arXiv subjects

G. M. Luke

Publications and source records attributed to G. M. Luke.

At least 55 records · Page 3Linked to original sources

arXiv2018

$μ$SR study of spin freezing and persistent spin dynamics in NaCaNi$_2$F$_7$

A new pyrochlore compound, NaCaNi$_2$F$_7$, was recently synthesized and has a single magnetic site with spin-1 Ni$^{2+}$. We present zero field (ZF) and longitudinal field (LF) muon spin rotation ($μ$SR) measurements on this pyrochlore. Density functional theory (DFT) calculations show that the most likely muon site is located between two fluorine ions, but off-centre. A characteristic F-$μ$-F muon spin polarization function is observed at high temperatures where Ni spin fluctuations are sufficiently rapid. The Ni$^{2+}$ spins undergo spin freezing into a disordered ground state below 4~K, with a characteristic internal field strength of 140~G. Persistent Ni spin dynamics are present to our lowest temperatures (75~mK), a feature characteristic of many geometrically frustrated magnetic systems.

cond-mat.str-el↗

arXiv2018

Dipolar-Octupolar Ising Antiferromagnetism in Sm$_2$Ti$_2$O$_7$: A Moment Fragmentation Candidate

Over the past two decades, the magnetic ground states of all rare earth titanate pyrochlores have been extensively studied, with the exception of Sm$_2$Ti$_2$O$_7$. This is, in large part, due to the very high absorption cross-section of naturally-occurring samarium, which renders neutron scattering infeasible. To combat this, we have grown a large, isotopically-enriched single crystal of Sm$_2$Ti$_2$O$_7$. Using inelastic neutron scattering, we determine that the crystal field ground state for Sm$^{3+}$ is a dipolar-octupolar doublet with Ising anisotropy. Neutron diffraction experiments reveal that Sm$_2$Ti$_2$O$_7$ orders into the all-in, all-out magnetic structure with an ordered moment of 0.44(7) $μ_B$ below $T_N=0.35$ K, consistent with expectations for antiferromagnetically-coupled Ising spins on the pyrochlore lattice. Zero-field muon spin relaxation measurements reveal an absence of spontaneous oscillations and persistent spin fluctuations down to 0.03 K. The combination of the dipolar-octupolar nature of the Sm$^{3+}$ moment, the all-in, all-out ordered state, and the low-temperature persistent spin dynamics make this material an intriguing candidate for moment fragmentation physics.

cond-mat.str-el↗

arXiv2018

Mermin-Wagner physics, (H,T) phase diagram, and candidate quantum spin-liquid phase in the spin-1/2 triangular-lattice antiferromagnet Ba8CoNb6O24

Ba$_8$CoNb$_6$O$_{24}$ presents a system whose Co$^{2+}$ ions have an effective spin 1/2 and construct a regular triangular-lattice antiferromagnet (TLAFM) with a very large interlayer spacing, ensuring purely two-dimensional character. We exploit this ideal realization to perform a detailed experimental analysis of the $S = 1/2$ TLAFM, which is one of the keystone models in frustrated quantum magnetism. We find strong low-energy spin fluctuations and no magnetic ordering, but a diverging correlation length down to 0.1 K, indicating a Mermin-Wagner trend towards zero-temperature order. Below 0.1 K, however, our low-field measurements show an nexpected magnetically disordered state, which is a candidate quantum spin liquid. We establish the $(H,T)$ phase diagram, mapping in detail the quantum fluctuation corrections to the available theoretical analysis. These include a strong upshift in field of the maximum ordering temperature, qualitative changes to both low- and high-field phase boundaries, and an ordered regime apparently dominated by the collinear "up-up-down" state. Ba$_8$CoNb$_6$O$_{24}$ therefore offers fresh input for the development of theoretical approaches to the field-induced quantum phase transitions of the $S = 1/2$ Heisenberg TLAFM.

cond-mat.str-el↗

arXiv2017

Experimental Approach to the Thermodynamics of the Pure Two-Dimensional Spin-1/2 Triangular Lattice Antiferromagnet in Ba8CoNb6O24

Frustrated quantum magnets pose well-defined questions concerning quantum fluctuation effects and the nature of the many-body wavefunction, which challenge theory, numerics, experiment and materials synthesis. The S = 1/2 triangular-lattice antiferromagnet (TLAF) presents a case where classical order is strongly suppressed by quantum fluctuations, leading to extensive renormalization of physical properties at all energy scales. However, purely two-dimensional (2D) models are difficult to realise in the 3D world and their physics is controlled by the Mermin-Wagner theorem, which describes the dominant effects of additional thermal fluctuations. Here we report the magnetic properties Ba8CoNb6O24, whose Co2+ions have an effective spin 1/2 and construct a regular TLAF with very large interlayer spacing. We find no magnetic ordering down to 0.028 K, strong low-energy spin fluctuations in qualitative agreement with theoretical analysis and a diverging correlation length, all indicating a Mermin-Wagner trend towards zero-temperature ordering in this ideal 2D system.

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arXiv2017

Phase Competition in the Palmer-Chalker XY Pyrochlore Er$_2$Pt$_2$O$_7$

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 $k=0$, $Γ_7$ magnetic structure (the Palmer-Chalker state), at $T_N = 0.38$ K. This contrasts with its sister XY pyrochlore antiferromagnets Er$_2$Ti$_2$O$_7$ and Er$_2$Ge$_2$O$_7$, both of which order into $Γ_5$ magnetic structures at much higher temperatures, $T_N=1.2$ K and $1.4$ K, respectively. In this temperature range, the magnetic heat capacity of Er$_2$Pt$_2$O$_7$ contains a broad anomaly centered at $T^*=1.5$ K. Our inelastic neutron scattering measurements reveal that this broad heat capacity anomaly sets the temperature scale for strong short-range spin fluctuations. Below $T_N = 0.38$ K, Er$_2$Pt$_2$O$_7$ displays a gapped spin wave spectrum with an intense, flat band of excitations at lower energy and a weak, diffusive band of excitations at higher energy. The flat band is well-described by classical spin wave calculations, but these calculations also predict sharp dispersive branches at higher energy, a striking discrepancy with the experimental data. This, in concert with the strong suppression of $T_N$, is attributable to enhanced quantum fluctuations due to phase competition between the $Γ_7$ and $Γ_5$ states that border each other within a classically predicted phase diagram.

cond-mat.str-el↗

arXiv2017

Gapped Excitations in the High-Pressure Antiferromagnetic Phase of URu$_2$Si$_2$

We report a neutron scattering study of the magnetic excitation spectrum in each of the three temperature and pressure driven phases of URu$_2$Si$_2$. We find qualitatively similar excitations throughout the (H0L) scattering plane in the hidden order and large moment phases, with no changes in the $\hbarω$-widths of the excitations at the $Σ$ = (1.407,0,0) and $Z$ = (1,0,0) points, within our experimental resolution. There is, however, an increase in the gap at the $Σ$ point from 4.2(2) meV to 5.5(3) meV, consistent with other indicators of enhanced antiferromagnetism under pressure.

cond-mat.str-el↗

arXiv2017

$μ$SR study of noncentrosymmetric superconductor PbTaSe$_2$

We present muon spin rotation ($μ$SR) measurements on the noncentrosymmetric superconductor PbTaSe$_2$. From measurements in an applied transverse field between $H_{c1}$ and $H_{c2}$, we extract the superfluid density as a function of temperature in the vortex state. This data can be fit with a fully gapped two-band model, consistent with previous evidence from ARPES, thermal conductivity, and resistivity. Furthermore, zero field measurements show no evidence for a time reversal symmetry breaking field greater than 0.05~G in the superconducting state. This makes exotic fully gapped spin-triplet states unlikely, and hence we contend that PbTaSe$_2$ is characterized by conventional BCS s-wave superconductivity in multiple bands.

cond-mat.supr-con↗

arXiv2017

Hidden Order Signatures in the Antiferromagnetic Phase of U(Ru$_{1-x}$Fe$_x$)$_2$Si$_2$

We present a comprehensive set of elastic and inelastic neutron scattering measurements on a range of Fe-doped samples of U(Ru$_{1-x}$Fe$_x$)$_2$Si$_2$ with 0.01~$\le~x~\le$~0.15. All of the samples measured exhibit long-range antiferromagnetic order, with the size of the magnetic moment quickly increasing to 0.51~$μ_B$ at 2.5\% doping and continuing to increase monotonically with doping, reaching 0.69~$μ_B$ at 15\% doping. Time-of-flight and inelastic triple-axis measurements show the existence of excitations at (1~0~0) and (1.4~0~0) in all samples, which are also observed in the parent compound. While the excitations in the 1\% doping are quantitatively identical to the parent material, the gap and width of the excitations change rapidly at 2.5\% Fe doping and above. The 1\% doped sample shows evidence for a separation in temperature between the hidden order and antiferromagnetic transitions, suggesting that the antiferromagnetic state emerges at very low Fe dopings. The combined neutron scattering data suggests not only discontinuous changes in the magnetic moment and excitations between the hidden order and antiferromagnetic phases, but that these changes continue to evolve up to at least $x$~=~0.15.

cond-mat.str-el↗

arXiv2017

Neutron Diffraction and $μ$SR Studies of Two Polymorphs of Nickel Niobate (NiNb$_2$O$_6$)

Neutron diffraction and muon spin relaxation ($μ$SR) studies are presented for the newly characterized polymorph of NiNb$_2$O$_6$ ($β$-NiNb$_2$O$_6$) with space group P4$_2$/n and $μ$SR data only for the previously known columbite structure polymorph with space group Pbcn. The magnetic structure of the P4$_2$/n form was determined from neutron diffraction using both powder and single crystal data. Powder neutron diffraction determined an ordering wave vector $\vec{k}$ = ($\frac{1}{2},\frac{1}{2},\frac{1}{2}$). Single crystal data confirmed the same $\vec{k}$-vector and showed that the correct magnetic structure consists of antiferromagnetically-coupled chains running along the a or b-axes in adjacent Ni$^{2+}$ layers perpendicular to the c-axis, which is consistent with the expected exchange interaction hierarchy in this system. The refined magnetic structure is compared with the known magnetic structures of the closely related tri-rutile phases, NiSb$_2$O$_6$ and NiTa$_2$O$_6$. $μ$SR data finds a transition temperature of $T_N \sim$ 15 K for this system, while the columbite polymorph exhibits a lower $T_N =$ 5.7(3) K. Our $μ$SR measurements also allowed us to estimate the critical exponent of the order parameter $β$ for each polymorph. We found $β=$ 0.25(3) and 0.16(2) for the $β$ and columbite polymorphs respectively. The single crystal neutron scattering data gives a value for the critical exponent $β=$~0.28(3) for $β$-NiNb$_2$O$_6$, in agreement with the $μ$SR value. While both systems have $β$ values less than 0.3, which is indicative of reduced dimensionality, this effect appears to be much stronger for the columbite system. In other words, although both systems appear to well-described by $S = 1$ spin chains, the interchain interactions in the $β$-polymorph are likely much larger.

cond-mat.other↗

arXiv2016

Spin Freezing in The Frustrated Disordered Quantum Magnet Ba$_{3}$(Mn$_{1-x}$V$_{x}$)$_{2}$O$_{8}$

Ba$_3$Mn$_2$O$_8$ is a geometrically frustrated spin dimer compound. We investigate the effect of site disorder on the zero field phase diagram of this material by considering the solid solution Ba$_{3}$(Mn$_{1-x}$V$_{x}$)$_{2}$O$_{8}$, where nonmagnetic V$^{5+}$ ions partially substitute magnetic Mn$^{5+}$ ions. This substitution results in unpaired $S=1$ moments for half-substituted dimers, which are ungapped and therefore susceptible to types of magnetic order not present in the parent compound. AC susceptibility measurements of compositions between $x=0.046$ and $x=0.84$ show a sharp frequency- and composition-dependent kink at temperatures below 210mK, suggesting that unpaired spins form a spin glass. The case for a glassy state is made clearer by the absence of any sharp features in the specific heat. However, Ba$_{3}$(Mn$_{1-x}$V$_{x}$)$_{2}$O$_{8}$ is not a paradigmatic spin glass. Whereas both the freezing temperature and the Weiss temperature (determined from susceptibility above 1K) vary strongly as a function of composition, the heat capacity per unpaired spin is found to be insensitive (above the glass transition) to the density of unpaired spins for the broad regime $0.18\leq x \leq 0.84$. This surprising result is consistent with a scenario in which nearest-neighbor unpaired spins form local, possibly fluctuating, spin-singlets prior to the eventual spin freezing. The spin glass state is only found for temperatures below the energy scale of single-ion anisotropy, suggestive this plays a significant role in determining the eventual ground state. Possible ground states in the "dilute" limit ($x < 0.04$ and $x > 0.9$) are also discussed.

cond-mat.str-el↗

arXiv2016

Magnetic dilution and domain selection in the XY pyrochlore antiferromagnet Er$_{2}$Ti$_2$O$_7$

Below $T_N = 1.1$K, the XY pyrochlore Er$_2$Ti$_2$O$_7$ orders into a $k=0$ non-collinear, antiferromagnetic structure referred to as the $ψ_2$ state. The magnetic order in Er$_2$Ti$_2$O$_7$ is known to obey conventional three dimensional (3D) percolation in the presence of magnetic dilution, and in that sense is robust to disorder. Recently, however, two theoretical studies have predicted that the $ψ_2$ structure should be unstable to the formation of a related $ψ_3$ magnetic structure in the presence of magnetic vacancies. To investigate these theories, we have carried out systematic elastic and inelastic neutron scattering studies of three single crystals of Er$_{2-x}$Y$_x$Ti$_2$O$_7$ with $x=0$ (pure), 0.2 (10$\%$-Y) and 0.4 (20$\%$-Y), where magnetic Er$^{3+}$ is substituted by non-magnetic Y$^{3+}$. We find that the $ψ_2$ ground state of pure Er$_2$Ti$_2$O$_7$ is significantly affected by magnetic dilution. The characteristic domain selection associated with the $ψ_2$ state, and the corresponding energy gap separating $ψ_2$ from $ψ_3$, vanish for Y$^{3+}$ substitutions between 10$\%$-Y and 20$\%$-Y, far removed from the 3D percolation threshold of $\sim$60$\%$-Y. The resulting ground state for Er$_2$Ti$_2$O$_7$ with magnetic dilutions from 20$\%$-Y up to the percolation threshold is naturally interpreted as a frozen mosaic of $ψ_2$ and $ψ_3$ domains.

cond-mat.str-el↗

arXiv2016

Relief of Frustration in the Heisenberg Pyrochlore Antiferromagnet Gd$_2$Pt$_2$O$_7$

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 $T_N = 1.6$ K. This transition is strongly first order, as indicated by the sharpness of the heat capacity anomaly, thermal hysteresis in the magnetic susceptibility, and a non-divergent relaxation rate in $μ$SR. The form of the heat capacity below $T_N$ suggests that the ground state is an anisotropic collinear antiferromagnet with an excitation spectrum that is gapped by 0.245(1) meV. The ordering temperature in Gd$_2$Pt$_2$O$_7$, $T_N = 1.6$ K, is a substantial 160% increase from other gadolinium pyrochlores, which have been found to order at 1 K or lower. We attribute this enhancement in $T_N$ to the $B$-site cation, platinum, which, despite being non-magnetic, has a filled $5d$ $t_{2g}$ orbital and an empty $5d$ $e_g$ orbital that can facilitate superexchange. Thus, the magnetic frustration in Gd$_2$Pt$_2$O$_7$ is partially "relieved", thereby promoting magnetic order.

cond-mat.str-el↗

arXiv2016

$μ$SR and magnetometry study of superconducting 5% Pt doped IrTe$_2$

We present magnetometry and muon spin rotation ($μ$SR) measurements of the superconducting dichalcogenide Ir$_{0.95}$Pt$_{0.05}$Te$_2$. From both sets of measurements we calculate the penetration depth and thence superfluid density as a function of temperature. The temperature dependence of the superfluid densities from both sets of data indicate fully gapped superconductivity that can be fit to a conventional s-wave model and yiel fitting parameters consistent with a BCS weak coupling superconductor. We therefore see no evidence for exotic superconductivity in Ir$_{0.95}$Pt$_{0.05}$Te$_2$.

cond-mat.supr-con↗

arXiv2016

Universal Dynamic Magnetism in Yb-Pyrochlores with Disparate Ground States

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. While the static properties of the ytterbium pyrochlores are distinct, inelastic neutron scattering measurements reveal a common character to their exotic spin dynamics. All three ytterbium pyrochlores show a gapless continuum of spin excitations, resembling over-damped ferromagnetic spin waves at low Q. Furthermore, the specific heat of the series also follows a common form, with a broad, high-temperature anomaly followed by a sharp low-temperature anomaly at T_C or T_N. The novel spin dynamics we report correlate strongly with the broad specific heat anomaly only, remaining unchanged across the sharp anomaly. This result suggests that the primary order parameter in the ytterbium pyrochlores associated with the sharp anomaly is "hidden" and not simple magnetic dipole order.

cond-mat.str-el↗

arXiv2016

XY Antiferromagnetic Ground State in the Effective S=1/2 Pyrochlore Yb2Ge2O7

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. muSR measurements reveal the presence of an ordering transition at T_N = 0.57 K with persistent weak dynamics in the ordered state. Finally, we present neutron diffraction measurements that reveal a clear phase transition to the k = (000) Gamma_5 ground state with an ordered magnetic moment of 0.3(1) mu_B per Yb ion. We compare and contrast this phenomenology with the low temperature behavior of Yb2Ti2O7 and Er2Ti2O7, the prototypical S_eff = 1/2 XY pyrochlore magnets.

cond-mat.str-el↗

arXiv2015

Inelastic x-ray scattering measurements on URu2Si2

We report high-resolution inelastic x ray scattering measurements of the acoustic phonons of URu2Si2. We observe minimal change in the phonon structure upon entering the Hidden Order phase. At all temperatures, the longitudinal acoustic phonons are anomalously broad at low q. There is no evidence to suggest that the increase in thermal conductivity in the Hidden Order phase is driven by a change in phonon dispersions or lifetimes. A more likely explanation is that the magnetic excitations in the Hidden Order state contribute to thermal transport.

cond-mat.str-el↗

arXiv2015

Spin frustration and magnetic ordering in triangular lattice antiferromagnet Ca$_3$CoNb$_2$O$_9$

We synthesized a quasi-two-dimensional distorted triangular lattice antiferromagnet Ca$_3$CoNb$_2$O$_9$, in which the effective spin of Co$^{2+}$ is 1/2 at low temperatures, whose magnetic properties were studied by dc susceptibility and magnetization techniques. The x-ray diffraction confirms the quality of our powder samples. The large Weiss constant $θ_{CW}\sim$ $-55$ K and the low Neel temperature($T_N\sim$ 1.45 K) give a frustration factor $f$ ($=\midθ_{CW}/T_N \mid$) $\approx$ 38, suggesting that Ca$_3$CoNb$_2$O$_9$ resides in strong frustration regime. Slightly below $T_N$, deviation between the susceptibility data under zero-field cooling (ZFC) and field cooling (FC) is observed. A new magnetic state with 1/3 of the saturate magnetization $M_s$ is suggested in the magnetization curve at 0.46 K. Our study indicates that Ca$_3$CoNb$_2$O$_9$ is an interesting material to investigate magnetism in triangular lattice antiferromagnets with weak anisotropy.

cond-mat.str-el↗

arXiv2015

Antiferromagnetism and Hidden Order in Isoelectronic Doping of URu$_2$Si$_2$

We present muon spin rotation ($μ$SR) and susceptibility measurements on single crystals of isoelectronically doped URu$_{2-x}$T$_x$Si$_2$ (T = Fe, Os) for doping levels up to 50\%. Zero Field (ZF) $μ$SR measurements show long-lived oscillations demonstrating that an antiferromagnetic state exists down to low doping levels for both Os and Fe dopants. The measurements further show an increase in the internal field with doping for both Fe and Os. Comparison of the local moment - hybridization crossover temperature from susceptibility measurements and our magnetic transition temperature shows that changes in hybridization, rather than solely chemical pressure, are important in driving the evolution of magnetic order with doping.

cond-mat.str-el↗