arXiv subjects

J. S. Brooks

Publications and source records attributed to J. S. Brooks.

At least 19 recordsLinked to original sources

arXiv2017

Doping evolution of the anisotropic upper critical fields in iron-based superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$

In-plane resistivity measurements as a function of temperature and magnetic field up to 35~T with precise orientation within the crystallographic $ac-$plane were used to study the upper critical field, $H_{c2}$, of the hole-doped iron-based superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$. Compositions of the samples studied were spanning from underdoped $x=$0.17 ($T_c$=12~K) and $x$=0.22 ($T_c$=20~K), both in the coexistence range of stripe magnetism and superconductivity, though optimal doping $x$=0.39 ($T_c$=38.4~K), $x$=0.47 ($T_c$=37.2~K), to overdoped $x$=0.65 ($T_c$=22~K), $x$=0.83 ($T_c$=10~K). We find notable doping asymmetry of the shapes of the anisotropic $H_{c2}(T)$ suggesting important role of paramagnetic limiting effects in $H \parallel a$ configuration in overdoped compositions and multi-band effects in underdoped compositions.

cond-mat.supr-con↗

arXiv2016

Inter-planar coupling dependent magnetoresistivity in high purity layered metals

The magnetic field-induced changes in the conductivity of metals are the subject of intense interest, both for revealing new phenomena and as a valuable tool for determining their Fermi surface. Here, we report a hitherto unobserved magnetoresistive effect in ultra-clean layered metals, namely a negative longitudinal magnetoresistance that is capable of overcoming their very pronounced orbital one. This effect is correlated with the inter-layer coupling disappearing for fields applied along the so-called Yamaji angles where the inter-layer coupling vanishes. Therefore, it is intrinsically associated with the Fermi points in the field-induced quasi-one-dimensional electronic dispersion, implying that it results from the axial anomaly among these Fermi points. In its original formulation, the anomaly is predicted to violate separate number conservation laws for left- and right-handed chiral- (e.g. Weyl) fermions. Its observation in PdCoO$_2$, PtCoO$_2$ and Sr$_2$RuO$_4$ suggests that the anomaly affects the transport of clean conductors, particularly near the quantum limit.

cond-mat.mes-hall↗

arXiv2016

Phase Separation and Superparamagnetism in the Martensitic Phase of $Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$

$Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$ shape memory alloys in the approximate range $5 \le x \le 10$ display desirable properties for applications as well as intriguing magnetism. These off-stoichiometric Heusler alloys undergo a martensitic phase transformation at a temperature $T_{M}$ of 300 - 400 K, from ferromagnetic (F) to nonferromagnetic, with unusually low thermal hysteresis and a large change in magnetization. The low temperature magnetic structures in the martensitic phase of such alloys, which are distinctly inhomogeneous, are of great interest but are not well understood. Our present use of spin echo NMR, in the large hyperfine fields at $^{55}Mn$ sites, provides compelling evidence that nanoscale magnetic phase separation into F and antiferromagnetic (AF) regions occurs below $T_{M}$ in alloys with x in the range 0 to 7. At finite Co substitution the F regions are found to be of two distinct types, corresponding to high and low local concentrations of Co on Ni sites. Estimates of the size distributions of both the F and AF nanoregions have been made. At x = 7 the AF component is not long-range ordered, even below 4 K, and is quite different to the AF component found at x = 0; by x = 14 the F phase is completely dominant. Of particular interest, we find, for x = 7, that field cooling leads to dramatic changes in the AF regions. These findings provide insight into the origins of magnetic phase separation and superparamagnetism in these complex alloys, particularly their intrinsic exchange bias, which is of considerable current interest.

cond-mat.mtrl-sci↗

arXiv2015

Correlation between Fermi surface transformations and superconductivity in the electron-doped high-$T_c$ superconductor Nd$_{2-x}$Ce$_x$CuO$_4$

Two critical points have been revealed in the normal-state phase diagram of the electron-doped cuprate superconductor Nd$_{2-x}$Ce$_x$CuO$_4$ by exploring the Fermi surface properties of high quality single crystals by high-field magnetotransport. First, the quantitative analysis of the Shubnikov-de Haas effect shows that the weak superlattice potential responsible for the Fermi surface reconstruction in the overdoped regime extrapolates to zero at the doping level $x_c = 0.175$ corresponding to the onset of superconductivity. Second, the high-field Hall coefficient exhibits a sharp drop right below optimal doping $x_{\mathrm{opt}} = 0.145$ where the superconducting transition temperature is maximum. This drop is most likely caused by the onset of long-range antiferromagnetic ordering. Thus, the superconducting dome appears to be pinned by two critical points to the normal state phase diagram.

cond-mat.str-el↗

arXiv2015

Spin-Orbit-Induced Orbital Excitations in Sr2RuO4 and Ca2RuO4: A Resonant Inelastic X-ray Scattering Study

High-resolution resonant inelastic X-ray scattering (RIXS) at the oxygen K-edge has been used to study the orbital excitations of Ca2RuO4 and Sr2RuO4. In combination with linear dichroism X-ray absorption spectroscopy, the ruthenium 4d-orbital occupation and excitations were probed through their hybridization with the oxygen p-orbitals. These results are described within a minimal model, taking into account crystal field splitting and a spin-orbit coupling λ_{so}=200~meV. The effects of spin-orbit interaction on the electronic structure and implications for the Mott and superconducting ground states of (Ca,Sr)2RuO4 are discussed.

cond-mat.str-el↗

arXiv2014

Magnetic and Structural Phase Transitions in the Spinel Compound Fe1+xCr2-xO4

Neutron and X-ray diffraction, magnetic susceptibility, and specific heat measurements have been used to investigate the magnetic and structural phase transitions of the spinel system Fe1+xCr2-xO4(0.0<=x<=1.0). The temperature versus Fe concentration (x) phase diagram features two magnetically ordered states and four structural states below 420 K. The complexity of the phase diagram is closely related to the change in the spin and orbital degrees of freedom induced by substitution of Fe ions for Cr ions. The systematic change in the crystal structure is explained by the combined effects of Jahn-Teller distortion, spin-lattice interaction, Fe2+-Fe3+ hopping, and disorder among Fe2+, Fe3+, and Cr3+ ions.

cond-mat.mtrl-sci↗

arXiv2013

Re-entrant magnetic field induced charge and spin gaps in the coupled dual-chain quasi-one dimensional organic conductor Perylene$_2$[Pt(mnt)$_2$]

An inductive method is used to follow the magnetic field-dependent susceptibility of the coupled charge density wave (CDW) and spin-Peierls (SP) ordered state behavior in the dual chain organic conductor Perylene$_2$[Pt(mnt)$_2$]. In addition to the coexisting SP-CDW state phase below 8 K and 20 T, the measurements show that a second spin-gapped phase appears above 20 T that coincides with a field-induced insulating phase. The results support a strong coupling of the CDW and SP order parameters even in high magnetic fields, and provide new insight into the nature of the magnetic susceptibility of dual-chain spin and charge systems.

cond-mat.str-el↗

arXiv2013

Effect of heavy-ion irradiation on London penetration depth in over-doped Ba(Fe,Co)2As2

Irradiation with 1.4 GeV $^{208}$Pb ions was used to induce artificial disorder in single crystals of iron-arsenide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and to study its effect on the temperature-dependent London penetration depth and transport properties. Study was undertaken on overdoped single crystals with $x$=0.108 and $x$=0.127 characterized by notable modulation of the superconducting gap. Irradiation with doses 2.22$\times10^{11}$$\textit{d}$/cm$^2$ and 2.4$\times10^{11}$$\textit{d}$/cm$^2$, corresponding to the matching fields of $B_ϕ = $6 T and 6.5 T, respectively, suppresses the superconducting $T_c$ by approximately 0.3 to 1 K. The variation of the low-temperature penetration depth in both pristine and irradiated samples is well described by the power-law, $Δλ(T)=AT^n$. Irradiation increases the magnitude of the pre-factor $A$ and decreases the exponent $n$, similar to the effect of irradiation in optimally doped samples. This finding supports universal $s_{\pm}$ pairing in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ compounds for the whole Co doping range.

cond-mat.supr-con↗

arXiv2013

Competition between the structural phase transition and superconductivity in Ir$_{1-x}$Pt$_x$Te$_2$ as revealed by pressure effects

Pressure-dependent transport measurements of Ir$_{1-x}$Pt$_x$Te$_2$ are reported. With increasing pressure, the structural phase transition at high temperatures is enhanced while its superconducting transition at low temperatures is suppressed. These pressure effects make Ir$_{1-x}$Pt$_x$Te$_2$ distinct from other studied $T$X$_2$ systems exhibiting a charge density wave (CDW) state, in which pressure usually suppresses the CDW state and enhances the superconducting state. The results reveal that the emergence of superconductivity competes with the stabilization of the low temperature monoclinic phase in Ir$_{1-x}$Pt$_x$Te$_2$.

cond-mat.str-el↗

arXiv2013

Angular-dependent upper critical field of overdoped Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$

In-plane resistivity measurements as a function of temperature, magnetic field and its orientation with respect to the crystallographic $ab-$plane were used to study the upper critical field, $H_{c2}$, of two overdoped compositions of the iron-based superconductor Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$, $x=$0.054 and $x$=0.072. Measurements were performed using precise alignment (with accuracy less than 0.1$^o$) of magnetic field with respect to the Fe-As-plane. The dependence of the $H_{c2}$ on angle $θ$ between the field and the $ab$-plane was measured in isothermal conditions in a broad temperature range. We found that the shape of $H_{c2}(θ)$, clearly deviates from Ginzburg-Landau functional form.

cond-mat.supr-con↗

arXiv2011

Angular dependent magnetothermopower of alpha-(ET)2KHg(SCN)4

The magnetic field and angular dependencies of the thermopower and Nernst effect of the quasi-two-dimensional organic conductor alpha-(ET)2KHg(SCN)4 are experimentally measured at temperatures below (4 K) and above (9 K) the transition temperature to fields of In addition, a theoretical model which involves a magnetic breakdown effect between the q1D and q2D bands is proposed in order to simulate the data. Analysis of the background components of the thermopower and Nernst effect imply that at low temperatures, in the CDW state, the properties of alpha-(ET)2KHg(SCN)4 are determined mostly by the orbits on the new open Fermi sheets. Quantum oscillations observed in the both thermoelectric effects, at fields above 8 T, originate only from the alpha orbit.

cond-mat.str-el↗

arXiv2010

Determination of the Upper Critical Field of a Single Crystal LiFeAs: The Magnetic Torque Study up to 35 Tesla

We report on the upper critical field B_c2 of a superconducting LiFeAs single crystal with T_c~16 K, determined from magnetic torque measurements in dc-magnetic fields up to 35 T and at temperatures down to 0.3 K. B_c2 at 0.3 K is obtained to be 26.4 T and 15.5 T for the applied field B_a||ab and B_a||c, respectively. The anisotropy parameter $Γ$=$B_c2^ab / B_c2^c$ is ~ 3 at T_c and decreases to 1.7 as $T \rightarrow 0$, showing rather isotropic superconductivity. While B_c2 is orbitally-limited for B_a||c, the spin-paramagnetic effect is evident in the temperature dependence of B_c2 for B_a||ab

cond-mat.supr-con↗

arXiv2010

$^{77}$Se and $^{63}$Cu NMR studies of the electronic correlations in Cu$_x$TiSe$_2$ ($x=0.05, 0.07$)

We report $^{77}$Se and $^{63}$Cu nuclear magnetic resonance (NMR) investigation on the charge-density-wave (CDW) superconductor Cu$_x$TiSe$_2$ ($x=0.05$ and 0.07). At high magnetic fields where superconductivity is suppressed, the temperature dependence of $^{77}$Se and $^{63}$Cu spin-lattice relaxation rates 1/T_{1}$ follow a linear relation. The slope of $^{77}1/T_{1}$ vs \emph{T} increases with the Cu doping. This can be described by a modified Korringa relation which suggests the significance of electronic correlations and the Se 4\emph{p}- and Ti 3\emph{d}-band contribution to the density of states at the Fermi level in the studied compounds.

cond-mat.str-el↗

arXiv2009

Tunnel magnetoresistance due to Coulomb blockade effects in quasi-one dimensional polymer nanofibers

We report on the low temperature tunnel magnetoresistance (MR) in quasi one-dimensional (1D) nanofibers made of conjugated polymers. The MR voltage bias dependence reveals an enhancement (at low biases) and the oscillatory behavior at temperatures below 10 K. The low temperature isotropic MR behavior has been attributed to the charging effects in the polymer nanofiber which considered as an array of small conducting regions separated by nano barriers. These effects at low temperatures lead to the single electron tunneling represented by the Coulomb blockade regime as well as to an enhancement and oscillation of the tunnel MR.

cond-mat.stat-mech↗

arXiv2009

Role of anion size, magnetic moment, and disorder on the properties of the organic conductor kappa-(BETS)_2Ga_{1-x}Fe_{x}Cl_{4-y}_Br_{y}

Shubnikov-de Haas and angular dependent magnetoresistance oscillations have been used to explore the role of anion size, magnetic moment, and disorder in the organic conductors kappa-(BETS)_2GaBr_{4} and kappa-(BETS)_2FeCl_{2}_Br_{2} in the isomorphic class kappa-(BETS)_2Ga_{1-x}Fe_{x}Cl_{4-y}_Br_{y}. The results, combined with previous work, show correlations between the anion composition (Ga_{1-x}Fe_{x}Cl_{4-y}_Br_{y}) and the superconducting transition temperature, effective mass, Fermi surface topology, and the mean free path.

cond-mat.str-el↗

arXiv2009

Geometrical and orbital effects in a quasi-one dimensional conductor

The angular dependent magnetoresistance of (Per)2[Au(mnt)2] under pressure has revealed geometrical effects associated with the crystallographic parameters. Pressure suppresses the charge density wave ground state of the material, and in the metallic state both geometrical and orbital quantum interference effects appear. Through magnetic field dependence and orientation, the orbital and geometrical effects are independently identified. We compare the results from (Per)2[Au(mnt)2] with previous studies of the well-known Bechgaard salts.

cond-mat.str-el↗

arXiv2009

Mossbauer spectroscopy study of the "mysterious" magnetic transition in lambda-(BETS)2FeCl4

The compound lambda-(BETS)2FeCl4 provides an effective demonstration of the interaction of pi-conduction electron and d-electron localized moment systems in molecular crystalline materials where antiferromagnetic insulating and magnetic field induced superconducting states can be realized. The metal-insulator transition has been thought to be cooperative, involving both the itinerant pi- electron and localized d-electron spins where antiferromagnetic order appears in both systems simultaneously. However, recent specific heat data has indicated otherwise [Akiba et al., J. Phys. Soc. Japan 78,033601(2009)]: although the pi-electron system orders antiferromagnetically and produces a metal-insulator transition, a "mysterious" paramagnetic d-electron state remains. We report 57Fe Mossbauer measurements that support the paramagnetic model, provided the d-electron spins remain in a fast relaxation state below the transition. From the measured hyperfine fields, we also determine the temperature dependence of the pi-d electron exchange field.

cond-mat.str-el↗

arXiv2009

Critical properties of the S=1 spin dimer compound Ba3Mn2O8

Ba3Mn2O8 is a hexagonally coordinated Mn5+ S=1 spin dimer system with small uniaxial single-ion anisotropy. 135,137Ba NMR spectroscopy is used to establish the lower critical field Hc1 of distinct field-induced phases for H parallel to c,H perpendicular to c, and measure the longitudinal (Ml) and transverse (Mt) magnetizations in the vicinity of the quantum critical point (QCP). Ml_parallel (T, Hc1), Ml_perpendicular (T, Hc1) are reproduced by solving a low-energy model for a dilute gas of interacting bosons. Ml_parallel(T goes to 0, H = Hc1) (Ml_perpendicular(T goes to 0, H = Hc1)) follows the expectation for a BEC (Ising-like) QCP.

cond-mat.str-el↗