arXiv subjects

P. Millet

Publications and source records attributed to P. Millet.

At least 19 recordsLinked to original sources

arXiv2005

Ground state of the Kagome-like S=1/2 antiferromagnet, Volborthite Cu3V2O7(OH)2.2H2O

Volborthite compound is one of the very few realizations of S=1/2 quantum spins on a highly frustrated kagome-like lattice. Low-T SQUID measurements reveal a broad magnetic transition below 2K which is further confirmed by a peak in the 51V nuclear spin relaxation rate (1/T1) at 1.4K$\pm$0.2K. Through 51V NMR, the ground state (GS) appears to be a mixture of different spin configurations, among which 20% correspond to a well defined short range order, possibly of the $\sqrt{3} \times \sqrt{3}$ type. While the freezing involve all the Cu$^{2+}$ spins, only 40% of the copper moment is actually frozen which suggests that quantum fluctuations strongly renormalize the GS.

cond-mat.str-el↗

arXiv2005

Spin Dilution in Frustrated Two-Dimensional s=1/2 Antiferromagnets on a Square Lattice

$^7$Li and $^{29}$Si NMR, $μ$SR and magnetization measurements in Li$_2$V$_{1-x}$OTi$_x$SiO$_4$, for $0 \leq x \leq 0.2$, are presented. The $x=0$ compound is a prototype of frustrated two-dimensional Heisenberg antiferromagnet on a square-lattice with competing nearest ($J_1$) and next-nearest ($J_2$) neighbour exchange interactions. Ti$^{4+}$ (S=0) for V$^{4+}$ (S=1/2) substitution yields the spin dilution of the antiferromagnetic layers. The analysis of the magnetization and of the nuclear spin-lattice relaxation rate shows that spin dilution not only reduces the spin-stiffness by a factor $\simeq (1-x)^2$, but also causes the decrease of the effective ratio $J_2(x)/J_1(x)$. Moreover, the sublattice magnetization curves derived from zero-field $μ$SR measurements in the collinear phase point out that, at variance with non-frustrated two-dimensional Heisenberg antiferromagnets, spin dilution affects the low-temperature staggered magnetization only to a minor extent. This observation is supported also by the $x$ dependence of the collinear ordering temperature. The results obtained for the Ti doped samples are discussed in the light of the results previously obtained in the pure $x=0$ compound and in non-frustrated two-dimensional Heisenberg antiferromagnets with spin-dilution.

cond-mat.str-el↗

arXiv2005

Unusual magnetic properties of the low-dimensional quantum magnet Na2V3O7

We report the results of low-temperature measurements of the specific heat Cp(T), ac susceptibility chi(T) and 23Na nuclear magnetic resonance NMR of Na2V3O7. At liquid He temperatures Cp(T)/T exhibits broad field-dependent maxima, which shift to higher temperatures upon increasing the applied magnetic field H. Below 1.5 K the ac magnetic susceptibility chi(T) follows a Curie-Weiss law and exhibits a cusp at 0.086 mK which indicates a phase transition at very low temperatures. These results support the previous conjecture that Na2V3O7 is close to a quantum critical point (QCP) at mu_{0}H = 0 T. The entire data set, including results of measurements of the NMR spin-lattice relaxation 1/T1(T), reveals a complex magnetic behavior at low temperatures. We argue that it is due to a distribution of singlet-triplet energy gaps of dimerized V moments. The dimerization process evolves over a rather broad temperature range around and below 100 K. At the lowest temperatures the magnetic properties are dominated by the response of only a minor fraction of the V moments.

cond-mat.str-el↗

arXiv2004

Anisotropic field dependence of the magnetic transition in Cu2Te2O5Br2

We present the results of measurements of the thermal conductivity of Cu2Te2O5Br2, a compound where tetrahedra of Cu^{2+} ions carrying S=1/2 spins form chains along the c-axis of the tetragonal crystal structure. The thermal conductivity kappa was measured along both the c- and the a-direction as a function of temperature between 3 and 300 K and in external magnetic fields H up to 69 kOe, oriented both parallel and perpendicular to the c-axis. Distinct features of kappa(T) were observed in the vicinity of T_N=11.4 K in zero magnetic field. These features are unaltered in external fields which are parallel to the c-axis, but are more pronounced when a field is applied perpendicularly to the c-axis. The transition temperature increases upon enhancing the external field, but only if the field is oriented along the a-axis.

cond-mat.str-el↗

arXiv2003

Longitudinal magnon in the tetrahedral spin system Cu2Te2O5Br2 near quantum criticality

We present a comprehensive study of the coupled tetrahedra-compound Cu2Te2O5Br2 by theory and experiments in external magnetic fields. We report the observation of a longitudinal magnon in Raman scattering in the ordered state close to quantum criticality. We show that the excited tetrahedral-singlet sets the energy scale for the magnetic ordering temperature T_N. This energy is determined experimentally. The ordering temperature T_N has an inverse-log dependence on the coupling parameters near quantum criticality.

cond-mat.str-el↗

arXiv2003

Crystal Structure and Magnetic Properties of a new two-dimensional S=1 quantum spin system Ni_5(TeO_3)_4X_2 (X= Cl, Br)

Two new transition-metal tellurium oxy-chlorides of the general formula Ni_5(TeO_3)_4X_2 (X=Cl, Br) have been isolated during the investigation of the ternary phase diagram NiO-NiCl2-TeO2. They crystallize in the monoclinic system, space group C2/c, and for the case of Ni_5(TeO_3)_4Cl_2 the unit cell is a=19.5674(2) A, b=5.2457(1) A, c=16.3084(1) A, with beta=125.289(1). The structure is layered and built up of corner connected [Ni_5O_17X_2] entities, made of five nickel(II) octahedra associated by edge and face sharing. The tellurium(IV) atoms are fixed upon the nickel layers. Their lone pairs E and the halogen atoms are packed in a double layer perpendicular to the [010] direction. From the magnetic point of view, this system provides a new 2D S=1 quantum spin system with antiferromagnetic superexchange interaction. The magnetic susceptibility shows anomalies pointing to magnetic ordering phenomena. The observed transition temperatures vary with the interlayer separation.

cond-mat.str-el↗

arXiv2002

A low-dimensional spin S = 1/2 system at the quantum critical limit: Na2V2O7

We report the results of measurements of the dc-susceptibility and the 23Na-NMR response of Na2V2O7, a recently synthesized, non metallic low dimensional spin system. Our results indicate that upon reducing the temperature to below 100 K, the V^{4+} moments are gradually quenched, leaving only one moment out of 9 active. The NMR data reveal a phase transition at very low temperatures. With decreasing applied field H, the critical temperature shifts towards T = 0 K, suggesting that Na2V2O7 may be regarded as an insulator reaching a quantum critical point at H = 0.

cond-mat.str-el↗

arXiv2002

Frustration driven structural distortion in VOMoO4

Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), magnetization measurements and electronic structure calculations in VOMoO4 are presented. It is found that VOMoO4 is a frustrated two-dimensional antiferromagnet on a square lattice with competing exchange interactions along the side J1 and the diagonal J2 of the square. From magnetization measurements J1+J2 is estimated around 155 K, in satisfactory agreement with the values derived from electronic structure calculations. Around 100 K a structural distortion, possibly driven by the frustration, is evidenced. This distortion induces significant modifications in the NMR and EPR spectra which can be accounted for by valence fluctuations. The analysis of the spectra suggests that the size of the domains where the lattice is distorted progressively grows as the temperature approaches the transition to the magnetic ground state at Tc=42 K.

cond-mat.str-el↗

arXiv2001

High Magnetic Field NMR Studies of LiVGe$_2$O$_6$, a quasi 1-D Spin $S = 1$ System

We report $^{7}$Li pulsed NMR measurements in polycrystalline and single crystal samples of the quasi one-dimensional S=1 antiferromagnet LiVGe$_2$O$_6$, whose AF transition temperature is $T_{\text{N}}\simeq 24.5$ K. The field ($B_0$) and temperature ($T$) ranges covered were 9-44.5 T and 1.7-300 K respectively. The measurements included NMR spectra, the spin-lattice relaxation rate ($T_1^{-1}$), and the spin-phase relaxation rate ($T_2^{-1}$), often as a function of the orientation of the field relative to the crystal axes. The spectra indicate an AF magnetic structure consistent with that obtained from neutron diffraction measurements, but with the moments aligned parallel to the c-axis. The spectra also provide the $T$-dependence of the AF order parameter and show that the transition is either second order or weakly first order. Both the spectra and the $T_1^{-1}$ data show that $B_0$ has at most a small effect on the alignment of the AF moment. There is no spin-flop transition up to 44.5 T. These features indicate a very large magnetic anisotropy energy in LiVGe$_2$O$_6$ with orbital degrees of freedom playing an important role. Below 8 K, $T_1^{-1}$ varies substantially with the orientation of $B_0$ in the plane perpendicular to the c-axis, suggesting a small energy gap for magnetic fluctuations that is very anisotropic.

cond-mat↗

arXiv2001

Low Energy Singlets in the Excitation Spectrum of the Spin Tetrahedra System Cu_2Te_2O_5Br_2

Low energy Raman scattering of the s=1/2 spin tetrahedra system Cu_2Te_2O_5Br_2 is dominated by an excitation at 18 cm^{-1} corresponding to an energy E_S=0.6Δ, with Δthe spin gap of the compound. For elevated temperatures this mode shows a soft mode-like decrease in energy pointing to an instability of the system. The isostructural reference system Cu_2Te_2O_5Cl_2 with a presumably larger inter-tetrahedra coupling does not show such a low energy mode. Instead its excitation spectrum and thermodynamic properties are compatible with long range Neel-ordering. We discuss the observed effects in the context of quantum fluctuations and competing ground states.

cond-mat.str-el↗

arXiv2001

Very low-frequency excitations in frustrated two-dimensional $S=1/2$ Heisenberg antiferromagnets

muSR and 7Li NMR relaxation measurements in frustrated two-dimensional S=1/2 Heisenberg antiferromagnets on a square lattice are presented. It is found that both in Li2VOSiO4 and Li2VOGeO4, spin dynamics at frequencies well below the Heisenberg exchange frequency are present. These dynamics are associated with the motions of walls separating coexisting collinear domains with a magnetic wave vector rotated by 90 degrees.

cond-mat.str-el↗

arXiv2001

Evidence for an unconventional magnetic instability in the spin-tetrahedra system Cu_2Te_2O_5Br_2

Thermodynamic experiments as well as Raman scattering have been used to study the magnetic instabilities in the spin-tetrahedra systems Cu_2Te_2O_5X_2, X=Cl and Br. While the phase transition observed in the Cl system at T_o=18.2 K is consistent with 3D AF ordering, the phase transition at T_o=11.3 K in the Br system has several unusual features. We propose an explanation in terms of weakly coupled tetrahedra with a singlet-triplet gap and low lying singlets.

cond-mat.str-el↗

arXiv2001

Spin-Peierls transition with strong structural fluctuations in the vanadium oxide VOSb$_{2}$O$_{4}$

We report on the magnetic susceptibility and electron spin resonance measurements on polycrystalline samples of the vanadium oxide VOSb$_{2}$O$_{4}$, a quasi-one dimensional S=1/2 Heisenberg system. We show that the susceptibility vanishes at zero temperature, as in a gapped system, and we argue that this is due to a spin-Peierls transition with strong structural fluctuations.

cond-mat.str-el↗

arXiv2001

Magnetic and thermodynamic properties of Li(2)VOSiO(4): a two-dimensional S=1/2 frustrated antiferromagnet on a square lattice

NMR, muSR, magnetization and specific heat measurements in Li(2)VOSiO(4) powders and single crystals are reported. Specific heat and magnetization measurements evidence that Li(2)VOSiO(4) is a frustrated two-dimensional S=1/2 Heisenberg antiferromagnet on a square lattice with a superexchange coupling J_1, along the sides of the square, almost equal to J_2, the one along the diagonal ($J_2/J_1=1.1\pm 0.1$ with $J_2 + J_1 = 8.2\pm 1$ K). At $T_c\simeq 2.8$ K a phase transition to a low temperature collinear order is observed. T_c and the sublattice magnetization, derived from NMR and muSR, were found practically independent on the magnetic field intensity up to 9 Tesla. The critical exponent of the sublattice magnetization was estimated $β\simeq 0.235$, nearly coincident with the one predicted for a two-dimensional XY system on a finite size. The different magnetic properties found above and below T_c are associated with the modifications in the spin hamiltonian arising from a structural distortion occurring just above T_c.

cond-mat.str-el↗

arXiv2000

LiVGe2O6, an anomalous quasi 1D, $S = 1$ system, as revealed by NMR

We report the results of $^{7}$Li nuclear magnetic resonance (NMR) studies of LiVGe$_{2}$O$_{6}$, a quasi one--dimensional spin $S = 1$ model system, at low temperatures. Our data, including NMR spectra and the temperature dependence of the spin-lattice relaxation rate $T_{1}^{-1}$, indicate a first order phase transition to occur at $T_{c}\simeq 23$ K. The NMR response of LiVGe$_{2}$O$_{6}$ below $T_{c}$ suggests that the ordered phase is antiferromagnetic and has unusual features. Possible reasons for this unexpected behavior are discussed.

cond-mat.str-el↗

arXiv2000

Li$_2$VO(Si,Ge)O$_4$, a prototype of a two-dimensional frustrated quantum Heisenberg antiferromagnet

NMR and magnetization measurements in Li$_2$VOSiO$_4$ and Li$_2$VOGeO$_4$ are reported. The analysis of the susceptibility shows that both compounds are two-dimensional $S=1/2$ Heisenberg antiferromagnets on a square lattice with a sizeable frustration induced by the competition between the superexchange couplings $J_1$ along the sides of the square and $J_2$ along the diagonal. Li$_2$VOSiO$_4$ undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for $J_2/J_1 > 0.5$. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion.

cond-mat.str-el↗

arXiv1999

Biquadratic interactions and spin-Peierls transition in the spin 1 chain LiVGe2O6

The magnetic susceptibility of a new one-dimensional, S=1 system, the vanadium oxide LiVGe2O6, has been measured. Contrary to previous S=1 chains, it exhibits an abrupt drop at 22 K typical of a spin-Peierls transition, and it is consistent with a gapless spectrum above this temperature. We propose that this behaviour is due to the presence of a significant biquadratic exchange interaction, a suggestion supported by quantum chemistry calculations that take into account the quasi-degeneracy of the t2g levels.

cond-mat.str-el↗

arXiv1998

Infrared study of spin-Peierls compound alpha'-NaV2O5

Infrared reflectance of alpha'-NaV2O5 single crystals in the frequency range from 50 cm-1 to 10000 cm-1 was studied for a, b and c-polarisations. In addition to phonon modes identification, for the a-polarised spectrum a broad continuum absorption in the range of 1D magnetic excitation energies was found. The strong near-IR absorption band at 0.8 eV shows a strong anisotropy with vanishing intensity in c-polarisation. Activation of new phonons due to the lattice dimerisation were detected below 35K as well as pretransitional structural fluctuations up to 65K.

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