arXiv subjects

L. Balicas

Publications and source records attributed to L. Balicas.

At least 73 records · Page 4Linked to original sources

arXiv2010

Fermi surface evolution through a heavy fermion superconductor-to-antiferromagnet transition: de Haas-van Alphen effect in Cd-substituted CeCoIn$_5$

We report the results of de-Haas-van-Alphen (dHvA) measurements in Cd doped CeCoIn$_5$ and LaCoIn$_5$. Cd doping is known to induce an antiferromagnetic order in the heavy fermion superconductor CeCoIn$_5$, whose effect can be reversed with applied pressure. We find a slight but systematic change of the dHvA frequencies with Cd doping in both compounds, reflecting the chemical potential shift due to the addition of holes. The frequencies and effective masses are close to those found in the nominally pure compounds with similar changes apparent in the Ce and La compounds with Cd substitution. We observe no abrupt changes to the Fermi surface in the high field paramagnetic state for $x \sim x_c$ corresponding to the onset of antiferromagnetic ordering at H=0 in CeCo(In$_{1-x}$Cd$_x$)$_5$. Our results rule out $f-$electron localization as the mechanism for the tuning of the ground state in CeCoIn$_5$ with Cd doping.

cond-mat.str-el↗

arXiv2009

Magnetic field induced quantum phase transition of the $S = 1/2$ antiferromagnet $K_2NaCrO_8$

The magnetic properties of alkali-metal peroxychromate K$_2$NaCrO$_8$ are governed by the $S = 1/2$ pentavalent chromium cation, Cr$^{5+}$. Specific heat, magnetocalorimetry, ac magnetic susceptibility, torque magnetometry, and inelastic neutron scattering data have been acquired over a wide range of temperature, down to 60 mK, and magnetic field, up to 18 T. The magnetic interactions are quasi-two-dimensional prior to long-range ordering, where $T_N = 1.66$ K in $H = 0$. In the $T \to 0$ limit, the magnetic field tuned antiferromagnetic-ferromagnetic phase transition suggests a critical field $H_c = 7.270$ T and a critical exponent $α= 0.481 \pm 0.004$. The neutron data indicate the magnetic interactions may extend over intra-planar nearest-neighbors and inter-planar next-nearest-neighbor spins.

cond-mat.str-el↗

arXiv2009

Zooming on the Quantum Critical Point in Nd-LSCO

Recent studies of the high-Tc superconductor La_(1.6-x)Nd_(0.4)Sr_(x)CuO_(4) (Nd-LSCO) have found a linear-T in-plane resistivity rho_(ab) and a logarithmic temperature dependence of the thermopower S / T at a hole doping p = 0.24, and a Fermi-surface reconstruction just below p = 0.24 [1, 2]. These are typical signatures of a quantum critical point (QCP). Here we report data on the c-axis resistivity rho_(c)(T) of Nd-LSCO measured as a function of temperature near this QCP, in a magnetic field large enough to entirely suppress superconductivity. Like rho_(ab), rho_(c) shows an upturn at low temperature, a signature of Fermi surface reconstruction caused by stripe order. Tracking the height of the upturn as it decreases with doping enables us to pin down the precise location of the QCP where stripe order ends, at p* = 0.235 +- 0.005. We propose that the temperature T_(rho) below which the upturn begins marks the onset of the pseudogap phase, found to be roughly twice as high as the stripe ordering temperature in this material.

cond-mat.supr-con↗

arXiv2009

Tracking anisotropic scattering in overdoped Tl$_2$Ba$_2$CuO$_{6+δ}$ above 100 K

This article describes new polar angle-dependent magnetoresistance (ADMR) measurements in the overdoped cuprate Tl$_2$Ba$_2$CuO$_{6+δ}$ over an expanded range of temperatures and azimuthal angles. These detailed measurements re-affirm the analysis of earlier data taken over a more restricted temperature range and at a single azimuthal orientation, in particular the delineation of the intraplane scattering rate into isotropic and anisotropic components. These new measurements also reveal additional features in the temperature and momentum dependence of the scattering rate, including anisotropy in the $T^2$ component and the preservation of both the $T$-linear and $T^2$ components up to 100 K. The resultant form of the scattering rate places firm constraints on the development of any forthcoming theoretical framework for the normal state charge response of high temperature superconducting cuprates.

cond-mat.supr-con↗

arXiv2009

Bose-Einstein Condensation of Triplons in Ba3Cr2O8

By performing heat capacity, magnetocaloric effect, torque magnetometry and force magnetometry measurements up to 33 T, we have mapped out the T-H phase diagram of the S = 1/2 spin dimer compound Ba3Cr2O8. We found evidence for field-induced magnetic order between Hc1 = 12.52(2) T and Hc2 = 23.65(5) T, with the maximum transition temperature Tc = 2.7 K at H = 18 T. The lower transition can likely be described by Bose-Einstein condensation of triplons theory, and this is consistent with the absence of any magnetization plateaus in our magnetic torque and force measurements. In contrast, the nature of the upper phase transition appears to be quite different as our measurements suggest that this transition is actually first order.

cond-mat.str-el↗

arXiv2009

High-field phase-diagram of Fe arsenide superconductors

Here, we report an overview of the phase diagram of single layered and double layered Fe arsenide superconductors at high magnetic fields. Our systematic magnetotransport measurements of polycrystalline SmFeAsO$_{1-x}$F$_x$ at different doping levels confirm the upward curvature of the upper critical magnetic field $H_{c2}(T)$ as a function of temperature $T$ defining the phase boundary between the superconducting and metallic states for crystallites with the ab planes oriented nearly perpendicular to the magnetic field. We further show from measurements on single crystals that this feature, which was interpreted in terms of the existence of two superconducting gaps, is ubiquitous among both series of single and double layered compounds. In all compounds explored by us the zero temperature upper critical field $H_{c2}(0)$, estimated either through the Ginzburg-Landau or the Werthamer-Helfand-Hohenberg single gap theories, strongly surpasses the weak coupling Pauli paramagnetic limiting field. This clearly indicates the strong coupling nature of the superconducting state and the importance of magnetic correlations for these materials. Our measurements indicate that the superconducting anisotropy, as estimated through the ratio of the effective masses $γ= (m_c/m_{ab})^{1/2}$ for carriers moving along the c-axis and the ab planes, respectively, is relatively modest as compared to the high-$T_c$ cuprates, but it is temperature, field and even doping dependent. Finally, our preliminary estimations of the irreversibility field $H_m(T)$, separating the vortex-solid from the vortex-liquid phase in the single layered compounds, indicates that it is well described by the melting of a vortex lattice in a moderately anisotropic uniaxial superconductor.

cond-mat.supr-con↗

arXiv2009

Small anisotropy, weak thermal fluctuations, and high field superconductivity in Co-doped iron pnictide Ba(Fe1-xCox)2As2

We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe1-xCox)2As2. Unlike the HTS cuprates and 1111-phase oxypnictides, Ba(Fe1-xCox)2As2 showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. The mass anisotropy gamma = Hc2ab/Hc2c deduced from the slopes of the upper critical field dHc2ab/dT = 4.9T/K and dHc2c/dT = 2.5T/K decreases from ~2 near Tc, to ~1.5 at lower temperatures. We observed the irreversibility field close to Hc2, and a rather unusual symmetric volume pinning force curve Fp(H) suggestive of strong pinning nano-structure.

cond-mat.mtrl-sci↗

arXiv2009

Probing multi-band superconductivity and magnetism in SmFeAsO$_{0.8}$F$_{0.2}$ single crystals by high-field vortex torque magnetometry

To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of magnetic torque $τ(θ)$ in the mixed state of SmO$_{0.8}$F$_{0.2}$FeAs single crystals as functions of temperature $T$ and high magnetic field $H$ up to 30 T. We show that the effective mass anisotropy parameter $γ$ extracted from $τ(θ)$, can be greatly overestimated if the strong paramagnetism of Sm or Fe ions is not properly taken into account. The correctly extracted $γ$ depends on both $T$ and $H$, saturating at $γ\simeq 9$ at lower temperatures. Neither the London penetration depth nor the superfluid density is affected by high fields fields up to the upper critical field. Our results indicate two strongly-coupled superconducting gaps of nearly equal magnitudes.

cond-mat.supr-con↗

arXiv2008

3:1 magnetization plateau and suppression of ferroelectric polarization in an Ising chain multiferroic

Ferroelectric Ising chain magnet Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ ($x\simeq$0.96) was studied in magnetic fields up to 33 T. Magnetization and neutron scattering measurements reveal successive metamagnetic transitions from the zero-field $\uparrow \uparrow \downarrow \downarrow$ spin configuration to the $\uparrow \uparrow \uparrow \downarrow$ state with a broad magnetization plateau, and then to the $\uparrow \uparrow \uparrow \uparrow$ state. The absence of hysteresis in these plateaus reveals an intriguing coupling between the intra-chain state and the three-dimensional geometrically frustrated magnetic system. Inversion symmetry, broken in the $\uparrow \uparrow \downarrow \downarrow$ state, is restored in the $\uparrow \uparrow \uparrow \downarrow$ state, leading to the complete suppression of the electric polarization driven by symmetric superexchange.

cond-mat.str-el↗

arXiv2008

The role of f -electrons in the Fermi surface of the heavy fermion superconductor beta-YbAlB4

We present a detailed quantum oscillation study of the Fermi surface of the recently discovered Yb-based heavy fermion superconductor beta-YbAlB4 . We compare the data, obtained at fields from 10 to 45 Tesla, to band structure calculations performed using the local density approximation. Analysis of the data suggests that f-holes participate in the Fermi surface up to the highest magnetic fields studied. We comment on the significance of these findings for the unconventional superconducting properties of this material.

cond-mat.str-el↗

arXiv2008

Linear-T resistivity and change in Fermi surface at the pseudogap critical point of a high-Tc superconductor

A fundamental question of high-temperature superconductors is the nature of the pseudogap phase which lies between the Mott insulator at zero doping and the Fermi liquid at high doping p. Here we report on the behaviour of charge carriers near the zero-temperature onset of that phase, namely at the critical doping p* where the pseudogap temperature T* goes to zero, accessed by investigating a material in which superconductivity can be fully suppressed by a steady magnetic field. Just below p*, the normal-state resistivity and Hall coefficient of La1.6-xNd0.4SrxCuO4 are found to rise simultaneously as the temperature drops below T*, revealing a change in the Fermi surface with a large associated drop in conductivity. At p*, the resistivity shows a linear temperature dependence as T goes to zero, a typical signature of a quantum critical point. These findings impose new constraints on the mechanisms responsible for inelastic scattering and Fermi surface transformation in theories of the pseudogap phase.

cond-mat.supr-con↗

arXiv2008

Upper critical fields and thermally-activated transport of Nd(O_0.7F_0.3)FeAs single crystal

We present measurements of the resistivity and the upper critical field H_c2 of Nd(O_0.7F_0.3)FeAs single crystals in strong DC and pulsed magnetic fields up to 45 T and 60 T, respectively. We found that the field scale of H_c2 is comparable to ~100 T of high T_c cuprates. H_c2(T) parallel to the c-axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline samples. Thus this behavior is indeed an intrinsic feature of oxypnictides, rather than manifestation of vortex lattice melting or granularity. The orientational dependence of H_c2 shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy decreases as T decreases, from 9.2 at 44K to 5 at 34K. Spin dependent magnetoresistance and nonlinearities in the Hall coefficient suggest contribution to the conductivity from electron-electron interactions modified by disorder reminiscent that of diluted magnetic semiconductors. The Ohmic resistivity measured below T_c but above the irreversibility field exhibits a clear Arrhenius thermally activated behavior over 4-5 decades. The activation energy has very different field dependencies for H||ab and H\perp ab. We discuss to what extent different pairing scenarios can manifest themselves in the observed behavior of H_{c2}, using the two-band model of superconductivity. The results indicate the importance of paramagnetic effects on H_c2(T),which may significantly reduce H_c2(0) as compared toH_c2(0)~200-300 T based on extrapolations of H_c2(T) near T_c down to low temperatures.

cond-mat.supr-con↗

arXiv2008

Nanoscale freezing of the 2D spin liquid Pr$_{3}$Ga$_{5}$SiO$_{14}$

In this letter, we report on the single crystal growth and physical characterization of the distorted kagomé system Pr$_3$Ga$_5$SiO$_{14}$. It is found that at zero magnetic field the system shows no magnetic order down to 0.035 K and exhibits a $T^{2}$ behavior for the specific heat at low temperatures, indicative of a gapless 2D spin liquid state. Application of an applied field induces nanoscale islands of ordered spins, with a concomitant reduction of the $T^{2}$ specific heat term. This state could be a possible ferro-spin nematic ordering stabilized out of an unusual spin liquid state.

cond-mat.str-el↗

arXiv2008

Electron pockets in the Fermi surface of hole-doped high-Tc superconductors

High-temperature superconductivity occurs as copper oxides are chemically tuned to have a carrier concentration intermediate between their metallic state at high doping and their insulating state at zero doping. The underlying evolution of the electron system in the absence of superconductivity is still unclear and a question of central importance is whether it involves any intermediate phase with broken symmetry. The Fermi surface of underdoped YBa2Cu3Oy and YBa2Cu4O8 was recently shown to include small pockets in contrast with the large cylinder characteristic of the overdoped regime1, pointing to a topological change in the Fermi surface. Here we report the observation of a negative Hall resistance in the magnetic field-induced normal state of YBa2Cu3Oy and YBa2Cu4O8, which reveals that these pockets are electron-like. We propose that electron pockets arise most likely from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped materials near the onset of antiferromagnetic order Comparison with materials of the La2CuO4 family that exhibit spin/charge density-wave order suggests that a Fermi surface reconstruction also occurs in those materials, pointing to a generic property of high-Tc superconductors.

cond-mat.supr-con↗

arXiv2008

Log-T divergence and Insulator-to-Metal Crossover in the normal state resistivity of fluorine doped SmFeAsO1-xFx

We report the resistivity of a series of fluorine-doped SmFeAsO1-xFx polycrystalline superconductors in magnetic fields up to 60T. For underdoped samples (x < 0.15), the low temperature resistive state is characterized by pronounced magneto-resistance and a resistive upturn at low temperatures. The "insulating behavior" is characterized by a log-T divergence observed over a decade in temperature. In contrast, the normal state for samples with doping x > 0.15 display metallic behavior with little magnetoresistance, where intense magnetic fields broaden the superconducting transition rather than suppress Tc. The location of the insulator-to metal crossover coincides with the reported suppression of the structural phase transition (SPT)in the phase diagram for SmFeAsO1-xFx series.

cond-mat.supr-con↗

arXiv2008

Itinerant spin excitations near the hidden order transition in URu2Si2

By means of neutron scattering we show that the high-temperature precursor to the hidden order state of the heavy fermion superconductor URu$_{2}$Si$_{2}$ exhibits heavily damped incommensurate paramagnons whose strong energy dispersion is very similar to that of the long-lived longitudinal f-spin excitations that appear below T$_{0}$. Since the underlying local f-exchange is preserved we expect only the f-d interactions to change across the phase transition and to cause the paramagnetic damping. The damping exhibits single-ion behavior independent of wave vector and vanishes below the hidden order transition. We suggest that this arises from a transition from valence fluctuations to a hybridized f-d state below T$_{0}$. Here we present evidence that the itinerant excitations, like those in chromium, are due to Fermi surface nesting of hole and electron pockets so that the hidden order phase likely originates from a Fermi-surface instability. We identify wave vectors that span nested regions of a band calculation and that match the neutron spin crossover from incommensurate to commensurate on approach to the hidden order phase.

cond-mat.str-el↗

arXiv2008

Ordered magnetic phases of the frustrated spin-dimer compound Ba3Mn2O8

Ba3Mn2O8 is a spin-dimer compound based on pairs of S=1, 3d^2, Mn^{5+} ions arranged on a triangular lattice. Antiferromagnetic intradimer exchange leads to a singlet ground state in zero-field. Here we present the first results of thermodynamic measurements for single crystals probing the high-field ordered states of this material associated with closing the spin gap to the excited triplet states. Specific heat, magnetocaloric effect, and torque magnetometry measurements were performed in magnetic fields up to 32 T and temperatures down to 20 mK. For fields above H_{c1} ~ 8.7 T, these measurements reveal a single magnetic phase for H parallel to c, but two distinct phases (approximately symmetric about the center of the phase diagram) for H perpendicular to c. Analysis of the simplest possible spin Hamiltonian describing this system yields candidates for these ordered states corresponding to a simple spiral structure for H parallel to c, and to two distinct modulated phases for H perpendicular to c. Both single-ion anisotropy and geometric frustration play crucial roles in defining the phase diagram.

cond-mat.str-el↗

arXiv2007

Local moment, itinerancy and deviation from Fermi liquid behavior in Na$_x$CoO$_2$ for $0.71 \leq x \leq 0.84$

Here we report the observation of Fermi surface (FS) pockets via the Shubnikov de Haas effect in Na$_x$CoO$_2$ for $x = 0.71$ and 0.84, respectively. Our observations indicate that the FS expected for each compound intersects their corresponding Brillouin zones, as defined by the previously reported superlattice structures, leading to small reconstructed FS pockets, but only if a precise number of holes per unit cell is \emph{localized}. For $0.71 \leq x < 0.75$ the coexistence of itinerant carriers and localized $S =1/2$ spins on a paramagnetic triangular superlattice leads at low temperatures to the observation of a deviation from standard Fermi-liquid behavior in the electrical transport and heat capacity properties, suggesting the formation of some kind of quantum spin-liquid ground state.

cond-mat.str-el↗