arXiv subjects

T. Lancaster

Publications and source records attributed to T. Lancaster.

At least 109 records · Page 6Linked to original sources

arXiv2008

Enhancement of Superfluid Stiffness, Suppression of Superconducting T_c and Field-induced Magnetism in the Pnictide Superconductor LiFeAs

Transverse-field muon-spin rotation measurements performed on two samples of LiFeAs demonstrate that the superfluid stiffness of the superconducting condensate in relation to its superconducting transition temperature is enhanced compared to other pnictide superconductors. Evidence is seen for a field-induced magnetic state in a sample with a significantly suppressed superconducting transition temperature. The results in this system highlight the role of direct Fe-Fe interactions in frustrating pairing mediated by antiferromagnetic fluctuations and suggest that, in common with other pnictide superconductors, the system is close to a magnetic instability.

cond-mat.supr-con↗

arXiv2008

Determination of characteristic muon precession and relaxation signals in FeAs and FeAs2, possible impurity phases in pnictide superconductors

We report muon-spin relaxation measurements of highly homogeneous samples of FeAs and FeAs2, both previously found as impurity phases in some samples of recently synthesized pnictide superconductors. We observe well defined muon precession in the FeAs sample with two precession frequencies of 38.2(3) and 22.7(9) MHz at 7.5 K, with the majority of the amplitude corresponding to the lower frequency component. In FeAs2 we confirm previous measurements showing that no long-ranged magnetic order occurs above 2 K and measure the muon spin relaxation rate, which increases on cooling. Our results exclude the possibility that previous muon-spin relaxation measurements of pnictide superconductors have been measuring the effect of these possible impurities.

cond-mat.str-el↗

arXiv2008

Effect of magnesium doping on the orbital and magnetic order in LiNiO2

In LiNiO2, the Ni3+ ions, with S=1/2 and twofold orbital degeneracy, are arranged on a trian- gular lattice. Using muon spin relaxation (MuSR) and electron spin resonance (ESR), we show that magnesium doping does not stabilize any magnetic or orbital order, despite the absence of interplane Ni2+. A disordered, slowly fluctuating state develops below 12 K. In addition, we find that magnons are excited on the time scale of the ESR experiment. At the same time, a g factor anisotropy is observed, in agreement with $| 3z^{2}-r^{2}>$ orbital occupancy.

cond-mat.str-el↗

arXiv2008

Coexistence of magnetic fluctuations and superconductivity in the pnictide high temperature superconductor SmFeAsO$_{1-x}$F$_{x}$ measured by muon spin rotation

Muon-spin rotation experiments were performed on the pnictide high temperature superconductor SmFeAsO$_{1-x}$F$_{x}$ with x=0.18 and 0.3. We observed an unusual enhancement of slow spin fluctuations in the vicinity of the superconducting which suggests that the spin fluctuations contribute to the formation of an unconventional superconducting state. An estimate of the in-plane penetration depth $λ_{ab}(0)=190(5)$\thinspace nm was obtained, which confirms that the pnictide superconductors obey an Uemura-style relationship between $T_{\mathrm{c}}$ and $λ_{ab}(0)^{-2}$.

cond-mat.supr-con↗

arXiv2008

Isotope effect in quasi-two-dimensional metal-organic antiferromagnets

Although the isotope effect in superconducting materials is well-documented, changes in the magnetic properties of antiferromagnets due to isotopic substitution are seldom discussed and remain poorly understood. This is perhaps surprising given the possible link between the quasi-two-dimensional (Q2D) antiferromagnetic and superconducting phases of the layered cuprates. Here we report the experimental observation of shifts in the Néel temperature and critical magnetic fields ($ΔT_{\rm N}/T_{\rm N}\approx 4%$; $ΔB_{\rm c}/B_{\rm c}\approx 4%$) in a Q2D organic molecular antiferromagnets on substitution of hydrogen for deuterium. These compounds are characterized by strong hydrogen bonds through which the dominant superexchange is mediated. We evaluate how the in-plane and inter-plane exchange energies evolve as the hydrogens on different ligands are substituted, and suggest a possible mechanism for this effect in terms of the relative exchange efficiency of hydrogen and deuterium bonds.

cond-mat.str-el↗

arXiv2008

Tuning the interlayer spacing in high T_c superconductors: penetration depth and two-dimensional superfluid density

Substantial control of the interlayer spacing in Bi-based high temperature superconductors has been achieved through the intercalation of guest molecules between the superconducting layers. Measurements using implanted muons reveal that the penetration depth increases with increasing layer separation while T_c does not vary appreciably, demonstrating that the bulk superfluid density is not the determining factor controlling T_c. Our results strongly suggest that the superfluid density appearing in the Uemura scaling relation n_s/m* \propto T_c should be interpreted as the two dimensional density within the superconducting layers, which we find to be constant for each class of system investigated.

cond-mat.supr-con↗

arXiv2008

Muon spin relaxation study of LaTiO3 and YTiO3

We report muon spin relaxation measurements on two Ti3+ containing perovskites, LaTiO3 and YTiO3, which display long range magnetic order at low temperature. For both materials, oscillations in the time-dependence of the muon polarization are observed which are consistent with three-dimensional magnetic order. From our data we identify two magnetically inequivalent muon stopping sites. The muon spin relaxation results are compared with the magnetic structures of these compounds previously derived from neutron diffraction and muon spin relaxation studies on structurally similar compounds.

cond-mat.str-el↗

arXiv2008

Exchange parameters of copper-based quasi-two-dimensional Heisenberg magnets measured using high magnetic fields and muon-spin rotation

Pulsed-field magnetization experiments (fields $B$ of up to 85 T and temperatures $T$ down to 0.4 K) are reported on nine organic Cu-based two-dimensional (2D) Heisenberg magnets. All compounds show a low-$T$ magnetization that is concave as a function of $B$, with a sharp ``elbow'' transition to a constant value at a field $B_{\rm c}$. Monte-Carlo simulations including a finite interlayer exchange energy $J_{\perp}$ quantitatively reproduce the data; the concavity indicates the effective dimensionality and $B_{\rm c}$ is an accurate measure of the in-plane exchange energy $J$. Using these values and Neél temperatures measured by muon-spin rotation, it is also possible to obtain a quantitative estimate of $|J_{\perp}/J|$. In the light of these results, it is suggested that in magnets of the form [Cu(HF$_2$)(pyz)$_2$]X, where X is an anion, the sizes of $J$ and $J_{\perp}$ are controlled by the tilting of the pyrazine (pyz) molecule with respect to the 2D planes.

cond-mat.str-el↗

arXiv2007

Anomalous temperature evolution of the internal magnetic field distribution in the charge-ordered triangular antiferromagnet AgNiO2

Zero-field muon-spin relaxation measurements of the frustrated triangular quantum magnet AgNiO2 are consistent with a model of charge disproportionation that has been advanced to explain the structural and magnetic properties of this compound. Below an ordering temperature of T_N=19.9(2) K we observe six distinct muon precession frequencies, due to the magnetic order, which can be accounted for with a model describing the probable muon sites. The precession frequencies show an unusual temperature evolution which is suggestive of the separate evolution of two opposing magnetic sublattices.

cond-mat.str-el↗

arXiv2007

Muon-fluorine entangled states in molecular magnets

The information accessible from a muon-spin relaxation experiment is often limited since we lack knowledge of the precise muon stopping site. We demonstrate here the possibility of localizing a spin polarized muon in a known stopping state in a molecular material containing fluorine. The muon-spin precession that results from the entangled nature of the muon-spin and surrounding nuclear spins is sensitive to the nature of the stopping site and we use this property to identify three classes of site. We are also able to describe the extent to which the muon distorts its surroundings.

cond-mat.str-el↗

arXiv2007

A muon-spin relaxation study of BiMnO3

We present the results of muon-spin relaxation measurements on ferromagnetic BiMnO3. Below T_C=98.0(1) K oscillations in the time-dependence of the muon polarization are observed, characteristic of a quasistatic magnetic field at a single muon site, allowing us to probe the critical behaviour associated with the magnetic phase transition. We are able to suggest candidate muon sites on the basis of dipole field calculations. Close to T_C, fluctuations of the Mn^3+ moments are characteristic of critical behaviour while there is a sharp crossover to a region of fast dynamic fluctuations at higher temperatures.

cond-mat.str-el↗

arXiv2007

Magnetism in Geometrically Frustrated YMnO3 Under Hydrostatic Pressure Studied with Implanted Muons

The rare-earth ferroelectromagnet YMnO3 consists of weakly coupled triangular layers of S=2 spins. Below T_N ~ 70 K muon-spin relaxation data show two oscillatory relaxing signals due to magnetic order, with no relaxing signal resolvable (which would be expected for different coexisting spin distributions). The transition temperature T_N increases with applied hydrostatic pressure, even though the ordered moment decreases. These results suggest that pressure increases both the exchange coupling between the layers and the frustration within the layers.

cond-mat.str-el↗

arXiv2007

Magnetism in the high-Tc analogue Cs2AgF4 studied with muon-spin relaxation

We present the results of a muon-spin relaxation study of the high-Tc analogue material Cs2AgF4. We find unambiguous evidence for magnetic order, intrinsic to the material, below T_C=13.95(3) K. The ratio of inter- to intraplane coupling is estimated to be |J'/J|=1.9 x 10^-2, while fits of the temperature dependence of the order parameter reveal a critical exponent beta=0.292(3), implying an intermediate character between pure two- and three- dimensional magnetism in the critical regime. Above T_C we observe a signal characteristic of dipolar interactions due to linear F-mu-F bonds, allowing the muon stopping sites in this compound to be characterized.

cond-mat.str-el↗

arXiv2006

Magnetic order in the S=1/2 two-dimensional molecular antiferromagnet, copper pyrazine perchlorate Cu(Pz)_2(ClO_4)_2

We present an investigation of magnetic ordering in the two-dimensional S=1/2 quantum magnet Cu(Pz)_2(ClO_4)_2 using specific heat and zero field muon-spin relaxation (μ^+SR). The magnetic contribution to the specific heat is consistent with an exchange strength of 17.7(3) K. We find unambiguous evidence for a transition to a state of three-dimensional long range order below a critical temperature T_N=4.21(1) K using μ^+SR even though there is no feature in the specific heat at that temperature. The absence of a specific heat anomaly at T_N is consistent with recent theoretical predictions. The ratio of T_N/J=0.24 corresponds to a ratio of intralayer to interlayer exchange constants of |J'/J|=6.8x10^-4, indicative of excellent two-dimensional isolation. The scaled magnetic specific heat of [Cu(Pz)_2(HF_2)]BF_4, a compound with an analogous structure, is essentially identical to that of Cu(Pz)_2(ClO_4)_2 although both differ slightly from the predicted value for an ideal 2D S=1/2 Heisenberg antiferromagnet.

cond-mat.str-el↗

arXiv2006

The kagome staircase compounds Ni$_{3}$V$_{2}$O$_{8}$ and Co$_{3}$V$_{2}$O$_{8}$ studied with implanted muons

We present the results of muon-spin relaxation ($μ^{+}$SR) measurements on the kagome staircase compounds Ni$_{3}$V$_{2}$O$_{8}$ and Co$_{3}$V$_{2}$O$_{8}$. The magnetic behavior of these materials may be described in terms of two inequivalent magnetic ion sites, known as spine sites and cross-tie sites. Our $μ^{+}$SR results allow us to probe each of these sites individually, revealing the distribution of the local magnetic fields near these positions. We are able not only to confirm the magnetic structures of the various phases proposed on the basis of bulk measurements but also to give an insight into the temperature evolution of the local field distribution in each phase.

cond-mat.str-el↗

arXiv2006

Muon-spin relaxation measurements on the dimerized spin-1/2 chains NaTiSi2O6 and TiOCl

We report muon spin relaxation (muSR) and magnetic susceptibility investigations of two Ti3+ chain compounds which each exhibit a spin gap at low temperature, NaTiSi2O6 and TiOCl. From these we conclude that the spin gap in NaTiSi2O6 is temperature independent, with a value of 2*Delta=660(50)K, arising from orbital ordering at Too = 210K; the associated structural fluctuations activate the muon spin relaxation rate up to temperatures above 270K. In TiOCl we find thermally activated spin fluctuations corresponding to a spin gap 2*Delta=420(40)K below Tc1=67K. We also compare the methods used to extract the spin gap and the concentration of free spins within the samples from muSR and magnetic susceptibility data.

cond-mat.str-el↗

arXiv2006

Magnetism in the S=1 frustrated antiferromagnet GeNi2O4 studied using implanted muons

We present the results of a muon-spin relaxation study of GeNi2O4. We provide further clarification of the two transitions to the antiferromagnetic state and measure the magnetic field dependence of the heat capacity up to 14 T. Both oscillatory and relaxing signals are observed below the lower transition (at temperature $T_{N2}) in the muon-decay positron asymmetry spectra, arising from two distinct types of magnetic environment. A possible explanation is suggested in terms of the separate ordering of two magnetic subsystems, one of which does not order fully down to the lowest measured temperature.

cond-mat.str-el↗

arXiv2005

Magnetic order in the quasi-one-dimensional spin 1/2 chain, copper pyrazine dinitrate

We present the first evidence of magnetic order in the quasi-one-dimensional spin 1/2 molecular chain compound, copper pyrazine dinitrate Cu(C4H4N2)(NO3)2}. Zero field muon-spin relaxation measurements made at dilution refrigerator temperatures show oscillations in the measured asymmetry, characteristic of a quasistatic magnetic field at the muon sites. Our measurements provide convincing evidence for long range magnetic order below a temperature T_N=107(1) mK. This leads to an estimate of the interchain coupling constant of |J'|/k_B=0.046 K and to a ratio |J'/J| = 4.4 x 10^-3.

cond-mat.str-el↗