arXiv subjects

J. Crocker

Publications and source records attributed to J. Crocker.

13 recordsLinked to original sources

arXiv2017

First-Order Reversal Curves of the Magnetostructural Phase Transition in FeTe

We apply the first-order reversal curve (FORC) method, borrowed from studies of ferromagnetic materials, to the magneto-structural phase transition of FeTe. FORC measurements reveal two features in the hysteretic phase transition, even in samples where traditional temperature measurements display only a single transition. For Fe1.13Te, the influence of magnetic field suggests that the main feature is primarily structural while a smaller, slightly higher-temperature transition is magnetic in origin. By contrast Fe1.03Te has a single transition which shows a uniform response to magnetic field, indicating a stronger coupling of the magnetic and structural phase transitions. We also introduce uniaxial stress, which spreads the distribution width without changing the underlying energy barrier of the transformation. The work shows how FORC can help disentangle the roles of the magnetic and structural phase transitions in FeTe.

cond-mat.str-el↗

arXiv2016

29Si nuclear magnetic resonance study of URu2Si2 under pressure

We report 29Si nuclear magnetic resonance measurements of single crystals and aligned powders of URu2Si2 under pressure in the hidden order and paramagnetic phases. We find that the Knight shift decreases with applied pressure, consistent with previous measurements of the static magnetic susceptibility. Previous measurements of the spin lattice relaxation time revealed a partial suppression of the density of states below 30 K. This suppression persists under pressure, and the onset temperature is mildly enhanced.

cond-mat.str-el↗

arXiv2015

Evolution of hyperfine parameters across a quantum critical point in CeRhIn$_5$

We report Nuclear Magnetic Resonance (NMR) data for both the In(1) and In(2) sites in the heavy fermion material CeRhIn$_5$ under hydrostatic pressure. The Knight shift data reveal a suppression of the hyperfine coupling to the In(1) site as a function of pressure, and the electric field gradient, $ν_{αα}$, at the In(2) site exhibits a change of slope, $dν_{αα}/dP$, at $P_{c1} = 1.75$ GPa. These changes to these coupling constants reflect alterations to the electronic structure at the quantum critical point.

cond-mat.str-el↗

arXiv2015

NMR evidence for inhomogeneous glassy behavior driven by nematic fluctuations in iron arsenide superconductors

We present $^{75}$As nuclear magnetic resonance spin-lattice and spin-spin relaxation rate data in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$ as a function of temperature, doping and magnetic field. The relaxation curves exhibit a broad distribution of relaxation rates, consistent with inhomogeneous glassy behavior up to 100 K. The doping and temperature response of the width of the dynamical heterogeneity is similar to that of the nematic susceptibility measured by elastoresistance measurements. We argue that quenched random fields which couple to the nematic order give rise to a nematic glass that is reflected in the spin dynamics.

cond-mat.str-el↗

arXiv2015

NMR Evidence of anisotropic Kondo liquid behavior in CeIrIn$_5$

We report detailed Knight shift measurements of the two indium sites in the heavy fermion compound CeIrIn$_5$ as a function of temperature and field orientation. We find that the Knight shift anomaly is orientation-dependent, with a crossover temperature $T^*$ that varies by 50% as the field is rotated from (001) to (100). This result suggests that the hybridization between the Ce 4f states and the itinerant conduction electrons is anisotropic, a result that reflects its collective origin, and may lead to anisotropic Kondo liquid behavior and unconventional superconductivity.

cond-mat.str-el↗

arXiv2013

Nuclear magnetic resonance studies of pseudospin fluctuations in URu$_2$Si$_2$

We report $^{29}$Si NMR measurements in single crystals and aligned powders of URu$_2$Si$_2$ in the hidden order and paramagnetic phases. The spin-lattice-relaxation data reveal evidence of pseudospin fluctuations of U moments in the paramagnetic phase. We find evidence for partial suppression of the density of states below 30 K, and analyze the data in terms of a two component spin-fermion model. We propose that this behavior is a realization of a pseudogap between the hidden order transition $T_{HO}$ and 30 K. This behavior is then compared to other materials that demonstrate precursor fluctuations in a pseudogap regime above a ground state with long-range order.

cond-mat.str-el↗

arXiv2013

Coexistence of cluster spin glass and superconductivity in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for $0.060 \leq x \leq 0.071$

We present 75As nuclear magnetic resonance data from measurements of a series of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ crystals with $0.00 \leq x \leq 0.075$ that reveals the coexistence of frozen antiferromagnetic domains and superconductivity for $0.060 \leq x \leq 0.071$. Although bulk probes reveal no long range antiferromagnetic order beyond $x = 0.06$, we find that the local spin dynamics reveal no qualitative change across this transition. The characteristic domain sizes vary by more than an order of magnitude, reaching a maximum variation at $x = 0.06$. This inhomogeneous glassy dynamics may be an intrinsic response to the competition between superconductivity and antiferromagnetism in this system.

cond-mat.str-el↗

arXiv2013

NMR evidence for spin fluctuations in the bilayer nickelate La3Ni2O6

We report nuclear magnetic resonance data in the bilayer nickelate La3Ni2O6. This material belongs to a family of low valence nickel oxides with square planar coordination of the Ni ions and is isoelectronic to the high temperature superconducting cuprates. Although the three layer nickelate compound exhibits a spin-state transition accompanied by antiferromagnetic order, the bilayer material shows no phase transition. The NMR data in the bilayer La3Ni2O6 reveal the presence of antiferromagnetic spin fluctuations down to 5K that are surprisingly similar to those present in the magnetically ordered trilayer La4Ni3O8, suggesting a similar electronic structure in the paramagnetic phase of both compounds.

cond-mat.str-el↗

arXiv2013

Nuclear magnetic resonance as a probe of electronic states of Bi2Se3

We present magnetotransport and Bi-209 nuclear magnetic resonance (NMR) data on a series of single crystals of Bi2Se3, Bi2Te2Se and Cu_xBi2Se3 with varying carrier concentrations. The Knight shift of the bulk nuclei is strongly correlated with the carrier concentration via a hyperfine coupling of 27 ueV, which may have important consequences for scattering of the protected surface states. Surprisingly we find that the NMR linewidths and the spin lattice relaxation rate appear to be dominated by the presence of localized spins, which may be related to the presence of Se vacancies.

cond-mat.mtrl-sci↗

arXiv2012

Long range order and two-fluid behavior in heavy electron materials

The heavy electron Kondo liquid is an emergent state of condensed matter that displays universal behavior independent of material details. Properties of the heavy electron liquid are best probed by NMR Knight shift measurements, which provide a direct measure of the behavior of the heavy electron liquid that emerges below the Kondo lattice coherence temperature as the lattice of local moments hybridizes with the background conduction electrons. Because the transfer of spectral weight between the localized and itinerant electronic degrees of freedom is gradual, the Kondo liquid typically coexists with the local moment component until the material orders at low temperatures. The two-fluid formula captures this behavior in a broad range of materials in the paramagnetic state. In order to investigate two-fluid behavior and the onset and physical origin of different long range ordered ground states in heavy electron materials, we have extended Knight shift measurements to URu$_2$Si$_2$, CeIrIn$_5$ and CeRhIn$_5$. In CeRhIn$_5$ we find that the antiferromagnetic order is preceded by a relocalization of the Kondo liquid, providing independent evidence for a local moment origin of antiferromagnetism. In URu$_2$Si$_2$ the hidden order is shown to emerge directly from the Kondo liquid and so is not associated with local moment physics. Our results imply that the nature of the ground state is strongly coupled with the hybridization in the Kondo lattice in agreement with phase diagram proposed by Yang and Pines.

cond-mat.str-el↗

arXiv2012

Evidence for filamentary superconductivity nucleated at antiphase domain walls in antiferromagnetic CaFe$_2$As$_2$

Resistivity, magnetization and microscopic $^{75}$As nuclear magnetic resonance (NMR) measurements in the antiferromagnetically ordered state of the iron-based superconductor parent material CaFe$_2$As$_2$ exhibit anomalous features that are consistent with the collective freezing of domain walls. Below $T^*\approx 10$ K, the resistivity exhibits a peak and downturn, the bulk magnetization exhibits a sharp increase, and $^{75}$As NMR measurements reveal the presence of slow fluctuations of the hyperfine field. These features in both the charge and spin response are strongly field dependent, are fully suppressed by $H^*\approx 15$ T, and suggest the presence of filamentary superconductivity nucleated at the antiphase domain walls in this material.

cond-mat.supr-con↗

arXiv2011

NMR Studies of pseudogap and electronic inhomogeneity in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

We report $^{17}$O NMR measurements in single crystals of overdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ with $T_c=82$ K. We measure the full anisotropy of the planar oxygen Knight shift, electric field gradient, and spin lattice relaxation rate tensors, and show that the entire temperature dependence is determined by the suppression of the density of states in the pseudogap below $T^*\sim 94$ K. The linewidth can be explained by a combination of magnetic and quadrupolar broadening as a result of an inhomogeneous distribution of local hole concentrations that is consistent with scanning tunneling microscopy measurements. This distribution is temperature independent, does not break $C_4$ symmetry, and exhibits no change below $T^*$ or $T_c$.

cond-mat.str-el↗

arXiv2010

Commensurate Antiferromagnetism in CePt2In7, a Nearly Two-Dimensional Heavy Fermion System

CePt$_2$In$_7$ is a new heavy fermion system with a structure similar to the CeMIn$_5$ system but with greater two-dimensional character. We report the synthesis and characterization of phase pure polycrystalline material, which reveals an antiferromagnetic transition at $T_N = 5.2$ K. Nuclear Quadrupolar Resonance (NQR) studies indicate that the antiferromagnetism is commensurate and exhibits strong antiferromagnetic fluctuations in the paramagnetic state.

cond-mat.str-el↗