arXiv subjects

P. Mendels

Publications and source records attributed to P. Mendels.

At least 55 records · Page 3Linked to original sources

arXiv2008

Dzyaloshinsky-Moriya Anisotropy in the Spin-1/2 Kagomé Compound ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$

We report the determination of the Dzyaloshinsky-Moriya interaction, the dominant magnetic anisotropy term in the \kagome spin-1/2 compound {\herbert}. Based on the analysis of the high-temperature electron spin resonance (ESR) spectra, we find its main component $|D_z|=15(1)$ K to be perpendicular to the \kagome planes. Through the temperature dependent ESR line-width we observe a building up of nearest-neighbor spin-spin correlations below $\sim$150 K.

cond-mat.str-el↗

arXiv2008

Easy-Axis Kagomé Antiferromagnet: Local-Probe Study of Nd$_3$Ga$_5$SiO$_{14}$

We report a local-probe investigation of the magnetically anisotropic kagomé compound Nd$_3$Ga$_5$SiO$_{14}$. Our zero-field $μ$SR results provide a direct evidence of a fluctuating collective paramagnetic state down to 60 mK, supported by a wipe-out of the Ga nuclear magnetic resonance (NMR) signal below 25 K. At 60 mK a dynamics crossover to a much more static state is observed by $μ$SR in magnetic fields above 0.5 T. Accordingly, the NMR signal is recovered at low $T$ above a threshold field, revealing a rapid temperature and field variation of the magnetic fluctuations.

cond-mat.str-el↗

arXiv2008

17O NMR study of the intrinsic magnetic susceptibility and spin dynamics of the quantum kagome antiferromagnet ZnCu3(OH)6Cl2

We report through 17O NMR, an unambiguous local determination of the intrinsic kagome lattice spin susceptibility as well as that created around non-magnetic defects issued from natural Zn/ Cu exchange in the S=1/2 (Cu2+) herbertsmithite ZnCu3(OH)6Cl2 compound. The issue of a singlet-triplet gap is addressed. The magnetic response around a defect is found to markedly differ from that observed in non-frustrated antiferromagnetic materials. Finally, we discuss our relaxation measurements in the light of Cu and Cl NMR data [cond-mat 070314] and suggest a flat q-dependence of the excitations.

cond-mat.str-el↗

arXiv2007

Low temperature magnetization of the S=1/2 kagome antiferromagnet ZnCu_3(OH)_6Cl_2

The dc-magnetization of the unique S=1/2 kagome antiferromagnet Herbertsmithite has been measured down to 0.1K. No sign of spin freezing is observed in agreement with former muSR and ac-susceptibility results. The low temperature magnetic response is dominated by a defect contribution which exhibits a new energy scale $\simeq 1$ K, likely reflecting the coupling of the defects. The defect component is saturated at low temperature by H>8T applied magnetic fields which enables us to estimate an upper bound for the non saturated intrinsic kagome susceptibility at T=1.7K.

cond-mat.str-el↗

arXiv2007

Interplay between magnetic properties and thermoelectricity in misfit and Na cobaltates

We present a comparative study of CoO2 layers in the Bi-misfit and NaxCoO2 cobaltates. Co NMR measures the intrinsic susceptibility of the Co layers and is not affected by spurious contributions. At low dopings where room-temperature thermopower (TEP) is large, Curie-Weiss susceptibilities are observed in both materials. But NMR and muSR experiments find neither charge nor spin order down to low temperatures in Bi-misfit, in contrast to the case of NaxCoO2. This demonstrates that metallicity, charge and magnetic orders are specific of the Na layers in NaxCoO2 whereas strong correlations are generic of the cobaltates physics and could explain the large TEP.

cond-mat.str-el↗

arXiv2007

Quantum magnetism in the paratacamite family: towards an ideal kagome lattice

We report MuSR measurements on the S=1/2 (Cu2+) paratacamite Zn_xCu_{4-x}(OH)_6Cl_2 family. Despite a Weiss temperature of -300 K, the x=1 compound is found to have no transition to a magnetic frozen state down to 50 mK as theoretically expected for the kagome Heisenberg antiferromagnet. We find that the limit between a dynamical and a frozen inhomogeneous ground state occurs around x=0.5. For x=1, we discuss the relevance to a singlet picture.

cond-mat.str-el↗

arXiv2006

Unconventional Dynamics in Triangular Heisenberg Antiferromagnet NaCrO2

We report magnetization, specific heat, muon spin rotation and Na NMR measurements on the S=3/2 rhombohedrally stacked Heisenberg antiferromagnet NaCrO2. This compound appears to be an ideal candidate for the study of triangular Heisenberg antiferromagnets with very weak interlayer coupling. While specific heat and magnetization measurements indicate the occurrence of a transition in the range 40-46 K, both muon spin rotation and NMR reveal a fluctuating regime extending well below T_c, with a peak of relaxation rate 1/T1 around 30 K. This novel finding is discussed within the context of excitations in the triangular Heisenberg antiferromagnets.

cond-mat.str-el↗

arXiv2006

Direct evidence for a dynamical ground state in the highly frustrated Tb$_2$Sn$_2$O$_7$ pyrochlore

MuSR experiments have been performed on powder sample of the "ordered spin ice" Tb$_2$Sn$_2$O$_7$ pyrochlore compound. At base temperature (T=35mK) the muon relaxation is found to be of dynamical nature which demonstrates that strong fluctuations persist below the ferromagnetic transition (T_C=0.87K). Hints of long range order appear as oscillations of the muon polarization when an external field is applied and also as a hysteretic behavior below T_C. We propose a dynamical and strongly correlated scenario where dynamics results from fluctuation of large spin clusters with the "ordered spin ice" structure.

cond-mat.str-el↗

arXiv2005

Evidence of a single nonmagnetic Co3+ state in the Na1CoO2 cobaltate

Macroscopic magnetization, muon spin rotation (muSR), and NMR measurements were carried out to study magnetism in the Na1CoO2 cobaltate. Using SQUID measurements, Na1CoO2 is shown to have a bulk magnetic susceptibility much lower and flatter than that of NaxCoO2 with x=0.7-0.9. In fact, muSR yields a signal of mostly nonmagnetic origin, which is attributed to the x=1 phase. The intrinsic cobalt spin susceptibility corresponding to this x=1 phase is measured using Na NMR. It is indeed found to be almost zero, in agreement with a low-spin 3+ charge state of all cobalt atoms. This single state of Co ions in CoO2 planes is confirmed by Co NMR, whose determination of cobalt shift and quadrupolar parameters allows us to give a reference value of the Co3+ orbital shift in cobaltates.

cond-mat.str-el↗

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

Series of Bulk Magnetic Phase Transitions in NaxCoO2: a muSR study

Using muon spin rotation, well-defined bulk ~ 100% magnetic phases in NaxCoO2 are revealed. A novel magnetic phase is detected for x = 0.85 with the highest transition temperature ever observed for x >= 0.75. This stresses the diversity of x >=0.75 magnetic phases and the link between magnetic and structural degrees of freedom. For the charge-ordered x = 0.50 compound, a cascade of transitions is observed below 85 K. From a detailed analysis of our data, we conclude that the ordered moment varies continuously with temperature and suggest that the two secondary transitions at 48 K and 29 K correspond to a moderate reorientation of antiferromagnetically coupled moments.

cond-mat.str-el↗

arXiv2004

$μ$SR study of the quantum dynamics in the frustrated $S=3/2$ kagome bilayers

We present $μ$SR experiments in the $S=3/2$ kagome bilayer compound Ba$_{2}$Sn$_{2}$ZnGa$_{10-7p}$Cr$_{7p}$O$_{22}$ (BSZCGO$(p)$) and compare it to the isostructural SrCr$_{9p}$Ga$_{12-9p}$O$_{19}$ (SCGO$(p)$), including for the latter new results for $p\geq0.89$. Quantum-dynamical low energy magnetic excitations are evidenced in this novel compound. We study the evolution of the muon relaxation rate with $p$, $T$ and field. A phenomenological model for the muon relaxation based on sporadic dynamics due to spin excitations in a singlet sea proposed by Uemura \emph{et al.} [Phys. Rev. Lett. \textbf{73}, 3306 (1994)]. is extended to all fields and $T$-range. Its connexion to the RVB picture is discussed, and we argue that such coherent states might mediate the interactions between "impurities" which induce the spin glass freezing.

cond-mat.str-el↗

arXiv2004

Antiferromagnetic properties of a water vapor-inserted $YBa_2Cu_3O_{6.5}$ compound studied by NMR, NQR and $μ$ SR

We present a detailed NQR, NMR and $μ$SR study of a magnetic phase obtained during a topotactic chemical reaction of YBa$_{2}$Cu$_{3}$O$_{6.5}$ high- temperature superconductor with low-pressure water vapor. Our studies give straightforward evidence that the ''empty'' Cu(1) chains play the role of an easy water insertion channel. It is shown that the NQR spectrum of the starting material transforms progressively under insertion of water, and completely disappears when one H$_{2}$O molecule is inserted per unit cell. Similarly, a Cu ZFNMR signal characteristic of this water inserted material appears and grows with increasing water content, which indicates that the products of the reaction are non-superconducting antiferromagnetic phases in which the bilayers are ordered. These antiferromagnetic phases are felt by proton NMR which reveals two sites with static internal fields of 150 and about 15 Gauss respectively. Two muon sites are also evidenced with similar local fields which vanish at $T\approx 400$ K. This indicates that the magnetic phases have similar Néel temperatures as the other bilayer undoped compounds. An analysis of the internal fields on different sites of the structure suggests that they can be all assigned to a single magnetic phase at large water content in which the Cu(1) electron spins order with those of the Cu(2). It appears that even samples packed in Stycast epoxy resin heated moderately at a temperature (200$^{0}$C) undergo a reaction with epoxy decomposition products which yield the formation of the same final compound. It is then quite clear that such effects should be considered quite seriously and avoided in experiments attempting to resolve tiny effects in such materials, as those performed in some recent neutron scattering experiments.

cond-mat.supr-con↗

arXiv2002

Absence of static phase separation in the high-Tc cuprate $YBa_{2}Cu_{3}O_{6+y}$

We use 89Y NMR in $YBa_{2}Cu_{3}O_{6+y}$ in order to evaluate with high sensitivity the distribution of hole content p in the CuO2 planes. For y=1 and y=0.6, this hole doping distribution is found narrow with a full width at half maximum smaller than Delta p=0.025. This rules out any large static phase separation between underdoped and optimally doped regions in contrast with the one observed by STM in Bi2212 and by NQR in LaSrCuO. This establishes that static electronic phase separation is not a generic feature of the cuprates.

cond-mat.supr-con↗

arXiv2002

Planar 17O NMR study of Pr_yY_{1-y}Ba_2Cu_3O_{6+x}

We report the planar ^{17}O NMR shift in Pr substituted YBa_{2}Cu_{3}O_{6+x}, which at x=1 exhibits a characteristic pseudogap temperature dependence, confirming that Pr reduces the concentration of mobile holes in the CuO_{2} planes. Our estimate of the rate of this counterdoping effect, obtained by comparison with the shift in pure samples with reduced oxygen content, is found insufficient to explain the observed reduction of T_c. From the temperature dependent magnetic broadening of the ^{17}O NMR we conclude that the Pr moment and the local magnetic defect induced in the CuO_2 planes produce a long range spin polarization in the planes, which is likely associated with the extra reduction of T_c. We find a qualitatively different behaviour in the oxygen depleted Pr_yY_{1-y}Ba_2Cu_3O_{6.6}, i.e. the suppression of T$_c$ is nearly the same, but the magnetic broadening of the ^{17}O NMR appears weaker. This difference may signal a weaker coupling of the Pr to the planes in the underdoped compound, which might be linked with the larger Pr to CuO_2 plane distance, and correspondingly weaker hybridization.

cond-mat.supr-con↗

arXiv2002

Susceptibility and dilution effects of the kagome bi-layer geometrically frustrated network. A Ga-NMR study of SrCr_(9p)Ga_(12-9p)O_(19)

We present an extensive gallium NMR study of the geometrically frustrated kagome bi-layer compound SrCr_(9p)Ga_(12-9p)O_(19) (Cr^3+, S=3/2) over a broad Cr-concentration range (.72<p<.95). This allows us to probe locally the kagome bi-layer susceptibility and separate the intrinsic properties due to the geometric frustration from those related to the site dilution. Our major findings are: 1) The intrinsic kagome bi-layer susceptibility exhibits a maximum in temperature at 40-50 K and is robust to a dilution as high as ~20%. The maximum reveals the development of short range antiferromagnetic correlations; 2) At low-T, a highly dynamical state induces a strong wipe-out of the NMR intensity, regardless of dilution; 3) The low-T upturn observed in the macroscopic susceptibility is associated to paramagnetic defects which stem from the dilution of the kagome bi-layer. The low-T analysis of the NMR lineshape suggests that the defect can be associated with a staggered spin-response to the vacancies on the kagome bi-layer. This, altogether with the maximum in the kagome bi-layer susceptibility, is very similar to what is observed in most low-dimensional antiferromagnetic correlated systems; 4) The spin glass-like freezing observed at T_g=2-4 K is not driven by the dilution-induced defects.

cond-mat.str-el↗

arXiv2001

Persistence of Li Induced Kondo Moments in the Superconducting State of Cuprates

We measure the magnetic susceptibility nearby Li spinless impurities in the superconducting phase of the high Tc cuprate YBaCuO. The induced moment which was found to exist above Tc persists below Tc. In the underdoped regime, it retains its Curie law below Tc. In contrast, near optimal doping, the large Kondo screening observed above Tc (T_K=135 K) is strongly reduced below Tc as expected theoretically when the superconducting gap develops. This moment still extends essentially on its 4 near neighbour Cu, showing the persistence of AF correlations in the superconducting state. A direct comparison with recent STM results of Pan et al. is proposed.

cond-mat.supr-con↗

arXiv2000

Reply to the comment of Tallon et al. Cond-Mat/0008295 about Phys. Rev. Lett. 83, 4381 (1999)

It is shown both from a qualitative and quantitative analysis that the T-dependence of the pure susceptibility detected by Tallon et al. in overdoped compounds cannot explain the T-dependence of the Li NMR shift. This confirms that the electronic correlations are still present in the overdoped regime as pointed out in Phys. Rev. Lett. 83, 4381 (1999).

cond-mat.supr-con↗