C. R. Wiebe

C. R. Wiebe
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C. R. Wiebe

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Physics - Strongly Correlated Electrons (29)
Physics - Superconductivity (8)
Physics - Statistical Mechanics (4)
Physics - Materials Science (3)
Physics - Other (2)

Publications Authored By C. R. Wiebe

We report neutron scattering measurements on Er$_2$Pt$_2$O$_7$, a new addition to the XY family of frustrated pyrochlore magnets. Symmetry analysis of our elastic scattering data shows that Er$_2$Pt$_2$O$_7$ is the first XY pyrochlore to order into the $k=0$, $\Gamma_7$ magnetic structure (the Palmer-Chalker state), at $T_N = 0.38$ K. Read More

The gadolinium pyrochlores, Gd$_2B_2$O$_7$, are amongst the best realizations of antiferromagnetically coupled Heisenberg spins on a pyrochlore lattice. We present a magnetic characterization of Gd$_2$Pt$_2$O$_7$, a unique member of this family. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements show that Gd$_2$Pt$_2$O$_7$ undergoes an antiferromagnetic ordering transition at $T_N = 1. Read More

The pyrochlore Ho2Ge2O7 is a new highly correlated spin ice material. Physical property measurements including x-ray diffraction, dc susceptibility and ac susceptibility, confirm that it shares the distinctive characteristics of other known spin ices. Polarized neutron scattering measurements on a powder sample, combined with reverse Monte Carlo (RMC) refinements, give unique information about the spin ice state in Ho2Ge2O7. Read More

The ytterbium pyrochlore magnets, Yb2B2O7 (B = Sn, Ti, Ge) are well described by S_eff = 1/2 quantum spins decorating a network of corner-sharing tetrahedra and interacting via anisotropic exchange. Structurally, only the non-magnetic B-site cation, and hence, primarily the lattice parameter, is changing across the series. Nonetheless, a range of magnetic behaviors are observed: the low temperature magnetism in Yb2Ti2O7 and Yb2Sn2O7 has ferromagnetic character, while Yb2Ge2O7 displays an antiferromagnetically ordered Neel state at low temperatures. Read More

We report neutron scattering and muon spin relaxation measurements (muSR) on the pyrochlore antiferromagnet Yb2Ge2O7. Inelastic neutron scattering was used to probe the transitions between crystal electric field levels, allowing us to determine the eigenvalues and eigenvectors appropriate to the J=7/2 Yb3+ ion in this environment. The crystal electric field ground state doublet in Yb2Ge2O7 corresponds primarily to m_J = +/- 1/2 with local XY anisotropy, consistent with an S_eff = 1/2 description for the Yb moments. Read More

MnTiO3 has been studied for many decades, but it was only in the last few years that its strong magnetoelectric behavior had been observed. Here, we use neutron scattering on two separately grown single crystals and two powder samples to show the presence of a supercell that breaks R-3 symmetry. We also present the temperature and field dependence of the dielectric constant and pyroelectric current, and show evidence of non-zero off-diagonal magnetoelectric tensor elements (forbidden by R-3 symmetry) followed by a polarization flop accompanying the spin flop transition at {\mu}0HSF = 6. Read More

Elastic neutron scattering, ac susceptibility, and specific heat experiments on the pyrochlores Er$_{2}$Ge$_{2}$O$_{7}$ and Yb$_{2}$Ge$_{2}$O$_{7}$ show that both systems are antiferromagnetically ordered in the $\Gamma_5$ manifold. The ground state is a $\psi_{3}$ phase for the Er sample and a $\psi_{2}$ or $\psi_{3}$ phase for the Yb sample, which suggests "Order by Disorder"(ObD) physics. Furthermore, we unify the various magnetic ground states of all known R$_{2}$B$_{2}$O$_{7}$ (R = Er, Yb, B = Sn, Ti, Ge) compounds through the enlarged XY type exchange interaction $J_{\pm}$ under chemical pressure. Read More

The rich phase diagrams of magnetically frustrated pyrochlores have maintained a high level of interest over the past 20 years. To experimentally explore these phase diagrams requires a means of tuning the relevant interactions. One approach to achieve this is chemical pressure, that is, varying the size of the non-magnetic cation. Read More

The origin of the spin liquid state in Tb$_2$Ti$_2$O$_7$ has challenged experimentalists and theorists alike for nearly 20 years. To improve our understanding of the exotic magnetism in Tb$_2$Ti$_2$O$_7$, we have synthesized a chemical pressure analog, Tb$_2$Ge$_2$O$_7$. Germanium substitution results in a lattice contraction and enhanced exchange interactions. Read More

The spin glass behavior of Y2Mo2O7 has puzzled physicists for nearly three decades. Free of bulk disorder within the resolution of powder diffraction methods, it is thought that this material is a rare realization of a spin glass resulting from weak disorder such as bond disorder or local lattice distortions. Here, we report on the single crystal growth of Y2Mo2O7. Read More

We report ac susceptibility and high-frequency electron spin resonance (ESR) measurements on the geometrically frustrated compound Ba$_3$NbFe$_3$Si$_2$O$_{14}$ with the N\'{e}el temperature $T_N=27 K$. An unusually large frequency-dependence of ac susceptibility in the temperature range of 20 - 100 K reveals a spin-glass-like behavior, signalling the presence of frustration related slow magnetic fluctuations. ESR experiments show a multi-step magnetic and spin chirality ordering process. Read More

Two B-site ordered double perovskites, La2LiMoO6 and Ba2YMoO6, based on the S = 1/2 ion, Mo5+, have been investigated in the context of geometric magnetic frustration. Powder neutron diffraction, heat capacity, susceptibility, muon spin relaxation(_SR), and 89Y NMR- including MAS NMR- data have been collected. La2LiMoO6 deviates strongly from simple Curie-Weiss paramagnetic behavior below 150K and zero-field cooled/ field cooled (ZFC/FC)irreversibility occurs below 20K with a weak, broad susceptibility maximum near 5K in the ZFC data. Read More

We have investigated the (0001) surfaces of several hexagonal manganite perovskites by low-energy electron diffraction (LEED) in order to determine if the surface periodicity is different from that of the bulk materials. These LEED studies were conducted using near-normal incidence geometry with a low energy electron microscope (LEEM)/LEED apparatus from room temperature to 1200 degrees Celsius and with an electron energy in the range of 15-50 eV. Diffraction patterns showed features of bulk-terminated periodicity as well as a 2\times2 surface reconstruction. Read More

The magnetic properties of alkali-metal peroxychromate K$_2$NaCrO$_8$ are governed by the $S = 1/2$ pentavalent chromium cation, Cr$^{5+}$. Specific heat, magnetocalorimetry, ac magnetic susceptibility, torque magnetometry, and inelastic neutron scattering data have been acquired over a wide range of temperature, down to 60 mK, and magnetic field, up to 18 T. The magnetic interactions are quasi-two-dimensional prior to long-range ordering, where $T_N = 1. Read More

$LiHo_xY_{1-x}F_4$ is an insulating system where the magnetic Ho$^{3+}$ ions have an Ising character, and interact mainly through magnetic dipolar fields. We used the muon spin relaxation technique to study the nature of the ground state for samples with x=0.25, 0. Read More

We report detailed 51V NMR spectra in a single crystal of MnV2O4. The vanadium spectrum reveals two peaks in the orbitally ordered state, which arise from different internal hyperfine fields at two different V sublattices. These internal fields evolve smoothly with externally applied field, and show no change in structure that would suggest a change of the orbital ordering. Read More

We report on the use of $^{69,71}$Ga nuclear magnetic resonance to probe spin dynamics in the rare-earth kagom\'{e} system Pr$_3$Ga$_5$SiO$_{14}$. We find that the spin-lattice relaxation rate $^{69}1/T_1$ exhibits a maximum around 30 K, below which the Pr$^{3+}$ spin correlation time $\tau$ shows novel field-dependent behavior consistent with a field-dependent gap in the excitation spectrum. The spin-spin relaxation rate $^{69}1/T_{2}$ exhibits a peak at a lower temperature (10 K) below which field-dependent power-law behavior close to $T^{2}$ is observed. Read More

In this letter, we report on the single crystal growth and physical characterization of the distorted kagom\'{e} system Pr$_3$Ga$_5$SiO$_{14}$. It is found that at zero magnetic field the system shows no magnetic order down to 0.035 K and exhibits a $T^{2}$ behavior for the specific heat at low temperatures, indicative of a gapless 2D spin liquid state. Read More

The recent discovery and subsequent developments of FeAs-based superconductors have presented novel challenges and opportunities in the quest for superconducting mechanisms in correlated-electron systems. Central issues of ongoing studies include interplay between superconductivity and magnetism as well as the nature of the pairing symmetry reflected in the superconducting energy gap. In the cuprate and RE(O,F)FeAs (RE = rare earth) systems, the superconducting phase appears without being accompanied by static magnetic order, except for narrow phase-separated regions at the border of phase boundaries. Read More

By means of neutron scattering we show that the high-temperature precursor to the hidden order state of the heavy fermion superconductor URu$_{2}$Si$_{2}$ exhibits heavily damped incommensurate paramagnons whose strong energy dispersion is very similar to that of the long-lived longitudinal f-spin excitations that appear below T$_{0}$. Since the underlying local f-exchange is preserved we expect only the f-d interactions to change across the phase transition and to cause the paramagnetic damping. The damping exhibits single-ion behavior independent of wave vector and vanishes below the hidden order transition. Read More

We report muon spin relaxation ($\mu$SR) and magnetic susceptibility measurements on Cu(Cl,Br)La(Nb,Ta)$_{2}$O$_{7}$, which demonstrate: (a) the absence of static magnetism in (CuCl)LaNb$_{2}$O$_{7}$ down to 15 mK confirming a spin-gapped ground state; (b) phase separation between partial volumes with a spin-gap and static magnetism in (CuCl)La(Nb,Ta)$_{2}$O$_{7}$; (c) history-dependent magnetization in the (Nb,Ta) and (Cl,Br) substitution systems; (d) a uniform long-range collinear antiferromagnetic state in (CuBr)LaNb$_{2}$O$_{7}$; and (e) a decrease of N\'eel temperature with decreasing Br concentration $x$ in Cu(Cl$_{1-x}$Br$_{x}$)LaNb$_{2}$O$_{7}$ with no change in the ordered Cu moment size for $0.33 \leq x \leq 1$. Together with several other $\mu$SR studies of quantum phase transitions in geometrically-frustrated spin systems, the present results reveal that the evolution from a spin-gap to a magnetically ordered state is often associated with phase separation and/or a first order phase transition. Read More

Transverse-field muon spin rotation measurements of overdoped La2-xSrxCuO4 reveal a large broadening of the local magnetic field distribution in response to applied field, persisting to high temperatures. The field-response is approximately Curie-Weiss like in temperature and is largest for the highest doping investigated. Such behaviour is contrary to the canonical Fermi-liquid picture commonly associated with the overdoped cuprates and implies extensive heterogeneity in this region of the phase diagram. Read More

One of the primary goals of modern condensed matter physics is to elucidate the nature of the ground state in various electronic systems. Many correlated electron materials, such as high temperature superconductors, geometrically frustrated oxides, and low-dimensional magnets are still the objects of fruitful study because of the unique properties which arise due to poorly understood many-body effects. Heavy fermion metals - materials which have high effective electron masses due to these effects - represent a class of materials with exotic properties, such as unusual magnetism, unconventional superconductivity, and "hidden order" parameters. Read More

We report $^{51}$V zero-field NMR of manganese vanadate spinel of MnV$_2$O$_4$, together with both ac and dc magnetization measurements. The field and temperature dependence of ac susceptibilities show a reentrant-spin-glass-like behavior below the ferrimagnetic(FEM) ordering temperature. The zero-field NMR spectrum consists of multiple lines ranging from 240 MHz to 320 MHz. Read More

We have performed $\mu$SR studies on single crystals of SrCu$_2$(BO$_3$)$_2$, a quasi-two-dimensional spin system with a spin singlet ground state. We observe two different muon sites which we associate with muons located adjacent to the two inequivalent O sites. One site, presumed to be located in the Cu-O-Cu superexchange path, exhibits a large increase in the frequency shift with decreasing temperature which is unaffected by the singlet formation, indicating that the muon has locally broken the singlet bond. Read More

The magnetic field dependent phase diagram of the ferroelectric material ${\rm Ho}_{1-x}Y_{x}MnO_{3}$ has been investigated for x = 0, 0.6, and 0.8 for fields directed along both the ab in-plane and c-axis basal plane directions. Read More

The intercalated graphite superconductor CaC6 with Tc ~ 11.5 K has been synthesized and characterized with magnetoresistance measurements. Above the transition, the resistivity follows a T^2 dependence up to 50 K, which suggests Fermi liquid behavior. Read More

muSR experiments on the geometrically frustrated spinel oxide, Li2Mn2O4, show the development of spin correlations over a range of length scales with decreasing temperature. Increased relaxation below 150 K is consistent with the onset of spin correlations. Below 50 K, spin order on a length scale, which is long range for the muSR probe, appears abruptly in temperature, consistent with prior neutron diffraction results. Read More

Muon spin relaxation ($\mu$SR) measurements in high transverse magnetic fields ($\parallel \hat c$) revealed strong field-induced quasi-static magnetism in the underdoped and Eu doped (La,Sr)$_{2}$CuO$_{4}$ and La$_{1.875}$Ba$_{0.125}$CuO$_{4}$, existing well above $T_{c}$ and $T_{N}$. Read More

Neutron scattering measurements on the spin-ice candidate material Ho$_2$Ru$_2$O$_7$ have revealed two magnetic transitions at T $\sim$ 95 K and T $\sim$ 1.4 K to long-range ordered states involving the Ru and Ho sublattices, respectively. Between these transitions, the Ho$^{3+}$ moments form short-ranged ordered spin clusters. Read More

Muon spin relaxation ($\mu$SR) measurements on the new layered cobalt oxide superconductor Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O and its parent, non-superconducting compounds, have revealed unconventional nature of superconductivity through: (1) a small superfluid energy which implies a surprisingly high effective mass of the charge carriers, approximately 100 times the bare electron mass; (2) the superconducting transition temperature $T_{c}$ scaling with the superfluid energy following the correlations found in high-$T_{c}$ cuprate and some other two-dimensional superconductors; (3) an anisotropic pairing without broken time-reversal symmetry; and (4) the proximity of a magnetically ordered insulating phase at Na$_{0. Read More

We have performed neutron scattering experiments on the heavy fermion superconductor URu$_2$Si$_2$ to search for the orbital currents predicted to exist in the ordered phase below {\Tn} = 17.5 K. Elastic scans in the (H, K, 0) and (H, 0, L) planes revealed no such order parameter at low temperatures. Read More

We report muon-spin rotation/relaxation (muSR) measurements on single crystals of the electron-doped high-T_c superconductor Pr$_{2-x}$Ce$_x$CuO$_4$. In zero external magnetic field, superconductivity is found to coexist with Cu spins that are static on the muSR time scale. In an applied field, we observe a Knight shift that is primarily due to the magnetic moment induced on the Pr ions. Read More