arXiv subjects

M. R. Lees

Publications and source records attributed to M. R. Lees.

At least 55 records · Page 3Linked to original sources

arXiv2016

Robust singlet dimers with fragile ordering in two-dimensional honeycomb lattice of Li$_2$RuO$_3$

When an electronic system has strong correlations and a large spin-orbit interaction, it often exhibits a plethora of mutually competing quantum phases. How a particular quantum ground state is selected out of several possibilities is a very interesting question. However, equally fascinating is how such a quantum entangled state breaks up due to perturbation. This important question has relevance in very diverse fields of science from strongly correlated electron physics to quantum information. Here we report that a quantum entangled dimerized state or valence bond crystal (VBC) phase of Li2RuO3 shows nontrivial doping dependence as we perturb the Ru honeycomb lattice by replacing Ru with Li. Through extensive experimental studies, we demonstrate that the VBC phase melts into a valence bond liquid phase of the RVB (resonating valence bond) type. This system offers an interesting playground where one can test and refine our current understanding of the quantum competing phases in a single compound.

cond-mat.str-el↗

arXiv2016

Observation of magnetic fragmentation in spin ice

Fractionalised excitations that emerge from a many body system have revealed rich physics and concepts, from composite fermions in two-dimensional electron systems, revealed through the fractional quantum Hall effect, to spinons in antiferromagnetic chains and, more recently, fractionalisation of Dirac electrons in graphene and magnetic monopoles in spin ice. Even more surprising is the fragmentation of the degrees of freedom themselves, leading to coexisting and a priori independent ground states. This puzzling phenomenon was recently put forward in the context of spin ice, in which the magnetic moment field can fragment, resulting in a dual ground state consisting of a fluctuating spin liquid, a so-called Coulomb phase, on top of a magnetic monopole crystal. Here we show, by means of neutron scattering measurements, that such fragmentation occurs in the spin ice candidate Nd$_2$Zr$_2$O$_7$. We observe the spectacular coexistence of an antiferromagnetic order induced by the monopole crystallisation and a fluctuating state with ferromagnetic correlations. Experimentally, this fragmentation manifests itself via the superposition of magnetic Bragg peaks, characteristic of the ordered phase, and a pinch point pattern, characteristic of the Coulomb phase. These results highlight the relevance of the fragmentation concept to describe the physics of systems that are simultaneously ordered and fluctuating.

cond-mat.str-el↗

arXiv2015

Antiferromagnetism at T > 500 K in the Layered Hexagonal Ruthenate SrRu2O6

We report an experimental and computational study of magnetic and electronic properties of the layered Ru(V) oxide SrRu2O6 (hexagonal, P-3 1m), which shows antiferromagnetic order with a Néel temperature of 563(2) K, among the highest for 4d oxides. Magnetic order occurs both within edge-shared octahedral sheets and between layers and is accompanied by anisotropic thermal expansivity that implies strong magnetoelastic coupling of Ru(V) centers. Electrical transport measurements using focused ion beam induced deposited contacts on a micron-scale crystallite as a function of temperature show p-type semiconductivity. The calculated electronic structure using hybrid density functional theory successfully accounts for the experimentally observed magnetic and electronic structure and Monte Carlo simulations reveals how strong intralayer as well as weaker interlayer interactions are a defining feature of the high temperature magnetic order in the material.

cond-mat.str-el↗

arXiv2015

Fluctuations and all-in$-$all-out ordering in dipole-octupole Nd2Zr2O7

We present an experimental study of the pyrochlore coumpound Nd2Zr2O7 by means of neutron scattering and magnetization measurements down to 90 mK. The Nd$^{3+}$ magnetic moments exhibit a strong local $\langle$111$\rangle$ Ising anisotropy together with a dipolar-octupolar nature, different from the standard Kramers-doublet studied so far. We show that, despite the positive Curie-Weiss temperature, \ndzr\ undergoes a transition around 285 mK towards an all-in$-$all-out antiferromagnetic state. We establish the $(H,T)$ phase diagram in the three directions of the applied field and reveal a metamagnetic transition around 0.1 T. The strongly reduced ordered magnetic moment as well as the unexpected shape of the magnetization curves demonstrate that Nd2Zr2O7 , is not a standard Ising antiferromagnet. We propose that the peculiar nature of the Nd doublet combined with competing interactions explain these findings.

cond-mat.str-el↗

arXiv2015

Magnetism in a family of $S = 1$ square lattice antiferromagnets Ni$X_2$(pyz)$_2$ ($X = $ Cl, Br, I, NCS; pyz = pyrazine)

The crystal structures of Ni$X_2$(pyz)$_2$ ($X$ = Cl (\textbf{1}), Br (\textbf{2}), I (\textbf{3}) and NCS (\textbf{4})) were determined at 298~K by synchrotron X-ray powder diffraction. All four compounds consist of two-dimensional (2D) square arrays self-assembled from octahedral NiN$_4$$X_2$ units that are bridged by pyz ligands. The 2D layered motifs displayed by \textbf{1}-\textbf{4} are relevant to bifluoride-bridged [Ni(HF$_2$)(pyz)$_2$]$Z$F$_6$ ($Z$ = P, Sb) which also possess the same 2D layers. In contrast, terminal $X$ ligands occupy axial positions in \textbf{1}-\textbf{4} and cause a staggering of adjacent layers. Long-range antiferromagnetic order occurs below 1.5 (Cl), 1.9 (Br and NCS) and 2.5~K (I) as determined by heat capacity and muon-spin relaxation. The single-ion anisotropy and $g$ factor of \textbf{2}, \textbf{3} and \textbf{4} are measured by electron spin resonance where no zero--field splitting was found. The magnetism of \textbf{1}-\textbf{4} crosses a spectrum from quasi-two-dimensional to three-dimensional antiferromagnetism. An excellent agreement was found between the pulsed-field magnetization, magnetic susceptibility and $T_\textrm{N}$ of \textbf{2} and \textbf{4}. Magnetization curves for \textbf{2} and \textbf{4} calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. \textbf{3} is characterized as a three-dimensional antiferromagnet with the interlayer interaction ($J_\perp$) slightly stronger than the interaction within the two-dimensional [Ni(pyz)$_2$]$^{2+}$ square planes ($J_\textrm{pyz}$).

cond-mat.str-el↗

arXiv2015

Structural and Magnetic Investigations of Single-Crystals of the Neodymium Zirconate Pyrochlore, Nd2Zr2O7

We report structural and magnetic properties studies of large high quality single-crystals of the frustrated magnet, Nd$_2$Zr$_2$O$_7$. Powder x-ray diffraction analysis confirms that Nd$_2$Zr$_2$O$_7$ adopts the pyrochlore structure. Room-temperature x-ray diffraction and time-of-flight neutron scattering experiments show that the crystals are stoichiometric in composition with no measurable site disorder. The temperature dependence of the magnetic susceptibility shows no magnetic ordering at temperatures down to 0.5 K. Fits to the magnetic susceptibility data using a Curie-Weiss law reveal a ferromagnetic coupling between the Nd moments. Magnetization versus field measurements show a local Ising anisotropy along the <111> axes of the Nd$^{3+}$ ions in the ground state. Specific heat versus temperature measurements in zero applied magnetic field indicate the presence of a thermal anomaly below $T\sim7$ K, but no evidence of magnetic ordering is observed down to 0.5 K. The experimental temperature dependence of the single-crystal bulk dc susceptibility and isothermal magnetization are analyzed using crystal field theory and the crystal field parameters and exchange coupling constants determined.

cond-mat.str-el↗

arXiv2015

Evidence for a hybridization gap in noncentrosymmetric CeRuSi3

Inelastic neutron scattering (INS) and specific heat measurements have been performed on the intermediate valence compound CeRuSi$_3$, which is isostructural to the noncentrosymmetric pressure-induced superconductors CeRhSi$_3$, CeIrSi$_3$ and CeCoGe$_3$. INS measurements at 7~K reveal a broad peak at (58.5~$\pm$~1.4)~meV, while at 300~K, broad quasielastic scattering is observed. This indicates a large Kondo temperature of $T_{\rm K}~\sim~680$~K. The magnetic contribution to the specific heat ($C_{\rm mag}$) has a value of $γ~=~62.5(1)$~mJ/mol~K$^2$ at low temperatures and above about 100~K can be well accounted for by the Coqlbin-Schrieffer model with a characteristic temperature of $T_0$~=~680~K, which is further evidence that CeRuSi$_3$ is in the intermediate valence regime.

cond-mat.str-el↗

arXiv2014

Studies of the superconducting properties of Sn1-xInxTe (x=0.38 to 0.45) using muon-spin spectroscopy

The superconducting properties of Sn1-xInxTe (x = 0.38 to 0.45) have been studied using magnetization and muon-spin rotation or relaxation (muSR) measurements. These measurements show that the superconducting critical temperature Tc of Sn1-xInxTe increases with increasing x, reaching a maximum at around 4.8 K for x = 0.45. Zero-field muSR results indicate that time-reversal symmetry is preserved in this material. Transverse-field muon-spin rotation has been used to study the temperature dependence of the magnetic penetration depth lambda(T) in the mixed state. For all the compositions studied, lambda(T) can be well described using a single-gap s-wave BCS model. The magnetic penetration depth at zero temperature lambda(0) ranges from 500 to 580 nm. Both the superconducting gap Delta(0) at 0 K and the gap ratio Delta(0)/kBTc indicate that Sn1-xInxTe (x = 0.38 to 0.45) should be considered as a superconductor with intermediate to strong coupling.

cond-mat.supr-con↗

arXiv2014

Magnetic properties of geometrically frustrated SrGd2O4

A study of the magnetic properties of the frustrated rare earth oxide SrGd2O4 has been completed using bulk property measurements of magnetization, susceptibility and specific heat on single crystal samples. Two zero-field phase transitions have been identified at 2.73 and 0.48 K. For the field, H, applied along the a and b axes, a single boundary is identified that delineates the transition from a low field, low temperature magnetically ordered regime to a high field, high temperature paramagnetic phase. Several field-induced transitions, however, have been observed with H || c. The measurements have been used to map out the magnetic phase diagram of SrGd2O4, suggesting that it is a complex system with several competing magnetic interactions. The low-temperature magnetic behavior of SrGd2O4 is very different compared to the other SrLn2O4 (Ln = Lanthanide) compounds, even though all of the SrLn2O4 compounds are isostructural, with the magnetic ions forming a low-dimensional lattice of zigzag chains that run along the c axis. The differences are likely to be due to the fact that in the ground state Gd3+ has zero orbital angular momentum and therefore the spin-orbit interactions, which are crucial for other SrLn2O4 compounds, can largely be neglected. Instead, given the relatively short Gd3+-Gd3+ distances in SrGd2O4, dipolar interactions must be taken into account for this antiferromagnet alongside the Heisenberg exchange terms.

cond-mat.str-el↗

arXiv2014

Probing the superconducting ground state of the noncentrosymmetric superconductors CaTSi3 (T = Ir, Pt) using muon-spin relaxation and rotation

The superconducting properties of CaTSi3 (where T = Pt and Ir) have been investigated using muon spectroscopy. Our muon-spin relaxation results suggest that in both these superconductors time-reversal symmetry is preserved, while muon-spin rotation data show that the temperature dependence of the superfluid density is consistent with an isotropic s-wave gap. The magnetic penetration depths and upper critical fields determined from our transverse-field muon-spin rotation spectra are found to be 448(6) and 170(6) nm, and 3800(500) and 1700(300) G, for CaPtSi3 and CaIrSi3 respectively. The superconducting coherence lengths of the two materials have also been determined and are 29(2) nm for CaPtSi3 and 44(4) nm for CaIrSi3.

cond-mat.supr-con↗

arXiv2014

Mapping the first order magnetic transition in Yb$_2$Ti$_2$O$_7$

The very nature of the ground state of the pyrochlore compound Yb$_2$Ti$_2$O$_7$ is much debated, as experimental results demonstrate evidence for both a disordered or a long-range ordered ground state. Indeed, the delicate balance of exchange interactions and anisotropy is believed to lead to competing states, such as a Quantum Spin Liquid state or a ferromagnetic state which may originate from an Anderson-Higgs transition. We present a detailed magnetization study demonstrating a first order ferromagnetic transition at 245 mK and 150 mK in a powder and a single crystal sample respectively. Its first-order character is preserved up to applied fields of $\sim$ 200 Oe. The transition stabilizes a ferromagnetic component and involves slow dynamics in the magnetization. Residual fluctuations are also evidenced, the presence of which might explain some of the discrepancies between previously published data for Yb$_2$Ti$_2$O$_7$.

cond-mat.str-el↗

arXiv2014

Structural and Magnetic Properties of Single Crystals of the Geometrically Frustrated Zirconium Pyrochlore, Pr2Zr2O7

We report the growth of large high quality crystals of the frustrated magnet, Pr2Zr2O7 by the floating zone technique. Several crystals have been produced starting with varying levels of excess of Pr6O11 to compensate for the loss of Pr2O3 during the growth process. X-ray diffraction analysis of the crystals shows that the Pr composition in our crystals, as determined by the Pr site occupancy in the pyrochlore structure, is always slightly less than 1. Fits to the magnetic sus-ceptibility data below 10 K using a Curie-Weiss law reveal an antiferromagnetic coupling between the Pr moments, with an estimated Weiss temperature of ΘW = -2.5 K and an effective moment for the Pr3+ of μeff = 2.5 μB. Magnetization versus field measurements show a spin-ice like anisotropy.

cond-mat.mtrl-sci↗

arXiv2014

Low-temperature magnetic fluctuations in the Kondo insulator SmB6

We present the results of a systematic investigation of the magnetic properties of the three-dimensional Kondo topological insulator SmB6 using magnetization and muon-spin relaxation/rotation (muSR) measurements. The muSR measurements exhibit magnetic field fluctuations in SmB6 below 15 K due to electronic moments present in the system. However, no evidence for magnetic ordering is found down to 19 mK. The observed magnetism in SmB6 is homogeneous in nature throughout the full volume of the sample. Bulk magnetization measurements on the same sample show consistent behavior. The agreement between muSR, magnetization, and NMR results strongly indicate the appearance of intrinsic bulk magnetic in-gap states associated with fluctuating magnetic fields in SmB6 at low temperature.

cond-mat.str-el↗

arXiv2014

Synthesis and characterisation of nanomaterials of the topological crystalline insulator SnTe

A new class of materials, Topological Crystalline Insulators (TCIs) have been shown to possess exotic surface state properties that are protected by mirror symmetry. These surface features can be enhanced if the surface-area-to-volume ratio of the material increases, or the signal arising from the bulk of the material can be suppressed. We report the experimental procedures to obtain high quality crystal boules of the TCI, SnTe, from which nanowires and microcrystals can be produced by the vapour-liquid-solid (VLS) technique. Detailed characterisation measurements of the bulk crystals as well as of the nanowires and microcrystals produced are presented. The nanomaterials produced were found to be stoichiometrically similar to the source material used. Electron back-scatter diffraction (EBSD) shows that the majority of the nanocrystals grow in the vicinal {001} direction to the growth normal. The growth conditions to produce the different nanostructures of SnTe have been optimised.

cond-mat.mtrl-sci↗

arXiv2014

Superconducting and magnetic properties of Sr3Ir4Sn13

Magnetization and muon spin relaxation or rotation (muSR) measurements have been performed to study the superconducting and magnetic properties of Sr3Ir4Sn13. From magnetization measurements the lower and upper critical fields of Sr3Ir4Sn13 are found to be 81(1) Oe and 14.4(2) kOe, respectively. Zero-field muSR data show no sign of any magnetic ordering or weak magnetism in Sr3Ir4Sn13. Transverse-field muSR measurements in the vortex state provided the temperature dependence of the magnetic penetration depth. The dependence of penetration depth with temperature is consistent with the existence of single s-wave energy gap in the superconducting state of Sr3Ir4Sn13 with a gap value of 0.82(2) meV at absolute zero temperature. The magnetic penetration depth at zero temperature is 291(3) nm. The gap to Tc ratio is 2.1(1), indicates that Sr3Ir4Sn13 should be considered as a strong-coupling superconductor.

cond-mat.supr-con↗

arXiv2014

Observation of Time-Reversal Symmetry Breaking in the Non-Centrosymmetric Superconductor Re6Zr

We have investigated the superconducting state of the non-centrosymmetric compound Re6Zr using magnetization, heat capacity, and muon-spin relaxation/rotation (muSR) measurements. Re6Zr has a superconducting transition temperature, Tc = 6.75 K. Transverse-field muSR experiments, used to probe the superfluid density, suggest an s-wave character for the superconducting gap. However, zero and longitudinal-field muSR data reveal the presence of spontaneous static magnetic fields below Tc indicating that time-reversal symmetry is broken in the superconducting state and an unconventional pairing mechanism. An analysis of the pairing symmetries identifies the ground states compatible with time-reversal symmetry breaking.

cond-mat.supr-con↗

arXiv2014

Investigations of the superconducting states of noncentrosymmetric LaPdSi3 and LaPtSi3

The noncentrosymmetric superconductors LaPdSi3 and LaPtSi3 have been studied with magnetization, specific heat, resistivity and muon spin rotation/relaxation (muSR) measurements. These crystallize in the tetragonal BaNiSn3 structure and superconductivity is observed at Tc = 2.65 K for LaPdSi3 and Tc = 1.52(6) K for LaPtSi3. The results are consistent with both compounds being weakly coupled, fully gapped superconductors but muSR measurements reveal that LaPdSi3 is a bulk type-I superconductor while LaPtSi3 is a type-II material with a Ginzburg-Landau parameter of kappa = 2.49(4). This is further supported by specific heat measurements, where the transition in an applied field is first-order in LaPdSi3 but second-order in LaPtSi3. The electronic specific heat in the superconducting state was analyzed using an isotropic s-wave model that gave Delta/kTc = 1.757(4) for LaPdSi3 and 1.735(5) for LaPtSi3. The temperature dependence of the effective penetration depth of LaPtSi3 was extracted from muSR measurements and was fitted giving Delta/kTc = 1.60(8) and lambda(0) = 239(3) nm. A critical field of Bc(0) = 182.7 G was obtained for LaPdSi3 from muSR measurements, which is in good agreement with the calculated thermodynamic critical field.

cond-mat.supr-con↗

arXiv2013

Large, high quality single-crystals of the new Topological Kondo Insulator, SmB6

SmB6 has recently been predicted to be a Topological Kondo Insulator, the first strongly correlated heavy fermion material to exhibit topological surface states. High quality crystals are necessary to investigate the topological properties of this material. Single crystal growth of the rare earth hexaboride, SmB6, has been carried out by the floating zone technique using a high power xenon arc lamp image furnace. Large, high quality single-crystals are obtained by this technique. The crystals produced by the floating zone technique are free of contamination from flux materials and have been characterised by resistivity and magnetisation measurements. These crystals are ideally suited for the investigation of both the surface and bulk properties of SmB6.

cond-mat.str-el↗