LMAF phase and it also explains why the HO and LMAF phases have the same Fermi surfaces yet different order parameters. Suitable derived order parameters for the HO are proposed to be the Fermi surface gap or the dynamic spin-spin correlation function. △ Less

Submitted 20 July, 2010; originally announced July 2010.

Comments: 23 pages, 20 figures

Hybridization gap and anisotropic far-infrared optical conductivity of URu2Si2

Authors: J. Levallois, F. Lévy-Bertrand, M. K. Tran, D. Stricker, J. A. Mydosh, Y. -K. Huang, D. van der Marel

Abstract: We performed far-infrared optical spectroscopy measurements on the heavy fermion compound URu 2 Si 2 as a function of temperature. The light's electric-field was applied along the a-axis or the c-axis of the tetragonal structure. We show that in addition to a pronounced anisotropy, the optical conductivity exhibits for both axis a partial suppression of spectral weight around 12 meV and below 30 K… ▽ More We performed far-infrared optical spectroscopy measurements on the heavy fermion compound URu 2 Si 2 as a function of temperature. The light's electric-field was applied along the a-axis or the c-axis of the tetragonal structure. We show that in addition to a pronounced anisotropy, the optical conductivity exhibits for both axis a partial suppression of spectral weight around 12 meV and below 30 K. We attribute these observations to a change in the bandstructure below 30 K. However, since these changes have no noticeable impact on the entropy nor on the DC transport properties, we suggest that this is a crossover phenomenon rather than a thermodynamic phase transition. △ Less

Submitted 28 October, 2011; v1 submitted 4 July, 2010; originally announced July 2010.

Comments: To be published in Physical Review B

Journal ref: Phys. Rev. B 84, 184420 (2011)

Visualizing the Formation of the Kondo Lattice and the Hidden Order in URu2Si2

Authors: Pegor Aynajian, Eduardo H. da Silva Neto, Colin V. Parker, Yingkai Huang, Abhay Pasupathy, John Mydosh, Ali Yazdani

Abstract: Heavy electronic states originating from the f atomic orbitals underlie a rich variety of quantum phases of matter. We use atomic scale imaging and spectroscopy with the scanning tunneling microscope (STM) to examine the novel electronic states that emerge from the uranium f states in URu2Si2. We find that as the temperature is lowered, partial screening of the f electrons' spins gives rise to a s… ▽ More Heavy electronic states originating from the f atomic orbitals underlie a rich variety of quantum phases of matter. We use atomic scale imaging and spectroscopy with the scanning tunneling microscope (STM) to examine the novel electronic states that emerge from the uranium f states in URu2Si2. We find that as the temperature is lowered, partial screening of the f electrons' spins gives rise to a spatially modulated Kondo-Fano resonance that is maximal between the surface U atoms. At T=17.5 K, URu2Si2 is known to undergo a 2nd order phase transition from the Kondo lattice state into a phase with a hidden order parameter. From tunneling spectroscopy, we identify a spatially modulated, bias-asymmetric energy gap with a mean-field temperature dependence that develops in the hidden order state. Spectroscopic imaging further reveals a spatial correlation between the hidden order gap and the Kondo resonance, suggesting that the two phenomena involve the same electronic states. △ Less

Submitted 26 May, 2010; v1 submitted 26 March, 2010; originally announced March 2010.

Journal ref: published online: PNAS 2010 http://www.pnas.org/content/early/2010/05/18/1005892107.abstract

Spinon localization in the heat transport of the spin-1/2 ladder compound (C$_5$H$_{12}$N)$_2$CuBr$_4$

Authors: A. V. Sologubenko, T. Lorenz, J. A. Mydosh, B. Thielemann, H. M. Ronnow, Ch. Ruegg, K. Kramer

Abstract: We present experiments on the magnetic field-dependent thermal transport in the spin-1/2 ladder system (C$_5$H$_{12}$N)$_2$CuBr$_4$. The thermal conductivity $κ(B)$ is only weakly affected by the field-induced transitions between the gapless Luttinger-liquid state realized for $B_{c1}< B < B_{c2}$ and the gapped states, suggesting the absence of a direct contribution of the spin excitations to t… ▽ More We present experiments on the magnetic field-dependent thermal transport in the spin-1/2 ladder system (C$_5$H$_{12}$N)$_2$CuBr$_4$. The thermal conductivity $κ(B)$ is only weakly affected by the field-induced transitions between the gapless Luttinger-liquid state realized for $B_{c1}< B < B_{c2}$ and the gapped states, suggesting the absence of a direct contribution of the spin excitations to the heat transport. We observe, however, that the thermal conductivity is strongly suppressed by the magnetic field deeply within the Luttinger-liquid state. These surprising observations are discussed in terms of localization of spinons within finite ladder segments and spinon-phonon umklapp scattering of the predominantly phononic heat transport. △ Less

Submitted 28 September, 2009; originally announced September 2009.

Comments: 4 pages, 3 figures

Journal ref: PHYSICAL REVIEW B 80, 220411(R) (2009)

Parasitic small-moment-antiferromagnetism and non-linear coupling of hidden order and antiferromagnetism in URu2Si2 observed by Larmor diffraction

Authors: P. G. Niklowitz, C. Pfleiderer, T. Keller, M. Vojta, Y. -K. Huang, J. A. Mydosh

Abstract: We report simultaneous measurements of the distribution of lattice constants and the antiferromagnetic moment in high-purity URu2Si2, using both Larmor and conventional neutron diffraction, as a function of temperature and pressure up to 18 kbar. We establish that the tiny moment in the hidden order (HO) state is purely parasitic and quantitatively originates from the distribution of lattice con… ▽ More We report simultaneous measurements of the distribution of lattice constants and the antiferromagnetic moment in high-purity URu2Si2, using both Larmor and conventional neutron diffraction, as a function of temperature and pressure up to 18 kbar. We establish that the tiny moment in the hidden order (HO) state is purely parasitic and quantitatively originates from the distribution of lattice constants. Moreover, the HO and large-moment antiferromagnetism (LMAF) at high pressure are separated by a line of first-order phase transitions, which ends in a bicritical point. Thus the HO and LMAF are coupled non-linearly and must have different symmetry, as expected of the HO being, e.g., incommensurate orbital currents, helicity order, or multipolar order. △ Less

Submitted 11 September, 2009; originally announced September 2009.

Comments: 4 pages, 4 figures

Journal ref: Phys. Rev. Lett. 104 (2010) 106406

Epitaxy, stoichiometry, and magnetic properties of Gd-doped EuO films on YSZ (001)

Authors: R. Sutarto, S. G. Altendorf, B. Coloru, M. Moretti Sala, T. Haupricht, C. F. Chang, Z. Hu, C. Schüßler-Langeheine, N. Hollmann, H. Kierspel, J. A. Mydosh, H. H. Hsieh, H. -J. Lin, C. T. Chen, L. H. Tjeng

Abstract: We have succeeded in preparing high-quality Gd-doped single-crystalline EuO films. Using Eu-distillation-assisted molecular beam epitaxy and a systematic variation in the Gd and oxygen deposition rates, we have been able to observe sustained layer-by-layer epitaxial growth on yttria-stabilized cubic zirconia (001). The presence of Gd helps to stabilize the layer-by-layer growth mode. We used sof… ▽ More We have succeeded in preparing high-quality Gd-doped single-crystalline EuO films. Using Eu-distillation-assisted molecular beam epitaxy and a systematic variation in the Gd and oxygen deposition rates, we have been able to observe sustained layer-by-layer epitaxial growth on yttria-stabilized cubic zirconia (001). The presence of Gd helps to stabilize the layer-by-layer growth mode. We used soft x-ray absorption spectroscopy at the Eu and Gd M4,5 edges to confirm the absence of Eu3+ contaminants and to determine the actual Gd concentration. The distillation process ensures the absence of oxygen vacancies in the films. From magnetization measurements we found the Curie temperature to increase smoothly as a function of doping from 70 K up to a maximum of 125 K. A threshold behavior was not observed for concentrations as low as 0.2%. △ Less

Submitted 24 September, 2009; v1 submitted 9 March, 2009; originally announced March 2009.

Comments: 8 pages, 9 figures

Journal ref: Phys. Rev. B 80, 085308 (2009)

Evidence for strongly coupled charge-density-wave ordering in three-dimensional RE5Ir4Si10 compounds by optical measurements

Authors: Riccardo Tediosi, Fabrizio Carbone, A. B. Kuzmenko, J. Teyssier, D. van der Marel, J. A. Mydosh

Abstract: We report optical spectra of Lu$_5$Ir$_4$Si$_{10}$ and Er$_5$Ir$_4$Si$_{10}$, exhibiting the phenomenon of coexisting superconductivity or antiferromagnetism and charge density wave (CDW) order. We measure the maximum value of the charge density wave gap present on part of the Fermi surface of Lu5Ir4Si10, corresponding to a ratio 2Δ/k_B T_CDW \approx 10, well above the value in the limit of weak… ▽ More We report optical spectra of Lu$_5$Ir$_4$Si$_{10}$ and Er$_5$Ir$_4$Si$_{10}$, exhibiting the phenomenon of coexisting superconductivity or antiferromagnetism and charge density wave (CDW) order. We measure the maximum value of the charge density wave gap present on part of the Fermi surface of Lu5Ir4Si10, corresponding to a ratio 2Δ/k_B T_CDW \approx 10, well above the value in the limit of weak electron-phonon coupling. Strong electron-phonon coupling was confirmed by analyzing the optical conductivity with the Holstein model describing the electron-phonon interactions, indicating the coupling to phonons centered at 30 meV, with a coupling constant λ\approx 2.6. Finally we provide evidence that approximately 16 % of the Fermi surface of Lu5Ir4Si10 becomes gapped in the CDW state. △ Less

Submitted 16 July, 2009; v1 submitted 5 December, 2008; originally announced December 2008.

Comments: 11 pages, 14 figures

Journal ref: Physical Review B 80 (2009) 035107

Uncovering the hidden order in URu2Si2: Identification of Fermi surface instability and gapping

Authors: S. Elgazzar, J. Rusz, M. Amft, P. M. Oppeneer, J. A. Mydosh

Abstract: Spontaneous, collective ordering of electronic degrees of freedom leads to second-order phase transitions that are characterized by an order parameter. The notion "hidden order" (HO) has recently been used for a variety of materials where a clear phase transition occurs to a phase without a known order parameter. The prototype example is the heavy-fermion compound URu2Si2 where a mysterious HO t… ▽ More Spontaneous, collective ordering of electronic degrees of freedom leads to second-order phase transitions that are characterized by an order parameter. The notion "hidden order" (HO) has recently been used for a variety of materials where a clear phase transition occurs to a phase without a known order parameter. The prototype example is the heavy-fermion compound URu2Si2 where a mysterious HO transition occurs at 17.5K. For more than twenty years this system has been studied theoretically and experimentally without a firm grasp of the underlying physics. Using state-of-the-art density-functional theory calculations, we provide here a microscopic explanation for the HO. We identify the Fermi surface "hot spots" where degeneracy induces a Fermi surface instability and quantify how symmetry breaking lifts the degeneracy, causing a surprisingly large Fermi surface gapping. As mechanism for the HO we propose spontaneous symmetry breaking through collective antiferromagnetic moment excitations. △ Less

Submitted 17 September, 2008; originally announced September 2008.

Comments: 14 pages, 4 figures

Magneto-thermal evidence of a partial gap at the Fermi surface of UPt_2Si_2

Authors: N. Johannsen, S. Suellow, A. V. Sologubenko, T. Lorenz, J. A. Mydosh

Abstract: Motivated by the observation of a giant Nernst effect in URu_2Si_2, the thermoelectric response of the related system UPt_2Si_2 was investigated using thermal and electric transport properties such as the Nernst and Seebeck effects, thermal conductivity, Hall effect and electrical resitivity. Unlike URu_2Si_2, UPt_2Si_2 is neither superconducting nor exhibits a ``hidden-order'' state. Neverthele… ▽ More Motivated by the observation of a giant Nernst effect in URu_2Si_2, the thermoelectric response of the related system UPt_2Si_2 was investigated using thermal and electric transport properties such as the Nernst and Seebeck effects, thermal conductivity, Hall effect and electrical resitivity. Unlike URu_2Si_2, UPt_2Si_2 is neither superconducting nor exhibits a ``hidden-order'' state. Nevertheless a pronounced Nernst effect anomaly is found to coincide with the onset of the antiferromagnetic order in UPt_2Si_2. Although the absolute values are substantially lower, its appearance and characteristics can favorably be compared to the giant Nernst effect in URu_2Si_2 indicating the common feature of a partial Fermi surface gap. △ Less

Submitted 8 April, 2008; originally announced April 2008.

Comments: 4 pages, 4 figures

Field-dependent thermal transport in the Haldane chain compound NENP

Authors: A. V. Sologubenko, T. Lorenz, J. A. Mydosh, A. Rosch, K. C. Shortsleeves, M. M. Turnbull

Abstract: We present a study of the magnetic field-dependent thermal transport in the spin S=1 chain material Ni(C2H8N2)2NO2(ClO4) (NENP). The measured thermal conductivity is found to be very sensitive to the field-induced changes in the spin excitation spectrum. The magnetic contribution to the total heat conductivity is analyzed in terms of a quasiparticle model, and we obtain a temperature and momentu… ▽ More We present a study of the magnetic field-dependent thermal transport in the spin S=1 chain material Ni(C2H8N2)2NO2(ClO4) (NENP). The measured thermal conductivity is found to be very sensitive to the field-induced changes in the spin excitation spectrum. The magnetic contribution to the total heat conductivity is analyzed in terms of a quasiparticle model, and we obtain a temperature and momentum independent mean free path. This implies that the motion of quasiparticles is effectively three dimensional despite the tiny interchain coupling. △ Less

Submitted 2 April, 2008; v1 submitted 24 December, 2007; originally announced December 2007.

Comments: 4 pages, 4 figures

Journal ref: Phys. Rev. Lett. 100, 137202 (2008)

Mott-Hubbard versus charge-transfer behavior in LaSrMnO4 studied via optical conductivity

Authors: A. Gössling, M. W. Haverkort, M. Benomar, H. Wu, D. Senff, T. Möller, M. Braden, J. A. Mydosh, M. Grüninger

Abstract: Using spectroscopic ellipsometry, we study the optical conductivity sigma(omega) of insulating LaSrMnO4 in the energy range of 0.75-5.8 eV from 15 to 330 K. The layered structure gives rise to a pronounced anisotropy. A multipeak structure is observed in sigma_1^a(omega) (2, 3.5, 4.5, 4.9, and 5.5 eV), while only one peak is present at 5.6 eV in sigma_1^c(omega). We employ a local multiplet calc… ▽ More Using spectroscopic ellipsometry, we study the optical conductivity sigma(omega) of insulating LaSrMnO4 in the energy range of 0.75-5.8 eV from 15 to 330 K. The layered structure gives rise to a pronounced anisotropy. A multipeak structure is observed in sigma_1^a(omega) (2, 3.5, 4.5, 4.9, and 5.5 eV), while only one peak is present at 5.6 eV in sigma_1^c(omega). We employ a local multiplet calculation and obtain (i) an excellent description of the optical data, (ii) a detailed peak assignment in terms of the multiplet splitting of Mott-Hubbard and charge-transfer absorption bands, and (iii) effective parameters of the electronic structure, e.g., the on-site Coulomb repulsion U_eff=2.2 eV, the in-plane charge-transfer energy Delta_a=4.5 eV, and the crystal-field parameters for the d^4 configuration (10 Dq=1.2 eV, Delta_eg=1.4 eV, and Delta_t2g=0.2 eV). The spectral weight of the lowest absorption feature (at 1-2 eV) changes by a factor of 2 as a function of temperature, which can be attributed to the change of the nearest-neighbor spin-spin correlation function across the Neel temperature. Interpreting LaSrMnO4 effectively as a Mott-Hubbard insulator naturally explains this strong temperature dependence, the relative weight of the different absorption peaks, and the pronounced anisotropy. By means of transmittance measurements, we determine the onset of the optical gap Delta^a_opt=0.4-0.45 eV at 15 K and 0.1-0.2 eV at 300 K. Our data show that the crystal-field splitting is too large to explain the anomalous temperature dependence of the c-axis lattice parameter by thermal occupation of excited crystal-field levels. Alternatively, we propose that a thermal population of the upper Hubbard band gives rise to the shrinkage of the c-axis lattice parameter. △ Less

Submitted 9 January, 2008; v1 submitted 24 September, 2007; originally announced September 2007.

Comments: 13 pages, 10 figures

Journal ref: Phys. Rev. B 77, 035109 (2008)

Pyroxenes: A new class of multiferroics

Authors: S. Jodlauk, P. Becker, J. A. Mydosh, D. I. Khomskii, T. Lorenz, S. V. Streltsov, D. C. Hezel, L. Bohaty

Abstract: Pyroxenes with the general formula $AM$Si$_2$O$_6$ ($A$ = mono- or divalent metal, $M$ = di- or trivalent metal) are shown to be a new class of multiferroic materials. In particular, we have found so far that NaFeSi$_2$O$_6$ becomes ferroelectric in a magnetically ordered state below $\simeq 6$ K. Similarly, magnetically driven ferroelectricity is also detected in the Li homologues, LiFeSi$_2$O… ▽ More Pyroxenes with the general formula $AM$Si$_2$O$_6$ ($A$ = mono- or divalent metal, $M$ = di- or trivalent metal) are shown to be a new class of multiferroic materials. In particular, we have found so far that NaFeSi$_2$O$_6$ becomes ferroelectric in a magnetically ordered state below $\simeq 6$ K. Similarly, magnetically driven ferroelectricity is also detected in the Li homologues, LiFeSi$_2$O$_6$ ($T_C \simeq 18$ K) and LiCrSi$_2$O$_6$ ($T_C \simeq 11$ K). In all these monoclinic systems the electric polarization can be strongly modified by magnetic fields. Measurements of magnetic susceptibility, pyroelectric current and dielectric constants (and their dependence on magnetic field) are performed using a natural crystal of aegirine (NaFeSi$_2$O$_6$) and synthetic crystals of LiFeSi$_2$O$_6$ and LiCrSi$_2$O$_6$ grown from melt solution. For NaFeSi$_2$O$_6$ a temperature versus magnetic field phase diagram for NaFeSi$_2$O$_6$ is proposed. Exchange constants are computed on the basis of {\it ab initio} band structure calculations. The possibility of a spiral magnetic structure caused by frustration as origin of the multiferroic behaviour is discussed. We propose that other pyroxenes may also be multiferroic, and that the versatility of this family offers an exceptional opportunity to study general conditions for and mechanisms of magnetically driven ferroelectricity. △ Less

Submitted 9 October, 2007; v1 submitted 7 September, 2007; originally announced September 2007.

Comments: 10 pages, 4 figures; calculated excange constants have been updated

Journal ref: J. Phys.: Condens. Matter 19, 432201 (2007)

Thermal expansion of the magnetically ordering intermetallics RTMg (R = Eu, Gd and T = Ag, Au)

Authors: J. Rohrkamp, O. Heyer, T. Fickenscher, R. Poettgen, S. Jodlauk, H. Hartmann, T. Lorenz, J. A. Mydosh

Abstract: We report measurements of the thermal expansion for two Eu$^{+2}$- and two Gd$^{+3}$-based intermetallics which exhibit ferro- or antiferromagnetic phase transitions. These materials show sharp positive (EuAgMg and GdAuMg) and negative (EuAuMg and GdAgMg) peaks in the temperature dependence of the thermal expansion coefficient $α$ which become smeared and/or displaced in an external magnetic fie… ▽ More We report measurements of the thermal expansion for two Eu$^{+2}$- and two Gd$^{+3}$-based intermetallics which exhibit ferro- or antiferromagnetic phase transitions. These materials show sharp positive (EuAgMg and GdAuMg) and negative (EuAuMg and GdAgMg) peaks in the temperature dependence of the thermal expansion coefficient $α$ which become smeared and/or displaced in an external magnetic field. Together with specific heat data we determine the initial pressure dependences of the transition temperatures at ambient pressure using the Ehrenfest or Clausius-Clapeyron relation. We find large pressure dependences indicating strong spin-phonon coupling, in particular for GdAgMg and EuAuMg where a quantum phase transition might be reached at moderate pressures of a few GPa. △ Less

Submitted 21 August, 2007; originally announced August 2007.

Comments: 6 pages, 3 figures

Journal ref: J. Phys.: Condens. Matter 19, 486204 (2007)

Insulator to semiconductor transition and magnetic properties of the one-dimensional S = 1/2 system In_2VO_5

Authors: A. Moller, T. Taetz, N. Hollmann, J. A. Mydosh, V. Kataev, M. Yehia, E. Vavilova, B. Buchner

Abstract: We report structural, magnetization, electrical resistivity and nuclear- and electron spin resonance data of the complex transition metal oxide In_2VO_5 in which structurally well-defined V-O chains are realized. An itinerant character of the vanadium d-electrons and ferromagnetic correlations, revealed at high temperatures, are contrasted with the insulating behavior and predominantly antiferro… ▽ More We report structural, magnetization, electrical resistivity and nuclear- and electron spin resonance data of the complex transition metal oxide In_2VO_5 in which structurally well-defined V-O chains are realized. An itinerant character of the vanadium d-electrons and ferromagnetic correlations, revealed at high temperatures, are contrasted with the insulating behavior and predominantly antiferromagnetic exchange between the localized V^{4+} S = 1/2-magnetic moments which develop below a certain characteristic temperature T* ~ 120 K. Eventually the compound exhibits short-range magnetic order at $T_SRO ~ 20 K. We attribute this crossover occurring around T* to the unusual anisotropic thermal contraction of the lattice which changes significantly the overlap integrals and the character of magnetic intra- and interchain interactions. △ Less

Submitted 15 August, 2007; originally announced August 2007.

Comments: submitted to Phys. Rev. B

Nernst Effect in NdBa_2[Cu_{1-y}Ni_y]_3O_{7-δ}

Authors: N. Johannsen, Th. Wolf, A. V. Sologubenko, T. Lorenz, A. Freimuth, J. A. Mydosh

Abstract: In NdBa_2[Cu_{1-y}Ni_y]_3O_{7-δ}, magnetic Ni-impurities suppress Tc but at the same time the pseudogap is strongly enhanced. This unique feature makes it an ideal system to study possible relations between the anomalous Nernst effect, superconductivity and the pseudogap. We present Nernst effect measurements on a series of optimally doped (O_7) and underdoped (O_{6.8}) samples with Ni contents… ▽ More In NdBa_2[Cu_{1-y}Ni_y]_3O_{7-δ}, magnetic Ni-impurities suppress Tc but at the same time the pseudogap is strongly enhanced. This unique feature makes it an ideal system to study possible relations between the anomalous Nernst effect, superconductivity and the pseudogap. We present Nernst effect measurements on a series of optimally doped (O_7) and underdoped (O_{6.8}) samples with Ni contents ranging from y=0 to 0.12. In all samples an onset of the Nernst signal is found at T^ν> Tc. For the optimally doped samples T^νand Tc decrease simultaneously with increasing Ni content. The underdoped samples show a different behavior, i.e. the onset of the Nernst signal is hardly affected by increasing the Ni content from y=0 to 0.03. Irrespective of the oxygen content, T^νclearly does not track the enhanced pseudogap temperature T*. △ Less

Submitted 11 June, 2007; originally announced June 2007.

Journal ref: Phys. Rev. B 76, 020512(R) (2007)

Anomalous thermal expansion and strong damping of the thermal conductivity of NdMnO$_3$ and TbMnO$_3$ due to 4f crystal-field excitations

Authors: K. Berggold, J. Baier, D. Meier, J. A. Mydosh, T. Lorenz, J. Hemberger, A. Balbashov, N. Aliouane, D. N. Argyriou

Abstract: We present measurements of the thermal conductivity $κ$ and the thermal expansion $α$ of NdMnO$_3$ and TbMnO$_3$. In both compounds a splitting of the $4f$ multiplet of the $R^{3+}$ ion causes Schottky contributions to $α$. In TbMnO$_3$ this contribution arises from a crystal-field splitting, while in NdMnO$_3$ it is due to the Nd-Mn exchange coupling. Another consequence of this coupling is a s… ▽ More We present measurements of the thermal conductivity $κ$ and the thermal expansion $α$ of NdMnO$_3$ and TbMnO$_3$. In both compounds a splitting of the $4f$ multiplet of the $R^{3+}$ ion causes Schottky contributions to $α$. In TbMnO$_3$ this contribution arises from a crystal-field splitting, while in NdMnO$_3$ it is due to the Nd-Mn exchange coupling. Another consequence of this coupling is a strongly enhanced canting of the Mn moments. The thermal conductivity is greatly suppressed in both compounds. The main scattering process at low temperatures is resonant scattering of phonons between different energy levels of the $4f$ multiplets, whereas the complex 3d magnetism of the Mn ions is of minor importance. △ Less

Submitted 8 March, 2007; originally announced March 2007.

Comments: 9 pages including 6 figures

Journal ref: Physical Review B 76, 094418 (2007)

New features in the phase diagram of TbMnO$_3$

Authors: D. Meier, N. Aliouane, D. N. Argyriou, J. A. Mydosh, T. Lorenz

Abstract: The (H,T)-phase diagram of the multiferroic perovskite TbMnO$_3$ was studied by high-resolution thermal expansion $α(T)$ and magnetostriction $ΔL(H)/L$ measurements. Below $T_{N}\simeq 42$ K, TbMnO$_3$ shows antiferromagnetic order, which changes at $T_{FE}\simeq 28$ K where simultaneously a spontaneous polarization $P||c$ develops. Sufficiently high magnetic fields applied along $a$ or $b$ indu… ▽ More The (H,T)-phase diagram of the multiferroic perovskite TbMnO$_3$ was studied by high-resolution thermal expansion $α(T)$ and magnetostriction $ΔL(H)/L$ measurements. Below $T_{N}\simeq 42$ K, TbMnO$_3$ shows antiferromagnetic order, which changes at $T_{FE}\simeq 28$ K where simultaneously a spontaneous polarization $P||c$ develops. Sufficiently high magnetic fields applied along $a$ or $b$ induce a polarization flop to $P||a$. We find that all of these transitions are strongly coupled to the lattice parameters. Thus, our data allow for a precise determination of the phase boundaries and also yield information about their uniaxial pressure dependencies. The strongly hysteretic phase boundary to the ferroelectric phase with $P||a$ is derived in detail. Contrary to previous reports, we find that even in high magnetic fields there are no direct transitions from this phase to the paraelectric phase. We also determine the various phase boundaries in the low-temperature region related to complex reordering transitions of the Tb moments. △ Less

Submitted 19 January, 2007; originally announced January 2007.

Comments: 17 pages including 9 figures

Journal ref: New Journal of Physics 9, 100 (2007)