arXiv subjects

I. R. Fisher

Publications and source records attributed to I. R. Fisher.

At least 91 records · Page 5Linked to original sources

arXiv2010

Persistence of magnons in a site-diluted dimerized frustrated antiferromagnet

We present inelastic neutron scattering and thermodynamic measurements characterizing the magnetic excitations in a disordered non-magnetic substituted spin-liquid antiferromagnet. The parent compound Ba3Mn2O8 is a dimerized, quasi-two-dimensional geometrically frustrated quantum disordered antiferromagnet. We substitute this compound with non-magnetic vanadium for the S = 1 manganese atoms, Ba3(Mn1-xVx)2O8, and find that the singlet-triplet excitations which dominate the spectrum of the parent compound persist for the full range of substitution examined, x = 0.02 to 0.3. We also observe additional low-energy magnetic fluctuations which are enhanced at the greatest substitution values. These excitations may be a precursor to a low-temperature random singlet phase which may exist in Ba3(Mn1-xVx)2O8

cond-mat.str-el↗

arXiv2010

Nonlinear order parameter oscillations and lattice coupling in strongly-driven charge-density-wave systems

The anharmonic response of charge-density wave (CDW) order to strong laser-pulse perturbations in 1T-TaS_2 and TbTe_3 is investigated by means of a multiple-pump-pulse time-resolved femtosecond optical spectroscopy. We observe remarkable anharmonic effects hitherto undetected in the systems exhibiting collective charge ordering. The efficiency for additional excitation of the amplitude mode by a laser pulse becomes periodically modulated after the mode is strongly excited into a coherently oscillating state. A similar effect is observed also for some other phonons, where the cross-modulation at the amplitude-mode frequency indicates anharmonic interaction of those phonons with the amplitude mode. By analyzing the observed phenomena in the framework of time-dependent Ginzburg-Landau theory we attribute the effects to the anharmonicity of the mode potentials inherent to the broken symmetry state of the CDW systems.

cond-mat.str-el↗

arXiv2010

Anomalous scattering in superconducting indium-doped tin telluride

Results of resistivity, Hall effect, magnetoresistance, susceptibility and heat capacity measurements are presented for single crystals of indium-doped tin telluride with compositions Sn$_{.988-x}$In$_x$Te where $0 \leq x \leq 8.4 %$, along with microstructural analysis based on transmission electron microscopy. For small indium concentrations, $x \leq 0.9 %$ the material does not superconduct above 0.3 K, and the transport properties are consistent with simple metallic behavior. For $x \geq 2.7 %$ the material exhibits anomalous low temperature scattering and for $x \geq 6.1 %$ bulk superconductivity is observed with critical temperatures close to 2 K. Intermediate indium concentrations $2.7% \leq x \leq 3.8%$ do not exhibit bulk superconductivity above 0.7 K. Susceptibility data indicate the absence of magnetic impurities, while magnetoresistance data are inconsistent with localization effects, leading to the conclusion that indium-doped SnTe is a candidate charge Kondo system, similar to thallium-doped PbTe.

cond-mat.supr-con↗

arXiv2010

Pinpointing Gap Minima in Ba(Fe$_{0.94}$Co$_{0.06})_{2}$As$_2$ \textit{via} Band Structure Calculations and Electronic Raman Scattering

A detailed knowledge of the gap structure for the Fe-pnictide superconductors is still rather rudimentary, with several conflicting reports of either nodes, deep gap minima, or fully isotropic gaps on the Fermi surface sheets, both in the $k_{x}-k_{y}$ plane and along the c-axis. In this paper we present considerations for electronic Raman scattering which can help clarify the gap structure and topology using different light scattering geometries. Using density functional calculations for the Raman vertices, it is shown that the location of the gap minima may occur on loops stretching over a portion of the c-axis in Ba(Fe$_{0.94}$Co$_{0.06})_{2}$As$_2$.

cond-mat.supr-con↗

arXiv2010

Anisotropic charge dynamics in detwinned Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

We investigate the optical conductivity as a function of temperature with light polarized along the in-plane orthorhombic $a$- and $b$-axes of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for $x$=0 and 2.5$\%$ under uniaxial pressure. The charge dynamics at low frequencies on these detwinned, single domain compounds tracks the anisotropic $dc$ transport properties across their structural and magnetic phase transitions. Our findings allow us to estimate the dichroism, which extends to relatively high frequencies. These results are consistent with a scenario in which orbital order plays a significant role in the tetragonal-to-orthorhombic structural transition.

cond-mat.supr-con↗

arXiv2010

Coherent dynamics of macroscopic electronic order through a symmetry-breaking transition

The temporal evolution of systems undergoing symmetry breaking phase transitions (SBTs) is of great fundamental interest not only in condensed matter physics, but extends from cosmology to brain function and finance \cite{topology,Kibble,Eltsov,Finance}. However, the study of such transitions is often hindered by the fact that they are difficult to repeat, or they occur very rapidly. Here we report for the first time on a high-time-resolution study of the evolution of both bosonic and fermionic excitations through a second order electronic charge-ordering SBT in a condensed matter system. Using a new three-pulse femtosecond spectroscopy technique, we periodically quench our model system into the high-symmetry state, detecting hitherto unrecorded coherent aperiodic undulations of the order parameter (OP), critical slowing down of the collective mode, and evolution of the particle-hole gap appearing through the Peierls-BCS mechanism as the system evolves through the transition. Numerical modeling based on Ginzburg-Landau theory is used to reproduce the observations without free parameters. The close analogy with other Higgs potentials in particle physics\cite{Higgs} gives new insight into hitherto unexplored dynamics of both single particle and collective excitations through a SBT. Of particular interest is the observation of spectro-temporal distortions caused by disturbances of the field arising from spontaneous annihilation of topological defects, similar to those discussed by the Kibble-Zurek cosmological model.

cond-mat.stat-mech↗

arXiv2010

Charge dynamics of the Co-doped BaFe$_2$As$_2$

We report on a thorough optical investigation over a broad spectral range and as a function of temperature of the charge dynamics in Ba(Co$_x$Fe$_{1-x}$)$_2$As$_2$ compounds for Co-doping ranging between 0 and 18%. For the parent compound as well as for $x$=0.025 we observe the opening of a pseudogap, due to the spin-density-wave phase transition and inducing a reshuffling of spectral weight from low to high frequencies. For compounds with 0.051$\le x \le$ 0.11 we detect the superconducting gap, while at $x$=0.18 the material stays metallic at all temperatures. We describe the effective metallic contribution to the optical conductivity with two Drude terms, representing the combination of a coherent and incoherent component, and extract the respective scattering rates. We establish that the $dc$ transport properties in the normal phase are dominated by the coherent Drude term for 0$\le x \le$0.051 and by the incoherent one for 0.061$\le x \le$0.18, respectively. Finally through spectral weight arguments, we give clear-cut evidence for moderate electronic correlations for 0$\le x \le$0.061, which then crossover to values appropriate for a regime of weak interacting and nearly-free electron metals for $x\ge$0.11.

cond-mat.str-el↗

arXiv2010

Anisotropic phase diagram of the frustrated spin dimer compound Ba3Mn2O8

Heat capacity and magnetic torque measurements are used to probe the anisotropic temperature-field phase diagram of the frustrated spin dimer compound Ba3Mn2O8 in the field range from 0T to 18T. For fields oriented along the c axis a single magnetically ordered phase is found in this field range, whereas for fields oriented along the a axis two distinct phases are observed. The present measurements reveal a surprising non-monotonic evolution of the phase diagram as the magnetic field is rotated in the [001]-[100] plane. The angle dependence of the critical field (Hc1) that marks the closing of the spin gap can be quantitatively accounted for using a minimal spin Hamiltonian comprising superexchange between nearest and next nearest Mn ions, the Zeeman energy and single ion anisotropy. This Hamiltonian also predicts a non-monotonic evolution of the transition between the two ordered states as the field is rotated in the a-c plane. However, the observed effect is found to be significantly larger in magnitude, implying that either this minimal spin Hamiltonian is incomplete or that the magnetically ordered states have a slightly different structure than previously proposed.

cond-mat.str-el↗

arXiv2010

Two-dimensional Dirac fermions in a topological insulator: transport in the quantum limit

Pulsed magnetic fields of up to 55T are used to investigate the transport properties of the topological insulator Bi_2Se_3 in the extreme quantum limit. For samples with a bulk carrier density of n = 2.9\times10^16cm^-3, the lowest Landau level of the bulk 3D Fermi surface is reached by a field of 4T. For fields well beyond this limit, Shubnikov-de Haas oscillations arising from quantization of the 2D surface state are observed, with the ν=1 Landau level attained by a field of 35T. These measurements reveal the presence of additional oscillations which occur at fields corresponding to simple rational fractions of the integer Landau indices.

cond-mat.mes-hall↗

arXiv2010

Temperature dependence of the excitation spectrum in the charge-density-wave ErTe$_3$ and HoTe$_3$ systems

We provide optical reflectivity data collected over a broad spectral range and as a function of temperature on the ErTe$_3$ and HoTe$_3$ materials, which undergo two consecutive charge-density-wave (CDW) phase transitions at $T_{CDW1}$= 265 and 288 K and at $T_{CDW2}$= 157 and 110 K, respectively. We observe the temperature dependence of both the Drude component, due to the itinerant charge carriers, and the single-particle peak, ascribed to the charge-density-wave gap excitation. The CDW gap progressively opens while the metallic component gets narrow with decreasing temperature. An important fraction of the whole Fermi surface seems to be affected by the CDW phase transitions. It turns out that the temperature and the previously investigated pressure dependence of the most relevant CDW parameters share several common features and behaviors. Particularly, the order parameter of the CDW state is in general agreement with the predictions of the BCS theory.

cond-mat.str-el↗

arXiv2010

Dispersive Spin Fluctuations in the near optimally-doped superconductor Ba(Fe1-xCox)2As2 ($x$=0.065)

Inelastic neutron scattering is used to probe the collective spin excitations of the near optimally-doped superconductor Ba(Fe1-xCox)2As2 ($x$=0.065). Previous measurements on the antiferromagnetically ordered parents of this material show a strongly anisotropic spin-wave velocity. Here we measure the magnetic excitations up to 80 meV and show that a similar anisotropy persists for superconducting compositions. The dispersive mode measured here connects directly with the spin resonance previously observed in this compound. When placed on an absolute scale, our measurements show that the local- or wavevector- integrated susceptibility is larger in magnitude than that of the ordered parents over the energy range probed.

cond-mat.supr-con↗

arXiv2010

Bulk electronic structure of optimally doped Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$

We report high-resolution, bulk Compton scattering measurements unveiling the Fermi surface of an optimally-doped iron-arsenide superconductor, Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. Our measurements are in agreement with first-principles calculations of the electronic structure, revealing both the $X$-centered electron pockets and the $Γ$-centered hole pockets. Moreover, our data are consistent with the strong three-dimensionality of one of these sheets that has been predicted by electronic structure calculations at the local-density-approximation-minimum As position. Complementary calculations of the noninteracting susceptibility, $χ_0({\bf q}, ω)$, suggest that the broad peak that develops due to interband Fermi-surface nesting, and which has motivated several theories of superconductivity in this class of material, survives the measured three dimensionality of the Fermi surface in this family.

cond-mat.str-el↗

arXiv2010

Enhanced Fermi surface nesting in superconducting BaFe$_2$(As$_{1-x}$P$_x$)$_2$ revealed by de Haas-van Alphen effect

The three-dimensional Fermi surface morphology of superconducting BaFe_2(As_0.37}P_0.63)_2 with T_c=9K, is determined using the de Haas-van Alphen effect (dHvA). The inner electron pocket has a similar area and k_z interplane warping to the observed hole pocket, revealing that the Fermi surfaces are geometrically well nested in the (π,π) direction. These results are in stark contrast to the Fermiology of the non-superconducting phosphides (x=1), and therefore suggests an important role for nesting in pnictide superconductivity.

cond-mat.supr-con↗

arXiv2009

Band and momentum dependent electron dynamics in superconducting ${\rm Ba(Fe_{1-x}Co_{x})_2As_2}$ as seen via electronic Raman scattering

We present details of carrier properties in high quality ${\rm Ba(Fe_{1-x}Co_{x})_2As_2}$ single crystals obtained from electronic Raman scattering. The experiments indicate a strong band and momentum anisotropy of the electron dynamics above and below the superconducting transition highlighting the importance of complex band-dependent interactions. The presence of low energy spectral weight deep in the superconducting state suggests a gap with accidental nodes which may be lifted by doping and/or impurity scattering. When combined with other measurements, our observation of band and momentum dependent carrier dynamics indicate that the iron arsenides may have several competing superconducting ground states.

cond-mat.supr-con↗

arXiv2009

STM imaging of electronic waves on the surface of Bi$_2$Te$_3$: topologically protected surface states and hexagonal warping effects

Scanning tunneling spectroscopy studies on high-quality Bi$_2$Te$_3$ crystals exhibit perfect correspondence to ARPES data, hence enabling identification of different regimes measured in the local density of states (LDOS). Oscillations of LDOS near a step are analyzed. Within the main part of the surface band oscillations are strongly damped, supporting the hypothesis of topological protection. At higher energies, as the surface band becomes concave, oscillations appear which disperse with a particular wave-vector that may result from an unconventional hexagonal warping term.

cond-mat.str-el↗

arXiv2009

Evidence for a charge Kondo effect in Pb(1-x)Tl(x)Te from measurements of thermoelectric power

We report measurements of the thermoelectric power (TEP) for a series of Pb(1-x)Tl(x)Te crystals with x = 0.0 to 1.3%. Although the TEP is very large for x = 0.0, using a single band analysis based on older work for dilute magnetic alloys we do find evidence for a Kondo contribution of 11 - 18 uV/K. This analysis suggests that Tk is ~ 50 - 70 K, a factor 10 higher than previously thought.

cond-mat.str-el↗

arXiv2009

Raman scattering evidence for a cascade-like evolution of the charge-density-wave collective amplitude mode

The two-dimensional rare-earth tri-tellurides undergo a unidirectional charge-density-wave (CDW) transition at high temperature and, for the heaviest members of the series, a bidirectional one at low temperature. Raman scattering experiments as a function of temperature on DyTe$_3$ and on LaTe$_3$ at 6 GPa provide a clear-cut evidence for the emergence of the respective collective CDW amplitude excitations. In the unidirectional CDW phase, we surprisingly discover that the amplitude mode develops as a succession of two mean-field, BCS-like transitions in different temperature ranges.

cond-mat.str-el↗

arXiv2009

Unconventional electronic reconstruction in undoped (Ba,Sr)Fe$_2$As$_2$ across the spin density wave transition

Through a systematic high resolution angle-resolved photoemission study of the iron pnictide compounds (Ba,Sr)Fe$_2$As$_2$, we show that the electronic structures of these compounds are significantly reconstructed across the spin density wave ordering, which cannot be described by a simple folding scenario of conventional density wave ordering. Moreover, we find that LDA calculations with an incorporated suppressed magnetic moment of 0.5$μ_{\tiny{\textrm{B}}}$ can match well the details in the reconstructed electronic structure, suggesting that the nature of magnetism in the pnictides is more itinerant than local, while the origin of suppressed magnetic moment remains an important issue for future investigations.

cond-mat.supr-con↗