A.A. Menovsky

Magnetic Excitations in the Heavy-Fermion Superconductor Uru2si2

Antiferromagnetic order and fluctuations in the heavy-fermion superconductor URu{sub 2}Si{sub 2} have been studied by magnetic neutron scattering. Below {ital T}{sub {ital N}}=17.5 K, URu{sub 2}Si{sub 2} is a type-I antiferromagnet with an anomalously small ordered moment of (0.04{plus minus}0.01){mu}{sub {ital B}} polarized along the tetragonal {ital c} axis. Dispersive resonant excitations exist in the ordered state with a zone-center gap of 0.43 THz. The excitations are polarized along the ordered moment and have a large dipolar matrix element, which suggests that they are coupled transitions between singlet crystal-field-like states. For energy transfer above 3 THz, peaks have not been identified in the magnetic excitation spectra, but instead a continuous spectrum of scattering peaked around the ordering wave vector indicates the presence of overdamped antiferromagnetically correlated spin fluctuations. Upon heating above {ital T}{sub {ital N}}, the resonant excitations abruptly become heavily damped but the magnetic scattering at higher energies does not change at {ital T}{sub {ital N}}. Instead, the disappearance of the antiferromagnetic modulation of the higher-energy scattering coincides with the maximum in the resistivity of URu{sub 2}Si{sub 2}.

Magnetic properties of the ternary compounds CeT2Si2 and UT2Si2

We have investigated the magnetic properties of the intermetallic compounds CeT2Si2 and UT2Si2, with T a transition metal. From our measurements we have determined a trend from Pauli-paramagnetism via antiferromagnetism to canted antiferromagnetism with increasing number of d-electrons. Superconductivity, like in CeCu2Si2, may be found near the paramagnetism/antiferromagnetism boundary.

Transition between Orbital Orderings in YVO3

Evidence has been found for a change in the ordered occupation of the vanadium d orbitals at the 77 K phase transition in YVO3, manifested by a change in the type of Jahn-Teller distortion. The orbital ordering above 77 K is not destroyed at the magnetic ordering temperature of 116 K, but is present as far as a second structural phase transition at 200 K. The transition between orbital orderings is caused by an increase in octahedral tilting with decreasing temperature.

Magnetic and transport properties of various UT2Ge2 (T = 3d element) intermetallic compounds

Abstract We have measured the magnetization and resistivity for five UT 2 Ge 2 intermetallic compounds with T = Mn, Fe, Co, Ni, and Cu. UMn 2 Ge 2 is an exceptional system possessing 3d local moments which order ferromagnetically at 380 K. In addition U moments are found, ordering ferromagnetically below 140 K. After elimination of impurity phases UFe 2 Ge 2 is a Pauli paramagnet. UCo 2 Ge 2 exhibits both enhanced Pauli paramagnetic and weak U local moment behavior. Both UNi 2 Ge 2 and UCu 2 Ge 2 have ordered U moments with T N = 77 K and T c = 100 K, respectively. No superconductivity is found in these materials down to 0.35 K. A simple band model is proposed to describe the trend in the magnetism.

Spin glass behavior in URh2Ge2

URh_2Ge_2 occupies an extraordinary position among the heavy-electron 122-compounds, by exhibiting a previously unidentified form of magnetic correlations at low temperatures, instead of the usual antiferromagnetism. Here we present new results of ac and dc susceptibilities, specific heat and neutron diffraction on single-crystalline as-grown URh_2Ge_2. These data clearly indicate that crystallographic disorder on a local scale produces spin glass behavior in the sample. We therefore conclude that URh_2Ge_2 is a 3D Ising-like, random-bond, heavy-fermion spin glass.

Heavy carriers and non-Drude optical conductivity in MnSi

metals. 3 However, the high value of the reflection coefficient close to the 100% line prevented a detailed analysis of the frequency-dependent optical conductivity in this range. In this Rapid Communication, we overcome this hurdle by using p-polarized light at a grazing angle of incidence of 80°, for which the reflection coefficient drops well below the 100% line. We show that the frequency-dependent-scattering rate and the effective mass deviate from the behavior expected for Fermi liquids which can be understood from the fact that the optical conductivity is best described with an expression that departs from the usual Drude model. Single crystals were grown using the traveling floatingzone technique. 3,4 The temperature dependence of the resistivity is shown in Fig. 1. Fitting the resistivity to r(T)

S29iNMR and Hidden Order inURu2Si2

Below T(N) approximately 17 K the 29Si NMR line in URu2Si2 exhibits a previously unobserved field-independent nearly isotropic contribution to the linewidth, which increases to approximately 12 G as T-->0. We argue that this feature does not arise from static freezing of the U-spin magnetization, but is due to coupling between 29Si spins and a hidden order parameter. We discuss time-reversal symmetry-breaking orbital antiferromagnetism and indirect nuclear spin-spin interactions as possible coupling mechanisms. Further NMR experiments and theoretical calculations are suggested.

Structural and magnetic properties of UCo2Ge2

We have studied the effect of annealing on the structural and magnetic properties of UCo2Ge2 and thereby find the existence of two different crystallographic structures. The as‐cast, high‐temperature (HT) phase adopts a crystal structure with symmetry lower than I4/mmm; after annealing at 750 °C for 5 days the ThCr2Si2 (I4/mmm) structure is formed. Additionally, the length of the c axis in these tetragonal structures changes dramatically: whereas the HT phase has a c parameter of ≊9.3 A, the low‐temperature (LT), annealed phase has a much larger c axis of ≊9.9 A. This expansion of the unit cell greatly influences the magnetic properties of the material: while the LT sample orders antiferromagnetically at 174 K, the HT one does not order magnetically down to 350 mK. The antiferromagnetic transition is clearly indicated by our magnetization, resistivity and specific heat measurements.

Observation of the transverse optical plasmon in SmLa0.8Sr0.2CuO4-delta

We present microwave and infrared measurements on SmLa0.8Sr0.2CuO4-delta, which are direct evidence for the existence of a transverse optical plasma mode, observed as a peak in the c-axis optical conductivity. This mode appears as a consequence of the existence of two different intrinsic Josephson couplings between the CuO2 layers, one with a Sm2O2 block layer, and the other one with a (La,Sr)2O2-delta block layer. From the frequencies and the intensities of the collective modes we determine the value of the compressibility of the two dimensional electron fluid in the copper oxygen planes.

Lattice instabilities and elastic response in the heavy electron system URu_2Si_2

We have performed thermal expansion and elastic constant measurements of the heavy electron system URu 2 Si 2 , focusing attention on the Γ 3 ( x 2 - y 2 ) and Γ 4 ( x y ) symmetry of the tetragonal group D 4 h . It is reconfirmed that there is no sizable uniform spontaneous distortion for these two types of symmetry through the puzzling phase transition at T 0 =17.5 K. On the other hand, a weak but significant tendency of softening are found below about 70 K in the transverse ( c 11 - c 12 )/2 mode. The results strongly suggest the presence of a lattice instability in the Γ 3 symmetry. From the results, proposed crystalline-electric-field models are also discussed.