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J. B. Betts

Publications and source records attributed to J. B. Betts.

30 records · Page 2Linked to original sources
arXiv2007

Using magnetostriction to measure the spin-spin correlation function and magnetoelastic coupling in the quantum magnet NiCl$_2$-4SC(NH$_2$)$_2$

We report a method for determining the spatial dependence of the magnetic exchange coupling, $dJ/dr$, from magnetostriction measurements of a quantum magnet. The organic Ni $S = 1$ system NiCl$_2$-4SC(NH$_2$)$_2$ exhibits lattice distortions in response to field-induced canted antiferromagnetism between $H_{c1} = 2.1$ T and $H_{c2} = 12.6$ T. We are able to model the magnetostriction in terms of uniaxial stress on the sample created by magnetic interactions between neighboring Ni atoms along the c-axis. The uniaxial strain is equal to $(1/E)dJ_c/dx_c < S_{\bf r} \cdot S_{{\bf r}+ {\bf e}_c} >$, where $E$, $J_c$, $x_c$ and ${\bf e}_c$ are the Young's modulus, the nearest neighbor (NN) exchange coupling, the variable lattice parameter, and the relative vector between NN sites along the c-axis. We present magnetostriction data taken at 25 mK together with Quantum Monte Carlo calculations of the NN spin-spin correlation function that are in excellent agreement with each other. We have also measured Young's modulus using resonant ultrasound, and we can thus extract $dJ_c/dx_c = 2.5$ K/$Å$, yielding a total change in $J_c$ between $H_{c1}$ and $H_{c2}$ of 5.5 mK or 0.25% in response to an 0.022% change in length of the sample.

cond-mat.str-el↗
arXiv2005

Ferromagnetism and possible heavy fermion behavior in single crystals of NdOs$_4$Sb$_{12}$

Single crystals of the filled-skutterudite compound NdOs$_4$Sb$_{12}$ have been investigated by means of electrical resistivity, magnetization, and specific heat measurements. The NdOs$_4$Sb$_{12}$ crystals have the LaFe$_4$P$_{12}$-type cubic structure with a lattice parameter of 9.3 Å. Possible heavy-fermion behavior is inferred from specific heat measurements, which reveal a large electronic specific heat coefficient $γ\approx 520$ mJ/mol-K$^2$, corresponding to an effective mass $m^* \sim$ 98 $m_e$. Features related to a ferromagnetic transition at {$\sim$ 0.9 K} can be observed in electrical resistivity, magnetization and specific heat. Conventional Arrott-plot analysis indicates that NdOs$_4$Sb$_{12}$ conforms to mean-field ferromagnetism.

cond-mat.str-el↗
arXiv2004

Heavy Fermion Behavior, Crystalline Electric Field Effects, and Weak Ferromagnetism in SmOs_{4}Sb_{12}

The filled skutterudite compound SmOs_{4}Sb_{12} was prepared in single crystal form and characterized. The SmOs_{4}Sb_{12} crystals have the LaFe_{4}P_{12}-type structure with lattice parameter a = 9.3085 Angstroms. Specific heat measurements indicate a large electronic specific heat coefficient of ~880 mJ/mol K^{2}, from which an enhanced effective mass m^{*} ~ 170 m_{e} is estimated. The specific heat data also suggest crystalline electric field (CEF) splitting of the Sm^{3+} J = 5/2 multiplet into a Gamma_{7} doublet ground state and a Gamma_{8} quartet excited state separated by 37 K. Electrical resistivity rho(T) measurements reveal a decrease in rho(T) below ~50 K that is consistent with CEF splitting of ~33 K between a Gamma_(7) doublet ground state and Gamma_{8} quartet excited state. Specific heat and magnetic susceptibility measurements display a possible weak ferromagnetic transition at ~2.6 K, which could be an intrinsic property of SmOs_4Sb_{12} or possibly due to an unknown impurity phase.

cond-mat.str-el↗
arXiv2004

High magnetic field thermal-expansion and elastic properties of CeRhIn$_5$

We report high magnetic field thermal-expansion and magnetostriction results on CeRhIn$_5$ single crystals. Several transitions, both first and second order, are observed when the field is applied perpendicular to the crystallographic c-axis. The magnetic field dependence of the thermal-expansion coefficient above 15 K, where the magnetic correlations are negligible, can be explained supposing an almost pure $| \pm 5/2>$ ground state doublet, in apparent contradiction with neutron scattering experiments. Although the spin-lattice interaction is relevant in this compound, the effect of the magnetic correlations on the elastic properties is relatively weak, as revealed by resonant ultrasound spectroscopy experiments.

cond-mat.str-el↗
arXiv2004

Spin reorientation and in-plane magnetoresistance of lightly doped La_{2-x}Sr_{x}CuO_{4} in magnetic fields up to 55 T

The magnetoresistance (MR) in the in-plane resistivity is measured in magnetic fields up to 55 T in lightly doped La_{2-x}Sr_{x}CuO_{4} in the Néel state (x = 0.01) and in the spin-glass state (x = 0.03) using high-quality untwinned single crystals. In both cases, a large negative MR is observed to appear when the magnetic order is established. For x = 0.01, it is found that the MR is indicative of a one-step transition into a high-field weak-ferromagnetic state at around 20 T when the magnetic field is applied from the spin easy axis (b axis), which means that there is no spin-flop transition in the Néel state of this material; this is contrary to a previous report, but is natural in light of the peculiar in-plane magnetic susceptibility anisotropy recently found in this system. In the spin-glass state, we observe that the large (up to \sim20%) negative MR saturates at around 40 T, and this MR is found to be essentially isotropic when the magnetic field is rotated within the ab plane. Our data show that the large negative MR is inherent to LSCO in a magnetically ordered state, in which the weak-ferromagnetic (WF) moment becomes well-defined; we discuss that the observed MR is essentially due to the reorientation of the WF moments towards the magnetic field direction both in the Néel state and in the spin-glass state.

cond-mat.supr-con↗
arXiv2004

Examination of the c-axis resistivity of Bi_{2}Sr_{2-x}La_xCuO_{6+δ} in magnetic fields up to 58 T

We measure the magnetic-field dependence of the c-axis resistivity, ρ_c(H), in a series of Bi_{2}Sr_{2-x}La_xCuO_{6+δ} (BSLCO) single crystals for a wide range of doping using pulsed magnetic fields up to 58 T. The behavior of ρ_c(H) is examined in light of the recent determination of the upper critical field H_{c2} for this material using Nernst effect measurements. We find that the peak in ρ_c(H) shows up at a field H_p that is much lower than H_{c2} and there is no discernable feature in ρ_c(H) at H_{c2}. Intriguingly, H_p shows a doping dependence similar to that of T_c, and there is an approximate relation k_{B}T_c \simeq {1/2}gμ_{B}H_p. Moreover, we show that the data for the lowest-T_c sample can be used to estimate the pseudogap closing field H_{pg}, but the method to estimate H_{pg} proposed by Shibauchi {\it et al.} [Phys. Rev. Lett. {\bf 86}, 5763 (2001)] must be modified to apply to the BSLCO system.

cond-mat.supr-con↗
arXiv2004

Quantum Phase Transitions in a Cuprate Superconductor Bi_{2}Sr_{2-x}La_{x}CuO_{6+δ}

To elucidate a quantum phase transition (QPT) in Bi_2Sr_{2-x}La_xCuO_{6+δ}, we measure charge and heat transport properties at very low temperatures and examine the following characteristics for a wide range of doping: normal-state resistivity anisotropy under 58 T, temperature dependence of the in-plane thermal conductivity K_{ab}, and the magnetic-field dependence of K_{ab}. It turns out that all of them show signatures of a QPT at the 1/8 hole doping. Together with the recent normal-state Hall measurements under 58 T that signified the existence of a QPT at optimum doping, the present results indicate that there are two QPTs in the superconducting doping regime of this material.

cond-mat.supr-con↗
arXiv2003

Experimental electronic heat capacities of $α-$ and $δ-$Plutonium; heavy-fermion physics in an element

We have measured the heat capacities of $δ-$Pu$_{0.95}$Al$_{0.05}$ and $α-$Pu over the temperature range 2-303 K. The availability of data below 10 K plus an estimate of the phonon contribution to the heat capacity based on recent neutron-scattering experiments on the same sample enable us to make a reliable deduction of the electronic contribution to the heat capacity of $δ-$Pu$_{0.95}$Al$_{0.05}$; we find $γ= 64 \pm 3$ mJK$^{-2}$mol$^{-1}$ as $T \to 0$. This is a factor $\sim 4$ larger than that of any element, and large enough for $δ-$Pu$_{0.95}$Al$_{0.05}$ to be classed as a heavy-fermion system. By contrast, $γ= 17 \pm 1$ mJK$^{-2}$mol$^{-1}$ in $α-$Pu. Two distinct anomalies are seen in the electronic contribution to the heat capacity of $δ-$Pu$_{0.95}$Al$_{0.05}$, one or both of which may be associated with the formation of the $α'-$ martensitic phase. We suggest that the large $γ$-value of $δ-$Pu$_{0.95}$Al$_{0.05}$ may be caused by proximity to a quantum-critical point.

cond-mat.str-el↗
arXiv2002

Effects of unreacted Mg impurities on the transport properties of MgB2

We synthesized polycrystalline MgB2 from a stoichiometric mixture of Mg and the 11 B isotope under different conditions. All the samples showed bulk superconductivity with Tc = 38~39 K. The samples containing the least amount of unreacted Mg showed the highest Tc and the sharpest transition width (Delta_Tc). A residual resistivity ratio (RRR) of ~ 5.8, and a magnetoresistance (MR), at 40 K, of 12% were obtained for these samples. Moreover, there was no upturn of resistivity in a low temperature region at 10 Tesla. The samples containing appreciable amounts of unreacted Mg showed quite different behaviors; the values of Delta_Tc, RRR, and MR were much larger. An upturn appeared in resistivity of the samples below about 50 K at 10 T and is thought to be due to the unreacted Mg.

cond-mat.supr-con↗
arXiv2000

Metal-to-Insulator Crossover in the Low-Temperature Normal State of Bi_{2}Sr_{2-x}La_{x}CuO_{6+δ}

We measure the normal-state in-plane resistivity of La-doped Bi-2201 single crystals at low temperatures by suppressing superconductivity with 60-T pulsed magnetic fields. With decreasing hole doping, we observe a crossover from a metallic to insulating behavior in the low-temperature normal state. This crossover is estimated to occur near 1/8 doping, well inside the underdoped regime, and not at optimum doping as reported for other cuprates. The insulating regime is marked by a logarithmic temperature dependence of the resistivity over two decades of temperature, suggesting that a peculiar charge localization is common to the cuprates.

cond-mat.supr-con↗
arXiv1998

Unconventional Transition from Metallic to Insulating Resistivity in the Spin-ladder Compound (Sr,Ca)$_{14}$Cu$_{24}$O$_{41}$

Spin-ladder compounds make interesting analogs of the high-temperature superconductors, because they contain layers of nearly one-dimensional "ladders" consisting of a square array of copper and oxygen atoms. Increasing the number of legs in the ladders provides a step-wise approach toward the two-dimensional copper-oxygen plane, that structure believed to be a key to high temperature superconductivity. Short-range spin correlations in ladders have been predicted to lead to formation of hole pairs favorable for superconductivity, once enough holes are introduced onto the ladders by doping. Indeed, superconductivity has been discovered in the two-leg ladder compound (Sr,Ca)$_{14}$Cu$_{24}$O$_{41}$ under high pressure. Here we show that charge transport in the non-superconducting state of (Sr,Ca)$_{14}$Cu$_{24}$O$_{41}$ shares three distinct regimes in common with high-temperature superconductors, including an unexplained insulating behavior at low temperatures in which the resistivity increases as the logarithm of the temperature. These observations suggest that the logarithmic divergence arises from a new localization mechanism common to the ladder compounds and the high-temperature superconductors, which may arise from nearly one-dimensional charge transport in the presence of a spin gap.

cond-mat.str-el↗