arXiv subjects

S. -H. Baek

Publications and source records attributed to S. -H. Baek.

30 records · Page 2Linked to original sources

arXiv2010

Anisotropic spin fluctuations and superconductivity in "115'' heavy fermion compounds: 59Co NMR study in PuCoGa5

We report results of $^{59}$Co nuclear magnetic resonance measurements on a single crystal of superconducting PuCoGa5 in its normal state. The nuclear spin-lattice relaxation rates and the Knight shifts as a function of temperature reveal an anisotropy of spin fluctuations with finite wave vector q. By comparison with the isostructural members, we conclude that antiferromagnetic XY-type anisotropy of spin fluctuations plays an important role in mediating superconductivity in these heavy fermion materials.

cond-mat.str-el↗

arXiv2010

Crystal field effects and quadrupole fluctuations in Ce$_3$Au$_3$Sb$_4$ detected by Sb NQR

We report $^{121,123}$Sb NQR studies on single crystals of the narrow gap semiconductor Ce3Au3Sb4. The temperature dependence of the nuclear quadrupole frequency ($ν_Q$), as well as the magnetic susceptibility, is well explained by crystal electric field effects. The nuclear spin-lattice relaxation rate ($T_1^{-1}$) of both $^{121}$Sb and $^{123}$Sb increases rapidly with decreasing temperature. The ratio of $T_1^{-1}$ for the two Sb isotopes is constant at high temperature but it decreases at low temperatures, indicating the important role of quadrupole fluctuations of the Ce ions. We propose that quadrupole fluctuations could be the origin of the large specific heat at low temperature via phonon-quadrupole moment coupling.

cond-mat.str-el↗

arXiv2010

$^{235}$U nuclear relaxation rates in an itinerant antiferromagnet USb$_2$

$^{235}$U nuclear spin-lattice ($T_1^{-1}$) and spin-spin ($T_2^{-1}$) relaxation rates in the itinerant antiferromagnet USb$_2$ are reported as a function of temperature in zero field. The heating effect from the intense rf pulses that are necessary for the $^{235}$U NMR results in unusual complex thermal recovery of the nuclear magnetization which does not allow measuring $T_1^{-1}$ directly. By implementing an indirect method, however, we successfully extracted $T_1^{-1}$ of the $^{235}$U. We find that the temperature dependence of $T_1^{-1}$ for both $^{235}$U and $^{121}$Sb follows the power law ($\propto T^n$) with the small exponent $n=0.3$ suggesting that the same relaxation mechanism dominates the on-site and the ligand nuclei, but an anomaly at 5 K was observed, possibly due to the change in the transferred hyperfine coupling on the Sb site.

cond-mat.str-el↗

arXiv2009

Two inequivalent sublattices and orbital ordering in MnV2O4 studied by 51V NMR

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. The result is consistent with the orbital ordering model recently proposed by Sarkar et al. [Phys. Rev. Lett. 102, 216405 (2009)] in which the same orbital that is a mixture of t_2g orbitals rotates by about 45$^\circ$ alternately within and between orbital chains in the I4_1/a tetragonal space group.

cond-mat.str-el↗

arXiv2009

Electric-field-controlled directional growth of ferroelectric domains in multiferroic BiFeO3 films

We describe the directional growth of ferroelectric domains in a multiferroic BiFeO3 thin film, which was grown epitaxially on a vicinal (001) SrTiO3 substrate. A detailed structural analysis of the film shows that a strain gradient, which can create a symmetry breaking in a ferroelectric double well potential, causes ferroelectric domains to grow with preferred directionality under the influence of an electric field. Our results suggest the possibility of controlling the direction of domain growth with an electric field by imposing constraints on ferroelectric films, such as a strain gradient.

cond-mat.mes-hall↗

arXiv2009

Uncovering the Hidden Order in URu2Si2 by Impurity Doping

We report the use of impurities to probe the hidden order parameter of the strongly correlated metal URu_2Si_2 below the transition temperature T_0 ~ 17.5 K. The nature of this order parameter has eluded researchers for more than two decades, but is accompanied by the development of a partial gap in the single particle density of states that can be detected through measurements of the electronic specific heat and nuclear spin-lattice relaxation rate. We find that impurities in the hidden order phase give rise to local patches of antiferromagnetism. An analysis of the coupling between the antiferromagnetism and the hidden order reveals that the former is not a competing order parameter but rather a parasitic effect of the latter.

cond-mat.str-el↗

arXiv2009

NMR investigation of superconductivity and antiferromagnetism in CaFe$_2$As$_2$ under pressure

We report $^{75}$As NMR measurements in CaFe$_2$As$_2$, made under applied pressures up to 0.83 GPa produced by a standard clamp pressure cell. Our data reveal phase segregation of paramagnetic (PM) and antiferromagnetic (AFM) phases over a range of pressures, with the AFM phase more than 90 % dominant at low temperatures. In situ RF susceptibility measurements indicate the presence of superconductivity. $^{75}$As spin-lattice relaxation experiments indicate that the $^{75}$As nuclei sample the superconductivity while in the magnetically-ordered environment.

cond-mat.supr-con↗

arXiv2009

Hybridization-driven gap in U3Bi4Ni3: a 209Bi NMR/NQR study

We report 209Bi NMR and NQR measurements on a single crystal of the Kondo insulator U3Bi4Ni3. The 209Bi nuclear spin-lattice relaxation rate ($T_1^{-1}$) shows activated behavior and is well-fit by a spin gap of 220 K. The 209Bi Knight shift (K) exhibits a strong temperature dependence arising from 5f electrons, in which K is negative at high temperatures and increases as the temperature is lowered. Below 50 K, K shows a broad maximum and decreases slightly upon further cooling. Our data provide insight into the evolution of the hyperfine fields in a fully gapped Kondo insulator based on 5f electron hybridization.

cond-mat.str-el↗

arXiv2008

First order magnetic transition in single crystal CaFe$_2$As$_2$ detected by $^{75}$As NMR

We report $^{75}$As Nuclear Magnetic Resonance data in a single crystal of CaFe$_2$As$_2$. The Knight shift, electric field gradient, and spin-lattice relaxation rate are strongly temperature dependent in the paramagnetic state, and change discontinuously at the structural transition temperature, $T_S=T_N=167$ K. Immediately below, the NMR spectra reveal an internal field at the As site associated with the presence of a commensurate magnetic order. These results indicate that the structural and magnetic transitions in CaFe$_2$As$_2$ are first order and strongly coupled, and that the electron density in the FeAs plane is highly sensitive to the out-of-plane structure.

cond-mat.str-el↗

arXiv2008

Microscopic study of the effect of impurities on the first order spin density wave transition in BaFe$_2$As$_2$

We report an $^{75}$As NMR study of BaFe$_2$As$_2$ in both single crystals and polycrystal forms. We find that Sn impurities in the single crystal dramatically alter the low energy spin fluctuations and suppress the ordering temperature from 138 K to 85 K. In contrast to Sn-free samples, we find that the temperature dependence of the $^{75}$As NMR spectra and spin lattice relaxation rates reveal a second order phase transition to a state of incommensurate magnetic order.

cond-mat.str-el↗

arXiv2007

AC susceptibility and $^{51}$V NMR study of MnV$_2$O$_4$

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. Its temperature dependence reveals that the ground state is given by the simultaneous formation of a long-range FEM order and a short-range order component. We attribute the spin-glass-like anomalies to freezing and fluctuations of the short-range ordered state caused by the competition between spin and orbital ordering of the V site.

cond-mat.str-el↗

arXiv2005

Dynamics of Magnetic Defects in Heavy Fermion LiV2O4 from Stretched Exponential 7Li NMR Relaxation

7Li NMR measurements on LiV2O4 from 0.5 to 4.2 K are reported. A small concentration of magnetic defects within the structure drastically changes the 7Li nuclear magnetization relaxation versus time from a pure exponential as in pure LiV2O4 to a stretched exponential, indicating glassy behavior of the magnetic defects. The stretched exponential function is described as arising from a distribution of 7Li nuclear spin-lattice relaxation rates and we present a model for the distribution in terms of the dynamics of the magnetic defects. Our results explain the origin of recent puzzling 7Li NMR literature data on LiV2O4 and our model is likely applicable to other glassy systems.

cond-mat.str-el↗