arXiv subjects

J. S. Gardner

Publications and source records attributed to J. S. Gardner.

At least 19 recordsLinked to original sources

arXiv2025

Excitation spectrum and low-temperature magnetism in disordered defect-fluorite Ho2Zr2O7

In this work, we report on the thermomagnetic characterization and crystalline-electric field (CEF) energy scheme of the disordered defect-fluorite Ho2Zr2O7. This structural phase is distinguished by the coexistence of magnetic frustration and extensive disorder, with Ho3+ and Zr4+ sharing randomly the same 4a site with even 50% occupancy, and an average 1/8 oxygen vacancy per unit cell. AC magnetic susceptibility measurements performed on powder samples down to 0.5 K revealed signs of slowing spin dynamics without glassy behavior, including a frequency dependent peak at 1 K. Yet, no evidence for long-range magnetic order is found down to 150 mK in specific heat. Inelastic neutron scattering measurements show a weak, low-lying CEF excitation around 2 meV, accompanied by a broad level centered at 60 meV. To fit our observations, we propose an approach to account for structural disorder in the crystal-field splitting of the non-Kramers Ho3+. Our model provides an explanation to the broadening of the high-energy, single-ion excitations and suggests that the zirconate ground-state wave function has zero magnetic moment. However, structural disorder acts as guarantor of the magnetism in Ho2Zr2O7, allowing the mixing of low lying states at finite temperatures. Finally, we show that this scenario is in good agreement with the bulk properties reported in this work.

cond-mat.str-el↗

arXiv2024

Embracing Disorder in Quantum Materials Design

Many of the most exciting materials discoveries in fundamental condensed matter physics are made in systems hosting some degree of intrinsic disorder. While disorder has historically been regarded as something to be avoided in materials design, it is often of central importance to correlated and quantum materials. This is largely driven by the conceptual and theoretical ease to handle, predict, and understand highly uniform systems that exhibit complex interactions, symmetries and band structures. In this perspective, we highlight how flipping this paradigm has enabled exciting possibilities in the emerging field of high entropy oxide (HEO) quantum materials. These materials host high levels of cation or anion compositional disorder while maintaining unexpectedly uniform single crystal lattices. The diversity of atomic scale interactions of spin, charge, orbital, and lattice degrees of freedom are found to emerge into coherent properties on much larger length scales. Thus, altering the variance and magnitudes of the atomic scale properties through elemental selection can open new routes to tune global correlated phases such as magnetism, metal-insulator transitions, ferroelectricity, and even emergent topological responses. The strategy of embracing disorder in this way provides a much broader pallet from which functional states can be designed for next-generation microelectronic and quantum information systems.

cond-mat.str-el↗

arXiv2021

Distinct magnetic ground states of $R_2$ZnIrO$_6$ ($R$ = La and Nd) determined by neutron powder diffraction

Double perovskite iridates $A_2$ZnIrO$_6$ ($A$ = alkaline or lanthanide) show complex magnetic behaviors ranging from weak ferromagnetism to successive antiferromagnetic transitions. Here we report the static ($dc$) and dynamic ($ac$) magnetic susceptibility, and neutron powder diffraction measurements for $A$ = La and Nd compounds to elucidate the magnetic ground state. Below 10~K, the $A$ = La compound is best described as canted iridium moments in an antiferromagnet arrangement with a propagation vector \textbf{k} = 0 and a net ferromagnetic component along the $c$-axis. On the other hand, Nd$_2$ZnIrO$_6$ is described well as an antiferromagnet with a propagation vector \textbf{k} = (1/2~1/2~0) below $T_\mathrm{N} \sim$ 17 K. Scattering from both the Nd and Ir magnetic sublattices were required to describe the data and both were found to lie almost completely within the $ab$-plane. $Dc$ susceptibility revealed a bifurcation between the zero-field-cooled and field-cooled curves below $\sim$13 K in Nd$_2$ZnIrO$_6$. A glassy state was ruled out by $ac$ susceptibility but detailed magnetic isotherms revealed the opening of the loop below 13~K. These results suggest a delicate balance exists between the Dzyaloshinskii-Moriya, crystal field schemes, and $d$-$f$ interaction in this series of compounds.

cond-mat.str-el↗

arXiv2019

Melting of Spin Ice state through structural disorder in Dy2Zr2O7

Neutron scattering, a.c. magnetic susceptibility and specific heat studies have been carried out on polycrystalline Dy2Zr2O7. Unlike the pyrochlore spin ice Dy2Ti2O7, Dy2Zr2O7 crystallizes into the fluorite structure and the magnetic Dy3+ moments randomly reside on the corner-sharing tetrahedral sublattice with non-magnetic Zr ions. Antiferromagnetic spin correlations develop below 10 K but remain dynamic down to 40 mK. These correlations extend over the length of two tetrahedra edges and grow to 6 nearest neighbors with the application of a 20 kOe magnetic field. No Pauling's residual entropy was observed and by 8 K the full entropy expected for a two level system is released. We propose that the disorder melts the spin ice state seen in the chemically ordered Dy2Ti2O7 compound, but the spins remain dynamic in a disordered, liquid-like state and do not freeze into a glass-like state that one might intuitively expect.

cond-mat.dis-nn↗

arXiv2018

Spin-glass ground state in a triangular-lattice compound YbZnGaO$_4$

We report on comprehensive results identifying the ground state of a triangular-lattice structured YbZnGaO$_4$ to be spin glass, including no long-range magnetic order, prominent broad excitation continua, and absence of magnetic thermal conductivity. More crucially, from the ultralow-temperature a.c. susceptibility measurements, we unambiguously observe frequency-dependent peaks around 0.1 K, indicating the spin-glass ground state. We suggest this conclusion to hold also for its sister compound YbMgGaO$_4$, which is confirmed by the observation of spin freezing at low temperatures. We consider disorder and frustration to be the main driving force for the spin-glass phase.

cond-mat.str-el↗

arXiv2017

Evidence for the Confinement of Magnetic Monopoles in Quantum Spin Ice

Magnetic monopoles are hypothesised elementary particles connected by Dirac strings that behave like infinitely thin solenoids. Despite decades of searches, free magnetic monopoles and their Dirac strings have eluded experimental detection, although there is substantial evidence for deconfined magnetic monopole quasiparticles in spin ice materials. Here we report the detection of a hierarchy of unequally-spaced magnetic excitations \emph{via} high resolution inelastic neutron spectroscopic measurements on the quantum spin ice candidate Pr$_{2}$Sn$_{2}$O$_{7}$. These excitations are well-described by a simple model of monopole pairs bound by a linear potential with an effective tension of 0.642(8) K~$\cdot$Å$^{-1}$ at 1.65~K. The success of the linear potential model suggests that these low energy magnetic excitations are direct spectroscopic evidence for the confinement of magnetic monopole quasiparticles in the quantum spin ice candidate Pr$_{2}$Sn$_{2}$O$_{7}$.

cond-mat.str-el↗

arXiv2017

Striped Magnetic Ground State of the Kagome Lattice in Fe4Si2Sn7O16

We have experimentally identified a new magnetic ground state for the kagome lattice, in the perfectly hexagonal Fe2+ (3d6, S = 2) compound Fe4Si2Sn7O16. Representational symmetry analysis of neutron diffraction data shows that below T_N = 3.5 K, the spins on 2/3 of the magnetic ions order into canted antiferromagnetic chains, separated by the remaining 1/3 which are geometrically frustrated and show no long-range order down to at least T = 0.1 K. Moessbauer spectroscopy confirms that there is no static order on the latter 1/3 of the magnetic ions - i.e., they are in a liquid-like rather than a frozen state - down to at least 1.65 K. A heavily Mn-doped sample Fe1.45Mn2.55Si2Sn7O16 has the same magnetic structure. Although the propagation vector q = (0, 1/2 , 1/2 ) breaks hexagonal symmetry, we see no evidence for magnetostriction in the form of a lattice distortion within the resolution of our data. We discuss the relationship to partially frustrated magnetic order on the pyrochlore lattice of Gd2Ti2O7, and to theoretical models that predict symmetry breaking ground states for perfect kagome lattices.

cond-mat.str-el↗

arXiv2017

Correlations and dynamics of spins in an XY-like spin-glass (Ni0.4Mn0.6)TiO3 single crystal system

Elastic and inelastic neutron scattering (ENS and INS) experiments were performed on a single crystal of (Ni0.4Mn0.6)TiO3 (NMTO) to study the spatial correlations and dynamics of spins in the XY-like spin-glass (SG) state. Magnetization measurements reveal signatures of SG behavior in NMTO with a freezing temperature of TSG ~ 9.1 K. The ENS experiments indicated that the intensity of magnetic diffuse scattering starts to increase around 12 K, which is close to TSG. Also, spin-spin correlation lengths (zeta) at 1.5 K are approximately (21) and (73) angstrom in the interlayer and the in-plane directions, respectively, demonstrating that magnetic correlations in NMTO exhibit quasi two-dimensional antiferromagnetic order. In addition, critical exponent (beta) is determined to be 0.37 from the intensity of magnetic diffuse scattering confirms the XY-like SG state of NMTO. INS results show quasi-elastic neutron scattering (QENS) profiles below TSG. The life-time of dynamic correlations obtained from the half width at half maximum of the Lorentzian QENS profiles, are approximately 16 and 16 ps at 10 K for two positions (0.00, 0.00, 1.52) and (0.01, 0.01, 1.50), respectively. Therefore, our experimental findings demonstrate that short-range-ordered antiferromagnetic clusters with short-lived spin correlations are present in the XY-like SG state of NMTO.

cond-mat.stat-mech↗

arXiv2017

The dynamics of linarite: Observations of magnetic excitations

Here we present inelastic neutron scattering measurements from the frustrated, quantum spin-1/2 chain material linarite, PbCuSO_4(OH)_2. Time of flight data, taken at 0.5K and zero applied magnetic field reveals low-energy dispersive spin wave excitations below 1.5meV both parallel and perpendicular to the Cu-chain direction. From this we confirm that the interchain couplings within linarite are around 10% of the nearest neighbour intrachain interactions. We analyse the data within both linear spin-wave theory and density matrix renormalisation group theories and establish the main magnetic exchange interactions and the simplest realistic Hamiltonian for this material.

cond-mat.str-el↗

arXiv2016

Emergence of the Isotropic Kitaev Honeycomb Lattice with Two-dimensional Ising Universality in α-RuCl$_3$

Anderson proposed structural topology in frustrated magnets hosting novel quantum spin liquids (QSLs). The QSL state is indeed exactly derived by fractionalizing the spin excitation into spinless Majorana fermions in a perfect two dimensional (2D) honeycomb lattice, the so-called Kitaev lattice, and its experimental realisation is eagerly being pursued. Here we, for the first time, report the Kitaev lattice stacking with van der Waals (vdW) bonding in a high quality α-RuCl$_3$ crystal using x-ray and neutron diffractions. Even in absence of apparent monoclinic distortion, the system exhibits antiferromagnetic (AFM) ordering below 6.5 K, likely due to minute magnetic interaction from trigonal distortion and/or interlayer coupling additionally to the Kitaev Hamiltonian. We also demonstrate 2D Ising-like critical behaviors near the Néel temperature in the order parameter and specific heat, capturing the characteristics of short-range spin-spin correlations underlying the Kitaev model. Our findings hold promise for unveiling enigmatic physics emerging from the Kitaev QSL.

cond-mat.mtrl-sci↗

arXiv2016

Statics and dynamics of the highly correlated spin ice Ho2Ge2O7

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. RMC refinements are used to fit the powder magnetic diffuse scattering and predict the single crystal magnetic scattering of Ho2Ge2O7, demonstrating consistency with spin ice behavior.

cond-mat.str-el↗

arXiv2014

Short-range magnetic correlations in the highly-correlated electron compound CeCu$_{4}Ga

We present experimental results for the heavy-electron compound CeCu$_{4}$Ga which show that it possesses short-range magnetic correlations down to a temperature of $T = 0.1$ K. Our neutron scattering data show no evidence of long-range magnetic order occurring despite a peak in the specific heat at $T^{*} =1.2$ K. Rather, magnetic diffuse scattering occurs which corresponds to short-range magnetic correlations occurring across two unit cells. The specific heat remains large as $T\sim0$ K resulting in a Sommerfeld coefficient of $γ_{0} = 1.44(2)$ J/mol-K$^{2}$, and, below $T^{*}$, the resistivity follows $T^{2}$ behavior and the ac magnetic susceptibility becomes temperature independent. A magnetic peak centered at an energy transfer of $E_{\rm{c}}=0.24(1)$ meV is seen in inelastic neutron scattering data which shifts to higher energies and broadens under a magnetic field. We discuss the coexistence of large specific heat, magnetic fluctuations, and short-range magnetic correlations at low temperatures and compare our results to those for materials possessing spin-liquid behavior.

cond-mat.str-el↗

arXiv2014

Liquid-like correlations in single crystalline Y2Mo2O7: an unconventional spin glass

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. Using neutron scattering, we present unique isotropic magnetic diffuse scattering arising beneath the spin glass transition despite having a well-ordered structure at the bulk level. Despite our attempts to model the diffuse scattering using a computationally exhaustive search of a class of simple spin Hamiltonians, we were unable to replicate the experimentally observed energy-integrated (diffuse) neutron scattering. A T^2-temperature dependence in the heat capacity and density functional theory calculations hint at significant frozen degeneracy in both the spin and orbital degrees of freedom resulting from spin-orbital coupling (Kugel-Khomskii type) and random fluctuations in the Mo environment at the local level.

cond-mat.mtrl-sci↗

arXiv2012

First order metamagnetic transition in Ho2Ti2O7 observed by vibrating coil magnetometry at milli-Kelvin temperatures

We report vibrating coil magnetometry of the spin ice system Ho2Ti2O7 down to ~0.04 K for magnetic fields up to 5 T applied parallel to the [111] axis. History dependent behavior emerges below T*_0 ~ 0.6 K near zero magnetic field, in common with other spin ice compounds. In large magnetic fields we observe a magnetization plateau followed by a hysteretic metamagnetic transition. The temperature dependence of the coercive fields as well as the susceptibility calculated from the magnetization identify the metamagnetic transition as a line of first order transitions terminating in a critical endpoint at T*_m ~ 0.37 K, B*_m ~ 1.5 T. The metamagnetic transition in Ho2Ti2O7 is strongly reminiscent of that observed in Dy2Ti2O7, suggestive of a general feature of the spin ices.

cond-mat.str-el↗

arXiv2012

Dimensional Evolution of Spin Correlations in the Magnetic Pyrochlore Yb2Ti2O7

The pyrochlore material Yb2Ti2O7 displays unexpected quasi-two-dimensional (2D) magnetic correlations within a cubic lattice environment at low temperatures, before entering an exotic disordered ground state below T=265mK. We report neutron scattering measurements of the thermal evolution of the 2D spin correlations in space and time. Short range three dimensional (3D) spin correlations develop below 400 mK, accompanied by a suppression in the quasi-elastic (QE) scattering below ~ 0.2 meV. These show a slowly fluctuating ground state with spins correlated over short distances within a kagome-triangular-kagome (KTK) stack along [111], which evolves to isolated kagome spin-stars at higher temperatures. Furthermore, low-temperature specific heat results indicate a sample dependence to the putative transition temperature that is bounded by 265mK, which we discuss in the context of recent mean field theoretical analysis.

cond-mat.str-el↗

arXiv2011

Crystal fields, disorder, and antiferromagnetic short-range order in Yb0.24Sn0.76Ru

We report extensive measurements on a new compound (Yb0.24Sn0.76)Ru that crystallizes in the cubic CsCl structure. Valence band photoemission and L3 x-ray absorption show no divalent component in the 4f configuration of Yb. Inelastic neutron scattering (INS) indicates that the eight-fold degenerate J-multiplet of Yb3+ is split by the crystalline electric field (CEF) into a Γ7 doublet ground state and a Γ8 quartet at an excitation energy 20 meV. The magnetic susceptibility can be fit very well by this CEF scheme under the assumption that a Γ6 excited state resides at 32 meV; however, the Γ8/Γ6 transition expected at 12 meV was not observed in the INS. The resistivity follows a Bloch- Grüneisen law shunted by a parallel resistor, as is typical of systems subject to phonon scattering with no apparent magnetic scattering. All of these properties can be understood as representing simple local moment behavior of the trivalent Yb ion. At 1 K, there is a peak in specific heat that is too broad to represent a magnetic phase transition, consistent with absence of magnetic reflections in neutron diffraction. On the other hand, this peak also is too narrow to represent the Kondo effect in the Γ7 ground state doublet. On the basis of the field-dependence of the specific heat, we argue that antiferromagnetic shortrange order (possibly co-existing with Kondo physics) occurs at low temperatures. The long-range magnetic order is suppressed because the Yb site occupancy is below the percolation threshold for this disordered compound.

cond-mat.str-el↗

arXiv2011

Lack of Evidence for a Singlet Crystal Field Ground State in the Tb2Ti2O7 Magnetic Pyrochlore

We present new high resolution inelastic neutron scattering data on the candidate spin liquid Tb2Ti2O7. We find that there is no evidence for a zero field splitting of the ground state doublet within the 0.2 K resolution of the instrument. This result contrasts with a pair of recent works on Tb2Ti2O7 claiming that the spin liquid behavior can be attributed to a 2 K split singlet-singlet single-ion spectrum at low energies. We also reconsider the entropy argument presented in Chapuis {\it et al.} as further evidence of a singlet-singlet crystal field spectrum. We arrive at the conclusion that estimates of the low temperature residual entropy drawn from heat capacity measurements are a poor guide to the single ion spectrum without understanding the nature of the correlations.

cond-mat.dis-nn↗

arXiv2010

Unusual signatures of the ferromagnetic transition in the heavy Fermion compound UMn$_2$Al$_{20}$

Magnetic susceptibility results for single crystals of the new cubic compounds UT$_2$Al$_{20}$ (T=Mn, V, and Mo) are reported. Magnetization, specific heat, resistivity, and neutron diffraction results for a single crystal and neutron diffraction and inelastic spectra for a powder sample are reported for UMn$_2$Al$_{20}$. For T = V and Mo, temperature independent Pauli paramagnetism is observed. For UMn$_2$Al$_{20}$, a ferromagnetic transition is observed in the magnetic susceptibility at $T_c$ = 20 K. The specific heat anomaly at $T_c$ is very weak while no anomaly in the resistivity is seen at $T_c$. We discuss two possible origins for this behavior of UMn$_2$Al$_{20}$: moderately small moment itinerant ferromagnetism, or induced local moment ferromagnetism.

cond-mat.str-el↗