arXiv subjects

I. R. Fisher

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

At least 55 records · Page 3Linked to original sources

arXiv2016

Determination of the resistivity anisotropy of orthorhombic materials via transverse resistivity measurements

Measurements of the resistivity anisotropy can provide crucial information about the electronic structure and scattering processes in anisotropic and low-dimensional materials, but quantitative measurements by conventional means often suffer very significant systematic errors. Here we describe a novel approach to measuring the resistivity anisotropy of orthorhombic materials, using a single crystal and a single measurement, that is derived from a $\fracπ{4}$ rotation of the measurement frame relative to the crystallographic axes. In this new basis the transverse resistivity gives a direct measurement of the resistivity anisotropy, which combined with the longitudinal resistivity also gives the in-plane elements of the conventional resistivity tensor via a 5-point contact geometry. This is demonstrated through application to the charge-density wave compound ErTe$_3$, and it is concluded that this method presents a significant improvement on existing techniques in many scenarios.

cond-mat.str-el↗

arXiv2016

Superconductivity and fluctuation in Ba$_{1-p}$K$_p$Fe$_2$As$_2$ and Ba(Fe$_{1-n}$Co$_n$)$_2$As$_2$

We study the interplay of fluctuations and superconductivity in BaFe$_2$As$_2$ (Ba-122) compounds with Ba and Fe substituted by K ($p$ doping) and Co ($n$ doping), respectively. To this end we measured electronic Raman spectra as a function of polarisation and temperature. We observe gap excitations and fluctuations for all doping levels studied. The response from fluctuations is much stronger for Co substitution and, according to the selection rules and the temperature dependence, originates from the exchange of two critical spin fluctuations with characteristic wave vectors $(\pmπ, 0)$ and $(0,\pmπ)$. At 22\% K doping ($p=0.22$), we find the same selection rules and spectral shape for the fluctuations but the intensity is smaller by a factor of 5. Since there exists no nematic region above the orthorhombic spin-density-wave (SDW) phase the identification of the fluctuations via the temperature dependence is not possible. The gap excitations in the superconducting state indicate strongly anisotropic near-nodal gaps for Co substitution which make the observation of collective modes difficult. The variation with doping of the spectral weights of the $A_{1g}$ and $B_{1g}$ gap features does not support the influence of fluctuations on Cooper pairing. Therefore, the observation of Bardasis-Schrieffer modes inside the nearly clean gaps on the K-doped side remains the only experimental evidence for the relevance of fluctuations for pairing.

cond-mat.supr-con↗

arXiv2016

Fermi surface evolution of Na-doped PbTe studied through density functional theory calculations and Shubnikov-de Haas measurements

We present a combined experimental and theoretical study of the evolution of the low-temperature Fermi surface of lead telluride, PbTe, when holes are introduced through sodium substitution on the lead site. Our Shubnikov-de-Haas measurements for samples with carrier concentrations up to $9.4\times10^{19}$cm$^{-3}$ (0.62 Na atomic $\%$) show the qualitative features of the Fermi surface evolution (topology and effective mass) predicted by our density functional (DFT) calculations within the generalized gradient approximation (GGA): we obtain perfect ellipsoidal L-pockets at low and intermediate carrier concentrations, evolution away from ideal ellipsoidicity for the highest doping studied, and cyclotron effective masses increasing monotonically with doping level, implying deviations from perfect parabolicity throughout the whole band. Our measurements show, however, that standard DFT calculations underestimate the energy difference between the L-point and $Σ$-line valence band maxima, since our data are consistent with occupation of a single Fermi surface pocket over the entire doping range studied, whereas the calculations predict an occupation of the $Σ$-pockets at higher doping. Our results for low and intermediate compositions are consistent with a non-parabolic Kane-model dispersion, in which the L-pockets are ellipsoids of fixed anisotropy throughout the band, but the effective masses depend strongly on Fermi energy.

cond-mat.mtrl-sci↗

arXiv2016

Measurement of the $B_{1g}$ and $B_{2g}$ components of the elastoresistivity tensor for tetragonal materials via transverse resistivity configurations

The elastoresistivity tensor $m_{ij,kl}$ relates changes in resistivity to strains experienced by a material. As a fourth-rank tensor, it contains considerably more information about the material than the simpler (second-rank) resistivity tensor; in particular, for a tetragonal material, the $B_{1g}$ and $B_{2g}$ components of the elastoresistivity tensor ($m_{xx,xx}-m_{xx,yy}$ and $2m_{xy,xy}$, respectively) can be related to its nematic susceptibility. Previous experimental probes of this quantity have focused exclusively on differential longitudinal elastoresistance measurements, which determine the induced resistivity anisotropy arising from anisotropic in-plane strain based on the difference of two longitudinal resistivity measurements. Here we describe a complementary technique based on \textit{transverse} elastoresistance measurements. This new approach is advantageous because it directly determines the strain-induced resistivity anisotropy from a single transverse measurement. To demonstrate the efficacy of this new experimental protocol, we present transverse elastoresistance measurements of the $2m_{xy,xy}$ elastoresistivity coefficient of BaFe$_2$As$_2$, a representative iron-pnictide that has previously been characterized via differential longitudinal elastoresistance measurements.

cond-mat.str-el↗

arXiv2015

NMR evidence for inhomogeneous nematic fluctuations in BaFe$_2$(As$_{1-x}$P$_x$)$_2$

We present evidence for nuclear spin-lattice relaxation driven by glassy nematic fluctuations in isovalent P-doped BaFe$_2$As$_2$ single crystals. Both the $^{75}$As and $^{31}$P sites exhibit stretched-exponential relaxation similar to the electron-doped systems. By comparing the hyperfine fields and the relaxation rates at these sites we find that the As relaxation cannot be explained solely in terms of magnetic spin fluctuations. We demonstrate that nematic fluctuations couple to the As nuclear quadrupolar moment and can explain the excess relaxation. These results suggest that glassy nematic dynamics are a universal phenomenon in the iron-based superconductors.

cond-mat.str-el↗

arXiv2015

Symmetry constraints on the elastoresistivity tensor

The elastoresistivity tensor $m_{ij,kl}$ characterizes changes in a material's resistivity due to strain. As a fourth-rank tensor, elastoresistivity can be a uniquely useful probe of the symmetries and character of the electronic state of a solid. We present a symmetry analysis of $m_{ij,kl}$ (both in the presence and absence of a magnetic field) based on the crystalline point group, focusing for pedagogic purposes on the $D_{4h}$ point group (of relevance to several materials of current interest). We also discuss the relation between $m_{ij,kl}$ and various thermodynamic susceptibilities, particularly where they are sensitive to critical fluctuations proximate to a critical point at which a point group symmetry is spontaneously broken.

cond-mat.str-el↗

arXiv2015

Transfer of spectral weight across the gap of Sr2IrO4 induced by La doping

We study with Angle Resolved PhotoElectron Spectroscopy (ARPES) the evolution of the electronic structure of Sr2IrO4, when holes or electrons are introduced, through Rh or La substitutions. At low dopings, the added carriers occupy the first available states, at bottom or top of the gap, revealing an anisotropic gap of 0.7eV in good agreement with STM measurements. At further doping, we observe a reduction of the gap and a transfer of spectral weight across the gap, although the quasiparticle weight remains very small. We discuss the origin of the in-gap spectral weight as a local distribution of gap values.

cond-mat.str-el↗

arXiv2015

Elastoconductivity as a probe of broken mirror symmetries

We propose the possible detection of broken mirror symmetries in correlated two-dimensional materials by elastotransport measurements. Using linear response theory we calculate the shearconductivity $Γ_{xx,xy}$, defined as the linear change of the longitudinal conductivity $σ_{xx}$ due to a shear strain $ε_{xy}$. This quantity can only be non-vanishing when in-plane mirror symmetries are broken and we discuss how candidate states in the cuprate pseudogap regime (e.g. various loop current or charge orders) may exhibit a finite shearconductivity. We also provide a realistic experimental protocol for detecting such a response.

cond-mat.str-el↗

arXiv2015

Wave vector dependent electron-phonon coupling drives charge-density-wave formation in TbTe3

We present a high energy-resolution inelastic x-ray scattering investigation of the soft phonon mode in the charge-density-wave system TbTe$_3$. We analyze our data based on lattice dynamical calculations using density-functional-perturbation-theory and find clear evidence that strongly momentum dependent electron-phonon-coupling defines the periodicity of the CDW superstructure: Our experiment reveals strong phonon softening and increased phonon line widths over a large part in reciprocal space adjacent to the CDW ordering vector $q_{CDW} = (0, 0, 0.3)$. Further, $q_{CDW}$ is clearly offset from the wave vector of (weak) Fermi surface nesting $q_{FS} = (0, 0, 0.25)$ and our detailed analysis indicates that electron-phonon-coupling is responsible for this shift. Hence, we can add TbTe$_3$, which was previously considered as a canonical CDW compound following the Peierls scenario, to the list of distinct charge-density-wave materials characterized by momentum dependent electron-phonon coupling.

cond-mat.str-el↗

arXiv2015

Classification of Collective Modes in a Charge Density Wave by Momentum-Dependent Modulation of the Electronic Band Structure

We present time- and angle-resolved photoemission spectroscopy measurements on the charge density wave system CeTe$_{3}$. Optical excitation transiently populates the unoccupied band structure and reveals a gap size of 2$Δ$ = 0.59 eV. The occupied Te-5p band dispersion is coherently modified by three modes at $Ω_{1}$ = 2.2 THz, $Ω_{2}$ = 2.7 THz and $Ω_{3}$ = 3 THz. All three modes lead to small rigid energy shifts whereas $Δ$ is only affected by $Ω_{1}$ and $Ω_{2}$. Their spatial polarization is analyzed by fits of a transient model dispersion and DFT frozen phonon calculations. We conclude that the modes $Ω_{1}$ and $Ω_{2}$ result from in-plane ionic lattice motions, which modulate the charge order, and that $Ω_{3}$ originates from a generic out-of-plane $A_{1g}$ phonon. We thereby demonstrate how the rich information from trARPES allows identification of collective modes and their spatial polarization, which explains the mode-dependent coupling to charge order.

cond-mat.mtrl-sci↗

arXiv2015

Pressure dependence of the charge-density-wave and superconducting states in GdTe$_3$, TbTe$_3$ and DyTe$_3$

We present electrical resistivity and ac-susceptibility measurements of GdTe$_3$, TbTe$_3$ and DyTe$_3$ performed under pressure. An upper charge-density-wave (CDW) is suppressed at a rate of $\mathrm{d}T_{\mathrm{CDW,1}}/\mathrm{d}P$ = $-$85 K/GPa. For TbTe$_3$ and DyTe$_3$, a second CDW below $T_{\mathrm{CDW,2}}$ increases with pressure until it reaches the $T_{\mathrm{CDW,1}}$($P$) line. For GdTe$_3$, the lower CDW emerges as pressure is increased above $\sim$ 1 GPa. As these two CDW states are suppressed with pressure, superconductivity (SC) appears in the three compounds at lower temperatures. Ac-susceptibility experiments performed on TbTe$_3$ provide compelling evidence for bulk SC in the low-pressure region of the phase diagram. We provide measurements of superconducting critical fields and discuss the origin of a high-pressure superconducting phase occurring above 5 GPa.

cond-mat.supr-con↗

arXiv2015

Origin of the resistive anisotropy in the electronic nematic phase of BaFe$_2$As$_2$ revealed by optical spectroscopy

We perform, as a function of uniaxial stress, an optical-reflectivity investigation of the representative 'parent' ferropnictide BaFe$_2$As$_2$ in a broad spectral range, across the tetragonal-to-orthorhombic phase transition and the onset of the long-range antiferromagnetic order (AFM). The infrared response reveals that the $dc$ transport anisotropy in the orthorhombic AFM state is determined by the interplay between the Drude spectral weight and the scattering rate, but that the dominant effect is clearly associated with the metallic spectral weight. In the paramagnetic tetragonal phase, though, the $dc$ resistivity anisotropy of strained samples is almost exclusively due to stress-induced changes in the Drude weight rather than in the scattering rate, definitively establishing the anisotropy of the Fermi surface parameters as the primary effect driving the $dc$ transport properties in the electronic nematic state.

cond-mat.supr-con↗

arXiv2014

Stripe-like nanoscale structural phase separation and optimal inhomogeneity in superconducting BaPb$_{1-x}$Bi$_x$O$_3$

Structural phase separation in the form of partially disordered stripes, with characteristic length scales in the nanometer range, is observed for superconducting BaPb$_{1-x}$Bi$_x$O$_3$. The evolution of the superconducting coherence length with composition relative to the size of these stripes suggests an important role of the nanostructure in determining the shape of the superconducting dome. It is proposed that the maximum $T_c$ is determined by a kind of "optimal inhomogeneity", characterized by a crossover from an inhomogeneous macroscopic superconductor to a granular superconductor for which phase fluctuations suppress $T_c$.

cond-mat.supr-con↗

arXiv2014

Evidence for a nematic component to the Hidden Order parameter in URu2Si2 from differential elastoresistance measurements

Measurements of the differential elastoresistance of URu$_2$Si$_2$ reveal that the fluctuations associated with the 17 K Hidden Order phase transition have a nematic component. Approaching the "Hidden Order" phase transition from above, the nematic susceptibility abruptly changes sign, indicating that while the Hidden Order phase has a nematic component, it breaks additional symmetries.

cond-mat.str-el↗

arXiv2014

Dynamic competition between spin density wave order and superconductivity in underdoped $Ba_{1-x}K_xFe_2As_2$

An intriguing aspect of unconventional superconductivity is that it always appears in the vicinity of other competing phases, whose suppression brings the full emergence of superconductivity. In the iron-pnictides, these competing phases are marked by a tetragonal-to-orthorhombic structural transition and a collinear spin-density-wave (SDW) transition. There has been macroscopic evidence for competition between these phases and superconductivity as the magnitude of both the orthorhombicity and magnetic moment are suppressed in the superconducting state. Here, using angle-resolved photoemission spectroscopy on detwinned underdoped $Ba_{1-x}K_xFe_2As_2$, we observe a coexistence of both the SDW gap and superconducting gap in the same electronic structure. Furthermore, our data reveal that following the onset of superconductivity, the SDW gap decreases in magnitude and shifts in a direction consistent with a reduction of the orbital anisotropy. This observation provides direct spectroscopic evidence for the dynamic competition between superconductivity and both SDW and electronic nematic orders in these materials.

cond-mat.supr-con↗

arXiv2014

Spectrally-resolved femtosecond reflectivity relaxation dynamics in undoped SDW 122-structure iron based pnictides

We systematically investigate temperature- and spectrally-dependent optical reflectivity dynamics in AAs$_{2}$Fe$_{2}$, (A=Ba, Sr and Eu), iron-based superconductors parent spin-density-wave (SDW) compounds. Two different relaxation processes are identified. The behavior of the slower process, which is strongly sensitive to the magneto-structural transition, is analyzed in the framework of the relaxation-bottleneck model involving magnons. The results are compared to recent time resolved angular photoemission results (TR-ARPES) and possible alternative assignment of the slower relaxation to the magneto-structural order parameter relaxation is discussed.

cond-mat.supr-con↗

arXiv2013

Alternative route to charge density wave formation in multiband systems

Charge and spin density waves, periodic modulations of the electron and magnetization densities, respectively, are among the most abundant and non-trivial low-temperature ordered phases in condensed matter. The ordering direction is widely believed to result from the Fermi surface topology. However, several recent studies indicate that this common view needs to be supplemented. Here, we show how an enhanced electron-lattice interaction can contribute to or even determine the selection of the ordering vector in the model charge density wave system ErTe3. Our joint experimental and theoretical study allows us to establish a relation between the selection rules of the electronic light scattering spectra and the enhanced electron-phonon coupling in the vicinity of band degeneracy points. This alternative proposal for charge density wave formation may be of general relevance for driving phase transitions into other broken-symmetry ground states, particularly in multiband systems such as the iron based superconductors.

cond-mat.str-el↗

arXiv2012

Phonon scattering in quasicrystalline i-Al_{72}Pd_{19.5}Mn_{8.5}: A study of the low-temperature thermal conductivity

We measured the thermal conductivity of an icosahedral quasicrystal i-Al_{72}Pd_{19.5}Mn_{8.5} in the temperature range between 0.4 K and 300 K. The analysis of the low-temperature results was based on a Debye-type model. The results of the analysis for the two temperature regions of 0.4 K <T<40 K and 0.4 K <T<1 K, are not consistent in the sense that a tunnelling-states contribution to phonon scattering is verified only for 0.4 K <T<1 K. The same fitting procedure indicates that structural defects of the stacking-fault type are an important source of phonon scattering. Their physical presence was cleary identified by a transmission electron microscopy experiment.

cond-mat↗