Determination of the zero-field magnetic structure of the helimagnet MnSi at low temperature

Below a temperature of approximately 29 K the manganese magnetic moments of the cubic binary compound MnSi order to a long-range incommensurate helical magnetic structure. Here, we quantitatively analyze a high-statistic zero-field muon spin rotation spectrum recorded in the magnetically ordered phase of MnSi by exploiting the result of representation theory as applied to the determination of magnetic structures. Instead of a gradual rotation of the magnetic moments when moving along a axis, we find that the angle of rotation between the moments of certain subsequent planes is essentially quenched. It is the magnetization of pairs of planes which rotates when moving along a axis, thus preserving the overall helical structure.Comment: 10 pages, 4 figure

Probing the Yb3+^{3+} spin relaxation in Y0.98_{0.98}Yb0.02_{0.02}Ba2_{2}Cu3_{3}Ox_{x} by Electron Paramagnetic Resonance

The relaxation of Yb$^{3+}$ in YBa$_{2}$Cu$_{3}$O$_{x}$ ($6<x<7$) was studied using Electron Paramagnetic Resonance (EPR). It was found that both electronic and phononic processes contribute to the Yb$^{3+}$ relaxation. The phononic part of the relaxation has an exponential temperature dependence, which can be explained by a Raman process via the coupling to high-energy ($\sim$500 K) optical phonons or an Orbach-like process via the excited vibronic levels of the Cu$^{2+}$ ions (localized Slonczewski-modes). In a sample with a maximum oxygen doping $x$=6.98, the electronic part of the relaxation follows a Korringa law in the normal state and strongly decreases below $T_{c}$. Comparison of the samples with and without Zn doping proved that the superconducting gap opening is responsible for the sharp decrease of Yb$^{3+}$ relaxation in YBa$_{2}$Cu$_{3}$O$_{6.98}$. It was shown that the electronic part of the Yb$^{3+}$ relaxation in the superconducting state follows the same temperature dependence as $^{63}$Cu and $^{17}$O nuclear relaxations despite the huge difference between the corresponding electronic and nuclear relaxation rates.Comment: 8 pages, 6 figure

Inhomogeneous broadening of the EPR signal of Yb3+ ions in domain walls of lightly doped antiferromagnetic cuprates

Distortion of the long-range antiferromagnetic order in the YBa2Cu3O6+y is investigated by the electron paramagnetic resonance (EPR) measurements for y = 0.1 - 0.4. In the case of the doping level y = 0.2, 0.3 the EPR signal consists of narrow and broad lines, which we relate to formation of the charged domain walls. Our theoretical analysis of the inhomogeneous EPR broadening due to the local antiferromagnetic order distortion in domain walls is well consistent with experimental results for the case of coplanar elliptical domain walls

Spin excitations and electron paramagnetic resonance in two-layer antiferromagnetic system Y1-xYbxBa2Cu 3O6+y

An experimental and theoretical study of the EPR spectrum of Y 0.98Yb0.02Ba2Cu3O6.1 is reported. A strong anisotropy of the EPR linewidth is revealed for the parallel and perpendicular orientations of the external magnetic field relative to the tetragonal symmetry axis. This anisotropy is related to the indirect spin-spin interaction between the ytterbium ions via antiferromagnetic spin-waves. The explicit expressions for the coupling of ytterbium ions with the spin waves and the corresponding indirect spin-spin interactions are derived. An estimate of the exchange coupling between the ytterbium and copper ions is found from the comparison of the theory with the experiment

Pressure effects on the transition temperature and the magnetic field penetration depth in the pyrochlore superconductor RbOs_2O_6

We report magnetization measurements under high hydrostatic pressure in the newly discovered pyrochlore superconductor RbOs_2O_6 (T_c\simeq6.3K at p=0). A pronounced and {\it positive} pressure effect (PE) on T_c with dT_c/dp =0.090(1)K/kbar was observed, whereas no PE on the magnetic penetration depth \lambda was detected. The relative pressure shift of T_c [ dlnT_c/dp \simeq 1.5%/kbar] is comparable with the highest values obtained for highly underdoped high-temperature cuprate superconductors. Our results suggest that RbOs_2O_6 is an adiabatic BCS-type superconductor.Comment: 11 pages, 4 figure

Superfluid Density and Energy Gap-Function of Superconducting PrPt4Ge12

The filled skutterudite superconductor PrPt4Ge12 was studied in muon-spin rotation (muSR), specific heat, and electrical resistivity experiments. The continuous increase of the superfluid density with decreasing temperature and the dependence of the magnetic penetration depth \lambda on the magnetic field obtained by means of muSR, as well as the observation of a T^3 dependence of the electronic specific heat establish the presence of point-like nodes in the superconducting energy gap. The energy gap was found to be well described by \Delta =\Delta_0| k_x - ik_y| or \Delta = \Delta_0(1 - k_y^4) functional forms, similar to that obtained for another skutterudite superconductor, PrOs4Sb12. The gap to T_c ratios were estimated to be \Delta_0/k_BT_c = 2.68(5) and 2.29(5), respectively.Comment: 4 pages, 5 figure

muSR investigation of magnetism and magnetoelectric coupling in Cu2OSeO3

A detailed zero and transverse field (ZF&TF) muon spin rotation (muSR) investigation of magnetism and the magneto-electric coupling in Cu2OSeO3 is reported. An internal magnetic field B_int(T=0) = 85.37(25) mT was found, in agreement with a ferrimagnetic state below Tc = 57.0(1) K. The temperature dependence of the magnetic order parameter is well described by the relation B_int = B(0)(1-(T/Tc)^2)^b with an effective exponent b = 0.39(1) which is close to the critical exponent B ~ 1/3 for a three dimensional (3D) magnetic system. Just above Tc the muon relaxation rate follows the power low \lambda (T)\propto (T/Tc - 1)^\omega with \omega = 1.06(9), which is characteristic for 3D ferromagnets. Measurements of B_int(T) with and without an applied electrostatic field E = 1.66 x 10^5 V/m suggest a possible electric field effect of magnitude \Delta Bv = Bv(0 V)-Bv(500 V) = - 0.4(4) mT.Comment: 7 pages, 5 figure

Probing the Yb3+ spin relaxation in Y0.98 Yb0.02 Ba2 Cu3 Ox by electron paramagnetic resonance

The relaxation of Yb3+ in YBa2 Cu3 Ox (6<x<7) was studied using electron paramagnetic resonance. It was found that both electronic and phononic processes contribute to the Yb3+ relaxation. The phononic part of the relaxation has an exponential temperature dependence, which can be explained by a Raman process via the coupling to high-energy (∼500 K) optical phonons or an Orbach-type process via the excited vibronic levels of the Cu2+ ions (localized Slonczewski-modes). In a sample with a maximum oxygen doping x=6.98, the electronic part of the relaxation follows a Korringa law in the normal state and strongly decreases below Tc. Comparison of the samples with and without Zn doping proved that the superconducting gap opening is responsible for the sharp decrease of Yb3+ relaxation in YBa2 Cu3 O6.98. It was shown that the electronic part of the Yb3+ relaxation in the superconducting state follows the same temperature dependence as C 63 u and O 17 nuclear relaxations despite the huge difference between the corresponding electronic and nuclear relaxation rates. © 2009 The American Physical Society

Muon-spin rotation and magnetization studies of chemical and hydrostatic pressure effects in EuFe_{2}(As_{1-x}P_{x})_{2}

The magnetic phase diagram of EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ was investigated by means of magnetization and muon-spin rotation studies as a function of chemical (isovalent substitution of As by P) and hydrostatic pressure. The magnetic phase diagrams of the magnetic ordering of the Eu and Fe spins with respect to P content and hydrostatic pressure are determined and discussed. The present investigations reveal that the magnetic coupling between the Eu and the Fe sublattices strongly depends on chemical and hydrostatic pressure. It is found that chemical and hydrostatic pressure have a similar effect on the Eu and Fe magnetic order.Comment: 11 pages, 10 figure

Discovering Conformational Sub-States Relevant to Protein Function

Background: Internal motions enable proteins to explore a range of conformations, even in the vicinity of native state. The role of conformational fluctuations in the designated function of a protein is widely debated. Emerging evidence suggests that sub-groups within the range of conformations (or sub-states) contain properties that may be functionally relevant. However, low populations in these sub-states and the transient nature of conformational transitions between these substates present significant challenges for their identification and characterization. Methods and Findings: To overcome these challenges we have developed a new computational technique, quasianharmonic analysis (QAA). QAA utilizes higher-order statistics of protein motions to identify sub-states in the conformational landscape. Further, the focus on anharmonicity allows identification of conformational fluctuations that enable transitions between sub-states. QAA applied to equilibrium simulations of human ubiquitin and T4 lysozyme reveals functionally relevant sub-states and protein motions involved in molecular recognition. In combination with a reaction pathway sampling method, QAA characterizes conformational sub-states associated with cis/trans peptidyl-prolyl isomerization catalyzed by the enzyme cyclophilin A. In these three proteins, QAA allows identification of conformational sub-states, with critical structural and dynamical features relevant to protein function. Conclusions: Overall, QAA provides a novel framework to intuitively understand the biophysical basis of conformational diversity and its relevance to protein function. © 2011 Ramanathan et al