arXiv subjects

J. M. Tranquada

Publications and source records attributed to J. M. Tranquada.

At least 73 records · Page 4Linked to original sources

arXiv2015

Dependence of superconductivity in CuxBi2Se3 on quenching conditions

Topological superconductivity, implying gapless protected surface states, has recently been proposed to exist in the compound CuxBi2Se3. Unfortunately, low diamagnetic shielding fractions and considerable inhomogeneity have been reported in this compound. In an attempt to understand and improve on the finite superconducting volume fractions, we have investigated the effects of various growth and post-annealing conditions. With a melt-growth (MG) method, diamagnetic shielding fractions of up to 56% in Cu0.3Bi2Se3 have been obtained, the highest value reported for this method. We investigate the efficacy of various quenching and annealing conditions, finding that quenching from temperatures above 560C is essential for superconductivity, whereas quenching from lower temperatures or not quenching at all is detrimental. A modified floating zone (FZ) method yielded large single crystals but little superconductivity. Even after annealing and quenching, FZ-grown samples had much less chance of being superconducting than MG-grown samples. From the low shielding fractions in FZ-grown samples and the quenching dependence, we suggest that a metastable secondary phase having a small volume fraction in most of the samples may be responsible for the superconductivity.

cond-mat.supr-con↗

arXiv2015

Surface-state-dominated transport in crystals of the topological crystalline insulator In-doped Pb$_{1-x}$Sn$_x$Te

Three-dimensional topological insulators and topological crystalline insulators represent new quantum states of matter, which are predicted to have insulating bulk states and spin-momentum-locked gapless surface states. Experimentally, it has proven difficult to achieve the high bulk resistivity that would allow surface states to dominate the transport properties over a substantial temperature range. Here we report a series of indium-doped Pb$_{1-x}$Sn$_x$Te compounds that manifest huge bulk resistivities together with strong evidence of topological surface states, based on thickness-dependent transport studies and magnetoresistance measurements. For these bulk-insulating materials, the surface states determine the resistivity for temperatures approaching 30 K.

cond-mat.mtrl-sci↗

arXiv2015

Substitution of Ni for Fe in superconducting Fe$_{0.98}$Te$_{0.5}$Se$_{0.5}$ depresses the normal-state conductivity but not the magnetic spectral weight

We have performed systematic resistivity and inelastic neutron scattering measurements on Fe$_{0.98-z}$Ni$_z$Te$_{0.5}$Se$_{0.5}$ samples to study the impact of Ni substitution on the transport properties and the low-energy ($\le$ 12 meV) magnetic excitations. It is found that, with increasing Ni doping, both the conductivity and superconductivity are gradually suppressed; in contrast, the low-energy magnetic spectral weight changes little. Comparing with the impact of Co and Cu substitution, we find that the effects on conductivity and superconductivity for the same degree of substitution grow systematically as the atomic number of the substituent deviates from that of Fe. The impact of the substituents as scattering centers appears to be greater than any contribution to carrier concentration. The fact that low-energy magnetic spectral weight is not reduced by increased electron scattering indicates that the existence of antiferromagnetic correlations does not depend on electronic states close to the Fermi energy.

cond-mat.supr-con↗

arXiv2014

Imaging Dirac-Mass Disorder from Magnetic Dopant-Atoms in the Ferromagnetic Topological Insulator Cr$_x$(Bi$_{0.1}$Sb$_{0.9}$)$_{2-x}$Te$_3$

To achieve and utilize the most exotic electronic phenomena predicted for the surface states of 3D topological insulators (TI),it is necessary to open a "Dirac-mass gap" in their spectrum by breaking time-reversal symmetry. Use of magnetic dopant atoms to generate a ferromagnetic state is the most widely used approach. But it is unknown how the spatial arrangements of the magnetic dopant atoms influence the Dirac-mass gap at the atomic scale or, conversely, whether the ferromagnetic interactions between dopant atoms are influenced by the topological surface states. Here we image the locations of the magnetic (Cr) dopant atoms in the ferromagnetic TI Cr$_{0.08}$(Bi$_{0.1}$Sb$_{0.9}$)$_{1.92}$Te$_3$. Simultaneous visualization of the Dirac-mass gap $Δ(r)$ reveals its intense disorder, which we demonstrate directly is related to fluctuations in $n(r)$, the Cr atom areal density in the termination layer. We find the relationship of surface-state Fermi wavevectors to the anisotropic structure of $Δ(r)$ consistent with predictions for surface ferromagnetism mediated by those states. Moreover, despite the intense Dirac-mass disorder, the anticipated relationship $Δ(r)\propto n(r)$ is confirmed throughout, and exhibits an electron-dopant interaction energy $J^*$=145$meV\cdot nm^2$. These observations reveal how magnetic dopant atoms actually generate the TI mass gap locally and that, to achieve the novel physics expected of time-reversal-symmetry breaking TI materials, control of the resulting Dirac-mass gap disorder will be essential.

cond-mat.mtrl-sci↗

arXiv2014

Low-energy phonons and superconductivity in Sn$_{0.8}$In$_{0.2}$Te

We present neutron scattering measurements on low-energy phonons from a superconducting ($T_{c} = 2.7$~K) Sn$_{0.8}$In$_{0.2}$Te single crystal sample. The longitudinal acoustic phonon mode and one transverse acoustic branch have been mapped out around the (002) Bragg peak for temperatures of 1.7 K and 4.2 K. We observe a substantial energy width of the transverse phonons at energies comparable to twice the superconducting gap; however, there is no change in this width between the superconducting and normal states. We also confirm that the compound is well ordered, with no indications of structural instability.

cond-mat.supr-con↗

arXiv2014

Neutron Scattering Evidence for a Periodically-Modulated Superconducting Phase in the Underdoped Cuprate La$_{1.905}$Ba$_{0.095}$CuO$_4$

The role of antiferromagnetic spin correlations in high-temperature superconductors remains a matter of debate. We present inelastic neutron scattering evidence that gapless spin fluctuations coexist with superconductivity in La$_{1.905}$Ba$_{0.095}$CuO$_4$. Furthermore, we observe that both the low-energy magnetic spectral weight and the spin incommensurability are enhanced by the onset of superconducting correlations. We propose that the coexistence occurs through intertwining of spatial modulations of the pair wave function and the antiferromagnetic correlations. This proposal is also directly relevant to sufficiently underdoped La$_{2-x}$Sr$_x$CuO$_4$ and YBa$_2$Cu$_3$O$_{6+x}$.

cond-mat.supr-con↗

arXiv2014

Unveiling the microscopic origin of the electronic smectic-nematic phase transition in La1/3Ca2/3MnO3

Electronic-liquid-crystal phases, in which part of the spatial symmetries are broken spontaneously, are considered to play a vital role in interpreting the structure-property relation in strongly-correlated materials. Although electronic-liquid-crystal phases have been inferred in studies of a wide range of materials, the nature of the transition between such phases has received little experimental attention. Here we report the direct observation of an electronic smectic-nematic phase transition in a doped manganite utilizing transmission electron microscopic techniques. We show that, in this case, the transition from the smectic to the nematic phase is driven by two mechanisms: the proliferation of dislocations and the development of mesoscale charge inhomogeneity. We observe the development of dislocation pairs within the commensurate smectic phase on approaching the transition (at 210 +/- 10 K) from below. Above the transition, the incommensurate nematic phase is accompanied by mesoscopic electronic phase separation.

cond-mat.str-el↗

arXiv2014

Superconductivity induced by In substitution into the topological crystalline insulator Pb(0.5)Sn(0.5)Te

Indium substitution turns the topological crystalline insulator (TCI) Pb$_{0.5}$Sn$_{0.5}$Te into a possible topological superconductor. To investigate the effect of the indium concentration on the crystal structure and superconducting properties of (Pb$_{0.5}$Sn$_{0.5}$)$_{1-x}$In$_{x}$Te, we have grown high-quality single crystals using a modified floating-zone method, and have performed systematic studies for indium content in the range $0\leq x\leq 0.35$. We find that the single crystals retain the rock salt structure up to the solubility limit of indium ($x\sim0.30$). Experimental dependences of the superconducting transition temperature ($T_c$) and the upper critical magnetic field ($H_{c2}$) on the indium content $x$ have been measured. The maximum $T_c$ is determined to be 4.7 K at $x=0.30$, with $μ_0H_{c2}(T=0)\approx 5$ T.

cond-mat.supr-con↗

arXiv2014

Low energy magnetic excitations from the Fe$_{1+y-z}$(Ni/Cu)$_{z}$Te$_{1-x}$Se$_{x}$ system

We report neutron scattering measurements on low energy ($\hbarω\sim 5$~meV) magnetic excitations from a series of Fe$_{1+y-z}$(Ni/Cu)$_{z}$Te$_{1-x}$Se$_{x}$ samples which belong to the "11" Fe-chalcogenide family. Our results suggest a strong correlation between the magnetic excitations near (0.5,0.5,0) and the superconducting properties of the system. The low energy magnetic excitations are found to gradually move away from (0.5,0.5,0) to incommensurate positions when superconductivity is suppressed, either by heating or chemical doping, confirming previous observations.

cond-mat.supr-con↗

arXiv2014

Evidence of Chiral Order in the Charge-Ordered Phase of Superconducting La1.875Ba0.125Cuo4 Single Crystals Using Polar Kerr-Effect Measurements

High resolution polar Kerr effect (PKE) measurements were performed on La1.875Ba0.125Cuo4 single crystals revealing that a finite Kerr signal is measured below an onset temperature-T_K that coincides with charge ordering transition temperature T_{CO}. We further show that the sign of the Kerr signal cannot be trained with magnetic field, is found to be the same of opposite sides of the same crystal, and is odd with respect to strain in the diagonal direction of the unit cell. These observations are consistent with a chiral "gyrotropic" order above T_c for La1.875Ba0.125Cuo4; similarities to other cuprates suggest that it is a universal property in the pseudogap regime.

cond-mat.str-el↗

arXiv2014

Direct observation of dynamic charge stripes in La2-xSrxNiO4

The insulator-to-metal transition continues to be a challenging subject, especially when electronic correlations are strong. In layered compounds, such as La2-xSrxNiO4 and La2-xBaxCuO4, the doped charge carriers can segregate into periodically-spaced charge stripes separating narrow domains of antiferromagnetic order. Although there have been theoretical proposals of dynamically fluctuating stripes, direct spectroscopic evidence of charge-stripe fluctuations has been lacking. Here we report the detection of critical lattice fluctuations, driven by charge-stripe correlations, in La2-xSrxNiO4 using inelastic neutron scattering. This scattering is detected at large momentum transfers where the magnetic form factor suppresses the spin fluctuation signal. The lattice fluctuations associated with the dynamic charge stripes are narrow in q and broad in energy. They are strongest near the charge stripe melting temperature. Our results open the way towards the quantitative theory of dynamic stripes and for directly detecting dynamical charge stripes in other strongly-correlated systems, including high-temperature superconductors such as La2-xSrxCuO4.

cond-mat.str-el↗

arXiv2013

Enhanced low-energy magnetic excitations via suppression of the itinerancy in Fe0.98-zCuzTe0.5Se0.5

We have performed resistivity and inelastic neutron scattering measurements on three samples of Fe0.98-zCuzTe0.5Se0.5 with z = 0, 0.02, and 0.1. It is found that with increasing Cu doping the sample's resistivity deviates progressively from that of a metal. However, in contrast to expectations that replacing Fe with Cu would suppress the magnetic correlations, the low-energy (no larger than 12 meV) magnetic scattering is enhanced in strength, with greater spectral weight and longer dynamical spin-spin correlation lengths. Such enhancements can be a consequence of either enlarged local moments or a slowing down of the spin fluctuations. In either case, the localization of the conduction states induced by the Cu doping should play a critical role. Our results are not applicable to models that treat 3d transition metal dopants simply as effective electron donors.

cond-mat.supr-con↗

arXiv2013

Optimizing the superconducting transition temperature and upper critical field of Sn(1-x)In(x)Te

Sn(1-x)In(x)Te is a possible candidate for topological superconductivity. Previous work has shown that substitution of In for Sn in the topological crystalline insulator SnTe results in superconductivity, with the transition temperature, Tc, growing with In concentration. We have performed a systematic investigation of Sn(1-x)In(x)Te for a broad range of x, synthesizing single crystals (by a modified floating zone method) as well as polycrystalline samples. The samples have been characterized by x-ray diffraction, resistivity, and magnetization. For the single crystals, the maximum Tc is obtained at x=0.45 with a value of of 4.5 K, as determined by the onset of diamagnetism.

cond-mat.supr-con↗

arXiv2013

Absolute cross-section normalization of magnetic neutron scattering data

We discuss various methods to obtain the resolution volume for neutron scattering experiments, in order to perform absolute normalization on inelastic magnetic neutron scattering data. Examples from previous experiments are given. We also try to provide clear definitions of a number of physical quantities which are commonly used to describe neutron magnetic scattering results, including the dynamic spin correlation function and the imaginary part of the dynamic susceptibility. Formulas that can be used for general purposes are provided and the advantages of the different normalization processes are discussed.

cond-mat.supr-con↗

arXiv2013

Evolution of superconducting correlations within magnetic-field-decoupled CuO(2) layers of La(1.905)Ba(0.095)CuO(4)

We explore the evolution of superconductivity in La(2-x)Ba(x)CuO(4) with x=0.095 in magnetic fields of up to 35 T applied perpendicular to the CuO(2) planes. Previous work on this material has shown that perpendicular fields enhance both charge and spin stripe order within the planes. We present measurements of the resistivity parallel and perpendicular to the planes, as well as the Hall effect. Measurements of magnetic susceptibility for fields of up to 15 T applied both parallel and perpendicular to the planes provide complementary measures of the superconductivity. We show that fields sufficient to destroy pair tunneling between the planes do not disrupt the superconducting correlations within the planes. In fact, we observe an onset of large amplitude but phase disordered superconductivity within the planes at approximately 30 K that is remarkably insensitive to field. With further cooling, we observe a phase-transition-like drop in the in-plane resistivity to an apparent state of superconductivity, despite the lack of phase coherence between the layers. These observations raise interesting questions concerning the identification of the upper critical field, where pairing is destroyed, in underdoped cuprates.

cond-mat.supr-con↗

arXiv2013

Energy-dependent crossover from anisotropic to isotropic magnetic dispersion in lightly-doped La1.96Sr0.04CuO4

Inelastic neutron scattering experiments have been performed on lightly-doped La$_{1.96}$Sr$_{0.04}$CuO$_{4}$, which shows diagonal incommensurate spin correlations at low temperatures. We previously reported that this crystal, with a single orthorhombic domain, exhibits the "hourglass" dispersion at low energies [Phys. Rev. Lett. 101, 197001 (2008)]. In this paper, we investigate in detail the energy evolution of the magnetic excitations up to 65 meV. It is found that the anisotropic excitations at low energies, dispersing only along the spin modulation direction, crossover to an isotropic, conical dispersion that resembles spin waves in the parent compound La$_2$CuO$_{4}$. The change from two-fold to full symmetry on crossing the waist of the hourglass reproduces behavior first identified in studies of underdoped YBa$_2$Cu$_3$O$_{6+x}$. We discuss the significance of these results.

cond-mat.str-el↗

arXiv2012

Enhanced charge stripe order of superconducting La(2-x)Ba(x)CuO(4) in a magnetic field

The effect of a magnetic field on the charge stripe order in La(2-x)Ba(x)CuO(4) has been studied by means of high energy (100 keV) x-ray diffraction for charge carrier concentrations ranging from strongly underdoped to optimally doped. We find that charge stripe order can be significantly enhanced by a magnetic field applied along the c-axis, but only at temperatures and dopings where it coexists with bulk superconductivity at zero field. The field also increases stripe correlations between the planes, which can result in an enhanced frustration of the interlayer Josephson coupling. Close to the famous x=1/8 compound, where zero field stripe order is pronounced and bulk superconductivity is suppressed, charge stripe order is independent of a magnetic field. The results imply that static stripe order and three-dimensionally coherent superconductivity are competing ground states.

cond-mat.supr-con↗

arXiv2012

Impact of Dynamic Orbital Correlations on Magnetic Excitations in the Normal State of Iron-Based Superconductors

We show here that orbital degrees of freedom produce a distinct signature in the magnetic excitation spectrum of iron-based superconductors above the magnetic ordering temperature. Because $d_{xz}$ and $d_{yz}$ orbitals are strongly connected with the Fermi surface topology, the nature of magnetic excitations can be modified significantly due to the presence of either static or fluctuating orbital correlations. Within a five-orbital itinerant model, we show that static orbital order generally leads to an enhancement of commensurate magnetic excitations even when the original Fermi surface lacks nesting at $(π,0)$ or $(0,π)$. When long-range orbital order is absent, Gaussian fluctuations beyond the standard random-phase approximation (RPA) capture the effects of fluctuating orbital correlations on the magnetic excitations. We find that commensurate magnetic excitations can also be enhanced if the orbital correlations are strong. Our results offer a natural explanation for the incommensurate-to-commensurate transformation observed in a recent neutron scattering measurement (Z. Xu, et. al., arXiv:1201.4404), and we propose that this unusual transformation is an important signature to distinguish orbital from spin physics in the normal state of the pnictides. Implications for the magnetic and superconducting states are discussed.

cond-mat.str-el↗