arXiv subjects

J. A. Quilliam

Publications and source records attributed to J. A. Quilliam.

At least 19 recordsLinked to original sources

arXiv2022

Magnetic order and spin liquid behavior in [Mo3+]^{11+} triangular magnets

Molecular magnets based on [Mo$_3$]$^{11+}$ units with one unpaired electron per trimer have attracted recent interest due to the identification of quantum spin liquid candidacy in some family members. Here, we present comprehensive measurements on polycrystalline samples of ZnScMo$_3$O$_8$, MgScMo$_3$O$_8$, and Na$_3$Sc$_2$Mo$_5$O$_{16}$ with the same Mo$_3$O$_{13}$ magnetic building blocks. The crystal structures are characterized with x-ray or neutron powder diffraction and the magnetic ground states are determined by performing ac and dc susceptibility, specific heat, neutron powder diffraction, and $μ$SR measurements. Our work indicates that ZnScMo$_3$O$_8$ and MgScMo$_3$O$_8$ have ferromagnetic Curie-Weiss temperatures of 18.5 K and 11.9 K, ordered ground states with net moments (low-moment ferromagnetism or canted antiferromagnetism), and zero field ordering temperatures of $T_c =$ 6 K and $<$ 2 K respectively. On the other hand, Na$_3$Sc$_2$Mo$_5$O$_{16}$ hosts a dynamical magnetic ground state with no evidence for magnetic ordering or spin freezing down to 20 mK despite an antiferromagnetic Curie-Weiss temperature of -36.2 K, and therefore is a candidate for quantum spin liquid behavior. By comparing the present results to past work on the same family of materials, we construct a phase diagram which illustrates that the magnetic ground states of these Mo-based molecular magnets are very sensitive to small changes in the nearest neighbor Mo-Mo distance.

cond-mat.str-el↗

arXiv2020

Field-angle dependence of sound velocity in the Weyl semimetal TaAs

The elastic modulus $c_{44}$ of a single crystal of the Weyl semimetal TaAs was investigated by measuring relative changes in the sound velocity under application of a magnetic field up to 10 T. Using an ultrasonic pulsed-echo technique, we studied the shear response of the crystal when the angle between the sound wave propagation and the magnetic field is changed. We observe a broken tetragonal symmetry at fields above 6 T, an anisotropy that is likely related to a longitudinal negative magnetoresistance and therefore might provide evidence of the chiral anomaly, one of the main topological signatures of this class of materials. We also observe quantum oscillations in the sound velocity whose frequencies vary with magnetic field orientation. A fan diagram of Landau level indices reveals topological and trivial Berry phases, depending on the field orientation, indicating a sensitivity to different Fermi surface pockets that do or do not enclose Weyl nodes respectively.

cond-mat.mtrl-sci↗

arXiv2020

Realization of the orbital-selective Mott state at the molecular level in Ba$_3$LaRu$_2$O$_9$

Molecular magnets based on heavy transition metals have recently attracted significant interest in the quest for novel magnetic properties. For systems with an odd number of valence electrons per molecule, high or low molecular spin states are typically expected in the double exchange or quasi-molecular orbital limits respectively. In this work, we use bulk characterization, muon spin relaxation, neutron diffraction, and inelastic neutron scattering to identify a rare intermediate spin-3/2 per dimer state in the 6H-perovskite Ba$_3$LaRu$_2$O$_9$ that cannot be understood in a double exchange or quasi-molecular orbital picture and instead arises from orbital-selective Mott insulating behavior at the molecular level. Our measurements are also indicative of collinear stripe magnetic order below $T_N$ = 26(1) K for these molecular spin-3/2 degrees-of-freedom, which is consistent with expectations for an ideal triangular lattice with significant next nearest neighbor in-plane exchange. Finally, we present neutron diffraction and Raman scattering data under applied pressure that reveal low-lying structural and spin state transitions at modest pressures P $\le$ 1 GPa, which highlights the delicate balance between competing energy scales in this system.

cond-mat.str-el↗

arXiv2020

Canted Antiferromagnetic Order in the Kagome Material Sr-Vesignieite

We report $^{51}$V NMR, $μ$SR and zero applied field $^{63,65}$Cu NMR measurements on powder samples of Sr-vesignieite, SrCu$_3$V$_2$O$_8$(OH)$_2$, a $S = 1/2$ nearly-kagome Heisenberg antiferromagnet. Our results demonstrate that the ground state is a $\mathbf{q} = 0$ magnetic structure with spins canting either in or out of the kagome plane, giving rise to weak ferromagnetism. We determine the size of ordered moments and the angle of canting for different possible $\mathbf{q} = 0$ structures and orbital scenarios, thereby providing insight into the role of the Dzyaloshinskii-Moriya (DM) interaction in this material.

cond-mat.str-el↗

arXiv2019

Novel Strongly Spin-Orbit Coupled Quantum Dimer Magnet: Yb$_2$Si$_2$O$_7$

The quantum dimer magnet (QDM) is the canonical example of quantum magnetism. The QDM state consists of entangled nearest-neighbor spin dimers and often exhibits a field-induced triplon Bose-Einstein condensate (BEC) phase. We report on a new QDM in the strongly spin-orbit coupled, distorted honeycomb-lattice material Yb$_2$Si$_2$O$_7$. Our single crystal neutron scattering, specific heat, and ultrasound velocity measurements reveal a gapped singlet ground state at zero field with sharp, dispersive excitations. We find a field-induced magnetically ordered phase reminiscent of a BEC phase, with exceptionally low critical fields of $H_{c1} \sim 0.4$ T and $H_{c2} \sim 1.4$ T. Using inelastic neutron scattering in an applied magnetic field we observe a Goldstone mode (gapless to within $δE$ = 0.037 meV) that persists throughout the entire field-induced magnetically ordered phase, suggestive of the spontaneous breaking of U(1) symmetry expected for a triplon BEC. However, in contrast to other well-known cases of this phase, the high-field ($μ$$_0$$H\geq1.2$T) part of the phase diagram in Yb$_2$Si$_2$O$_7$ is interrupted by an unusual regime signaled by a change in the field dependence of the ultrasound velocity and magnetization, as well as the disappearance of a sharp anomaly in the specific heat. These measurements raise the question of how anisotropy in strongly spin-orbit coupled materials modifies the field induced phases of QDMs.

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

Tunable quantum spin liquidity in the 1/6th-filled breathing kagome lattice

We present measurements on a series of materials, Li$_2$In$_{1-x}$Sc$_x$Mo$_3$O$_8$, that can be described as a 1/6th-filled breathing kagome lattice. Substituting Sc for In generates chemical pressure which alters the breathing parameter non-monotonically. $μ$SR experiments show that this chemical pressure tunes the system from antiferromagnetic long range order to a quantum spin liquid phase. A strong correlation with the breathing parameter implies that it is the dominant parameter controlling the level of magnetic frustration, with increased kagome symmetry generating the quantum spin liquid phase. Magnetic susceptibility measurements suggest that this is related to distinct types of charge order induced by changes in lattice symmetry, in line with the theory of Chen et al. [Phys. Rev. B 93, 245134 (2016)]. The specific heat for samples at intermediate Sc concentration and with minimal breathing parameter, show consistency with the predicted $U(1)$ quantum spin liquid.

cond-mat.str-el↗

arXiv2017

Anisotropic phase diagram and spin fluctuations of the hyperkagome magnet Gd3Ga5O12 as revealed by sound velocity measurements

Sound velocity and attenuation measurements on the frustrated garnet material Gd3Ga5O12 (GGG) are presented as a function of field and temperature, using two different acoustic modes and with two different magnetic field orientations: [100] and [110]. We demonstrate that the phase diagram is highly anisotropic, with two distinct field-induced ordered phases for H||[110] and only one for H||[100]. Extensive lattice softening is found to occur at low fields, which can be associated with spin fluctuations. However, deep within the spin liquid phase a low-temperature stiffening of the lattice and reduced attenuation provides evidence for a spin gap which may be related to short-range antiferromagnetic correlations over minimal 10-spin loops.

cond-mat.str-el↗

arXiv2017

Frustrated spin-1/2 molecular magnetism in the mixed-valence antiferromagnets Ba3MRu2O9 (M = In, Y, Lu)

We have performed magnetic susceptibility, heat capacity, muon spin relaxation, and neutron scattering measurements on three members of the family Ba3MRu2O9, where M = In, Y and Lu. These systems consist of mixed-valence Ru dimers on a triangular lattice with antiferromagnetic interdimer exchange. Although previous work has argued that charge order within the dimers or intradimer double exchange plays an important role in determining the magnetic properties, our results suggest that the dimers are better described as molecular units due to significant orbital hybridization, resulting in one spin-1/2 moment distributed equally over the two Ru sites. These molecular building blocks form a frustrated, quasi-two-dimensional triangular lattice. Our zero and longitudinal field muSR results indicate that the molecular moments develop a collective, static magnetic ground state, with oscillations of the zero field muon spin polarization indicative of long-range magnetic order in the Lu sample. The static magnetism is much more disordered in the Y and In samples, but they do not appear to be conventional spin glasses.

cond-mat.str-el↗

arXiv2016

Dimensionality and irreversibility of field-induced transitions in SrDy2O4

Low temperature ultrasound velocity measurements are presented on the frustrated spin system SrDy2O4 that allow us to define high resolution phase diagrams with the magnetic field applied along all three principal axes. For H||b, a region of field-induced long range order is delimited by a dome of first-order phase transitions. An unusual magnetization process is observed with significant irreversibility at very low temperatures when passing between the low-field spin liquid phase and the long range ordered phase which we attribute to large energy barriers. For H||c, the system appears to remain effectively one-dimensional, exhibiting two transitions as a function of magnetic field, but no finite-temperature long range order.

cond-mat.str-el↗

arXiv2013

Juxtaposition of Spin Freezing and Long Range Order in a Series of Geometrically Frustrated Antiferromagnetic Gadolinium Garnets

Specific heat measurements in zero magnetic field are presented on a homologous series of geometrically frustrated, antiferromagnetic, Heisenberg garnet systems. Measurements of Gd3Ga5O12, grown with isotopically pure Gd, agree well with previous results on samples with naturally abundant Gd, showing no ordering features. In contrast, samples of Gd3Te2Li3O12 and Gd3Al5O12 are found to exhibit clear ordering transitions at 243 mK and 175 mK respectively. The effects of low level disorder are studied through dilution of Gd3+ with non-magnetic Y3+ in Gd3Te2Li3O12. A thorough structural characterization, using X-ray diffraction, is performed on all of the samples studied. We discuss possible explanations for such diverse behavior in very similar systems.

cond-mat.dis-nn↗

arXiv2013

Lightly stuffed pyrochlore structure of single-crystalline Yb2Ti2O7 grown by the optical floating zone technique

Recent neutron scattering and specific heat studies on the pyrochlore Yb2Ti2O7 have revealed variations in its magnetic behavior below 265mK. In the best samples, a sharp anomaly in the specific heat is observed at T=265mK. Other samples, especially single crystals, have broad features in the specific heat which vary in sharpness and temperature depending on the sample, indicating that the magnetic ground state may be qualitatively different in such samples. We performed detailed comparisons of the chemical structure of a pulverised single crystal of Yb2Ti2O7, grown by the floating zone technique, to a sintered powder sample of Yb2Ti2O7. Rietveld refinements of neutron powder diffraction data on these samples reveal that the crushed single crystal is best described as a "stuffed" pyrochlore, Yb2(Ti(2-x)Yb(x))O7-(x/2) with x = 0.046(4), despite perfectly stoichiometric starting material. Substituting magnetic Yb3+ on the non-magnetic Ti4+ sublattice would introduce random exchange bonds and local lattice deformations. These are expected to be the mechanism leading to of the variation of the delicate magnetic ground state of Yb2Ti2O7. Determination of the cubic cell length, a, could be useful as a method for characterizing the stoichiometry of non- pulverised single crystals at room temperature.

cond-mat.str-el↗

arXiv2012

Singlet Ground State of the Quantum Antiferromagnet Ba3CuSb2O9

We present local probe results on the honeycomb lattice antiferromagnet Ba3CuSb2O9. Muon spin relaxation measurements in zero field down to 20 mK show unequivocally that there is a total absence of spin freezing in the ground state. Sb NMR measurements allow us to track the intrinsic susceptibility of the lattice, which shows a maximum at around 55 K and drops to zero in the low-temperature limit. The spin-lattice relaxation rate shows two characteristic energy scales, including a field-dependent crossover to exponential low-temperature behavior, implying gapped magnetic excitations.

cond-mat.str-el↗

arXiv2012

Experimental Phase Diagram and Dynamics of a Dilute Dipolar-Coupled Ising System

We present ac susceptibility and specific heat measurements taken on samples of LiHo$_x$Y$_{1-x}$F$_4$ in the dilute limit: x = 0.018, 0.045, 0.080 and 0.12. Susceptibility measurements show glassy behavior including wide absorption spectra that continually broaden with decreasing temperature. Dynamical scaling analyses show evidence of finite-temperature spin-glass transitions, the temperatures of which match those of recent theoretical work. A surprisingly long intrinsic time constant is observed in these samples and is found to be inversely correlated with the concentration of magnetic moments, x. Our results support the picture that this behavior is largely a single-ion effect, related to the random transverse fields generated by the off-diagonal component of the dipolar interaction and significantly slowed by the important nuclear hyperfine interaction. Specific heat measurements show broad features due to the electronic spins on top of a large Schottky-like nuclear contribution. Unusually, the peak position of the electronic component is found to be largely concentration independent, unlike the glass transition temperature.

cond-mat.dis-nn↗

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

Ground State and Intrinsic Susceptibility of the Kagome Antiferromagnet Vesignieite as seen by 51V NMR

The intrinsic magnetic susceptibility and local magnetization of the near-kagome quantum magnet vesignieite, Cu3BaV2O8(OH)2, are presented as measured using 51V NMR. The NMR line shift gives an accurate measurement of the intrinsic susceptibility of the kagome sites which closely resembles that of the quantum spin liquid herbertsmithite [A. Olariu et al. Phys. Rev. Lett. 100, 087202 (2008)]. It is therefore surprising that, at Tc ~ 9 K, a transition to a heterogeneous ground state is observed. A gradual wipeout of half the NMR intensity indicates a slowly fluctuating spin liquid component and a detailed analysis of the linewidth reveals the onset of static magnetism at the remaining half of the sites. It is proposed that this transition and unusual ground state originate from a nearby quantum critical point induced by the Dzyaloshinskii-Moriya interaction.

cond-mat.str-el↗

arXiv2011

Dynamics of the Magnetic Susceptibility Deep in the Coulomb Phase of the Dipolar Spin Ice Material Ho2Ti2O7

Low temperature measurements of the ac magnetic susceptibility along the [110] direction of a single crystal of the dipolar spin ice material Ho2Ti2O7, in zero static field, are presented. While behavior that is qualitatively consistent with previous work on Ho2Ti2O7 and the related material Dy2Ti2O7 is observed, this work extends measurements to appreciably lower temperatures and frequencies. In the freezing regime, below 1 K, the dynamics are found to be temperature activated, thus well described by an Arrhenius law with an activation energy close to 6J_eff, a result that is not easily explained with the current model of magnetic monopole excitations in dipolar spin ice. The form and temperature dependence of the ac susceptibility spectra are found to be nontrivial and distinct from standard glassy relaxation. Particular attention has been paid to correcting for the demagnetization effect, which is quite significant in these materials and has important, even qualitative, effects on the susceptibility spectra.

cond-mat.mtrl-sci↗

arXiv2008

Observation of Spin Glass Dynamics in Dilute LiHo$_x$Y$_{1-x}$F$_4$

AC susceptibility measurements are presented on the dilute, dipolar coupled, Ising magnet LiHo$_x$Y$_{1-x}$F$_4$ for a concentration $x = 0.045$. The frequency and temperature dependences of the susceptibility show characteristic glassy relaxation. The absorption spectrum is found to broaden with decreasing temperature suggesting that the material is behaving as a spin glass and not as an exotic spin liquid as was previously observed. A dynamical scaling analysis suggests a spin glass transition temperature of 42.6 mK $\pm 2$ mK with an exponent $zν= 7.8 \pm 0.23$.

cond-mat.dis-nn↗