arXiv subjects

Genda Gu

Publications and source records attributed to Genda Gu.

At least 127 records · Page 7Linked to original sources

arXiv2017

Direct observation of self energy signatures of the resonant collective mode in Bi2Sr2CaCu2O8

We use high resolution ARPES to study the resonant, collective excitation mode in the superconducting state of Bi2212. By collecting very high quality data we found new features in the self energy in the antinodal region, where the interaction of electrons with the mode is the strongest. This interaction leads to pronounced peak in the scattering rate and we demonstrate that this feature is directly responsible for well known hump-dip-peak structure in cuprates. By studying how the weight of this peak changes with temperature we unequivocally demonstrate that interaction of electrons with resonant mode in cuprates vanishes at Tc and is very much localized in the momentum space close to the antinode. These findings present a consistent picture of line shape and self energy signatures of the electron-boson coupling in cuprates and resolve long standing controversy surrounding this issue. The momentum dependence of the strength of electron-mode interaction enables development of quantitative theory of this phenomenon in cuprates.

cond-mat.supr-con↗

arXiv2017

Indium substitution effect on the topological crystalline insulator family (Pb$_{1-x}$Sn$_{x}$)$_{1-y}$In$_{y}$Te: Topological and superconducting properties

Topological crystalline insulators (TCIs) have been of great interest in the area of condensed matter physics. We investigated the effect of indium substitution on the crystal structure and transport properties in the TCI system (Pb$_{1-x}$Sn$_{x}$)$_{1-y}$In$_{y}$Te. For samples with a tin concentration $x\le50\%$, the low-temperature resisitivities show a dramatic variation as a function of indium concentration: with up to ~2% indium doping the samples show weak-metallic behavior, similar to their parent compounds; with ~6% indium doping, samples have true bulk-insulating resistivity and present evidence for nontrivial topological surface states; with higher indium doping levels, superconductivity was observed, with a transition temperature, Tc, positively correlated to the indium concentration and reaching as high as 4.7 K. We address this issue from the view of bulk electronic structure modified by the indium-induced impurity level that pins the Fermi level. The current work summarizes the indium substitution effect on (Pb,Sn)Te, and discusses the topological and superconducting aspects, which can be provide guidance for future studies on this and related systems.

cond-mat.supr-con↗

arXiv2017

Lifshitz transition mediated electronic transport anomaly in bulk ZrTe5

Zirconium pentatelluride ZrTe$_5$, a fascinating topological material platform, hosts exotic chiral fermions in its highly anisotropic three-dimensional Dirac band and holds great promise advancing the next-generation information technology. However, the origin underlying its anomalous resistivity peak has been under debate for decades. Here we provide transport evidence substantiating the anomaly to be a direct manifestation of a Lifshitz transition in the Dirac band with an ultrahigh carrier mobility exceeding 3$\times$10$^5$ cm$^2$ V$^{-1}$ s$^{-1}$. We demonstrate that the Lifshitz transition is readily controllable by means of carrier doping, which sets the anomaly peak temperature $T_p$. $T_p$ is found to scale approximately as $n_H^{0.27}$, where the Hall carrier concentration $n_H$ is linked with the Fermi level by $ε_F$ $\propto$ $n_H^{1/3}$ in a linearly dispersed Dirac band. This relation indicates $T_p$ monotonically increases with $ε_F$, which serves as an effective knob for fine tuning transport properties in pentatelluride-based Dirac semimetals.

cond-mat.mtrl-sci↗

arXiv2016

Polarized neutron scattering on HYSPEC: the HYbrid SPECtrometer at SNS

We describe some of the first polarized neutron scattering measurements performed at HYSPEC spectrometer at the Spallation Neutron Source, Oak Ridge National Laboratory. We discuss details of the instrument setup and the experimental procedures in the mode with full polarization analysis. Examples of polarized neutron diffraction and polarized inelastic neutron data obtained on single crystal samples are presented.

cond-mat.str-el↗

arXiv2016

Superconductivity with two-fold symmetry in topological superconductor SrxBi2Se3

Topological superconductivity is the quantum condensate of paired electrons with an odd parity of the pairing function. One of the candidates is the triplet pairing superconductor derived from topological insulator Bi2Se3 by chemical doping. Theoretically it was predicted that a two-fold superconducting order parameter may exist in this kind of bulk topological superconductor. Earlier experiments by nuclear magnetic resonance and angle-resolved magnetocaloric measurements seem to support this picture. By using a Corbino-shape like electrode configuration, we measure the c-axis resistivity without the influence of the vortex motion and find clear evidence of two-fold superconductivity in the recently discovered topological superconductor SrxBi2Se3. The Laue diffraction measurements on these samples show that the maximum gap direction is either parallel or perpendicular to the main crystallographic axis.

cond-mat.supr-con↗

arXiv2016

Combined single crystal polarized XAFS and XRD at high pressure: probing the interplay between lattice distortions and electronic order at multiple length scales in high $T_c$ cuprates

Some of the most exotic material properties derive from electronic states with short correlation length (~10-500 Å), suggesting that the local structural symmetry may play a relevant role in their behavior. Here we discuss the combined use of polarized x-ray absorption fine structure and x-ray diffraction at high pressure as a powerful method to tune and probe structural and electronic orders at multiple length scales. Besides addressing some of the technical challenges associated with such experiments, we illustrate this approach with results obtained in the cuprate La$_{1.875}$Ba$_{0.125}$CuO$_4$, in which the response of electronic order to pressure can only be understood by probing the structure at the relevant length scales.

cond-mat.mtrl-sci↗

arXiv2016

"Forbidden" phonon: dynamical signature of bond symmetry breaking in the iron chalcogenides

Investigation of the inelastic neutron scattering spectra in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ near a signature wave vector $\mathbf{Q} = (1,0,0)$ for the bond-order wave (BOW) formation of parent compound Fe$_{1+y}$Te [Phys. Rev. Lett. 112, 187202 (2014)] reveals an acoustic-phonon-like dispersion present in all structural phases. While a structural Bragg peak accompanies the mode in the low-temperature phase of Fe$_{1+y}$Te, it is absent in the high-temperature tetragonal phase, where Bragg scattering at this $\mathbf{Q}$ is forbidden by symmetry. Notably, this mode is also observed in superconducting FeTe$_{0.55}$Se$_{0.45}$, where structural and magnetic transitions are suppressed, and no BOW has been observed. The presence of this "forbidden" phonon indicates that the lattice symmetry is dynamically or locally broken by magneto-orbital BOW fluctuations, which are strongly coupled to lattice in these materials.

cond-mat.str-el↗

arXiv2016

Low vibration high numerical aperture automated variable temperature Raman microscope

Raman micro-spectroscopy is well suited for studying a variety of properties and has been applied to wide- ranging areas. Combined with tuneable temperature, Raman spectra can offer even more insights into the properties of materials. However, previous designs of variable temperature Raman microscopes have made it extremely challenging to measure samples with low signal levels due to thermal and positional instability as well as low collection efficiencies. Thus, contemporary Raman microscope has found limited applicability to probing the subtle physics involved in phase transitions and hysteresis. This paper describes a new design of a closed-cycle, Raman microscope with full polarization rotation. High collection efficiency, thermal and mechanical stability are ensured by both deliberate optical, cryogenic, and mechanical design. Measurements on two samples, Bi2Se3 and V2O3, which are known as challenging due to low thermal conductivities, low signal levels and/or hysteretic effects, are measured with previously undemonstrated temperature resolution.

cond-mat.mtrl-sci↗

arXiv2016

Thermal evolution of antiferromagnetic correlations and tetrahedral bond angles in superconducting FeTe$_{1-x}$Se$_x$

It has recently been demonstrated that dynamical magnetic correlations measured by neutron scattering in iron chalcogenides can be described with models of short-range correlations characterized by particular {choices of four-spin plaquettes, where the appropriate choice changes as the} parent material is doped towards superconductivity. Here we apply such models to describe measured maps of magnetic scattering as a function of two-dimensional wave vectors obtained for optimally superconducting crystals of FeTe$_{1-x}$Se$_x$. We show that the characteristic antiferromagnetic wave vector evolves from that of the bicollinear structure found in underdoped chalcogenides (at high temperature) to that associated with the stripe structure of antiferromagnetic iron arsenides (at low temperature); {these can both be described with the same local plaquette, but with different inter-plaquette correlations}. While the magnitude of the low-energy magnetic spectral weight is substantial at all temperatures, it actually weakens somewhat at low temperature, where the charge carriers become more itinerant. The observed change in spin correlations is correlated with the dramatic drop in the electronic scattering rate and the growth of the bulk nematic response on cooling. Finally, we also present powder neutron diffraction results for lattice parameters in FeTe$_{1-x}$Se$_x$ indicating that the tetrahedral bond angle tends to increase towards the ideal value on cooling, in agreement with the increased screening of the crystal field by more itinerant electrons and the correspondingly smaller splitting of the Fe $3d$ orbitals.

cond-mat.supr-con↗

arXiv2015

Study of magneto-transport properties and quantum oscillations in PbSe single crystals

PbSe is a low-gap semiconductor with excellent infrared photo-detection properties. Here we report our high magnetic field and low temperature electrical properties measurement performed on a moderately doped PbSe single crystals with p-type bulk carrier density of around $1\times10^{18}$ cm$^{-3}$. Longitudinal resistance (R$_{xx}$) and Hall resistance (R$_{xy}$) were simultaneously measured between 0 T--18 T at several temperatures between 0.8 K--25 K. These transport measurements start showing oscillatory behavior around and above 6 T of magnetic field. The quantum oscillation frequency is ~15 T, giving an estimate for the carrier density of each L pocket in the BZ participating in these oscillations. The effective mass of the free carriers is estimated from the temperature dependence of oscillation amplitudes. Measurements as the magnetic fields is rotated reveal the magneto-transport properties of a 3D-like fermi surface. Free carrier scattering rate from the transport data has been estimated which also gave an estimation of Dingle temperature in PbSe. Room temperature optical measurements has been conducted and other optical properties has been estimated. The Drude-Lorentz model fit has been performed which show a low frequency phonon mode around 45 cm$^{-1}$ and bandgap of around 0.2 eV along with other interband electronic transitions. Some of the optical and transport results has been compared against other members of class IV-VI chalcogenides.

cond-mat.mes-hall↗

arXiv2015

Kondo-like zero-bias conductance anomaly in a three-dimensional topological insulator nanowire

Zero bias anomalies in topological nanowires have recently captured significant attention, as they are possible signatures of Majorana modes. Yet there are many other possible origins of zero bias peaks in nanowires, for example, weak localization, Andreev bound states, or the Kondo effect. Here, we discuss observations of differential conductance peaks at zero bias voltage in nonsuperconducting electronic transport through a 3D topological insulator nanowire. The zero bias conductance peaks show logarithmic temperature dependence and often linear splitting with magnetic fields, both of which are signatures of the Kondo effect in quantum dots. Here, we characterize the zero bias peaks and discuss their origin.

cond-mat.mes-hall↗

arXiv2015

Imaging atomic-scale effects of high-energy ion irradiation on superconductivity and vortex pinning in Fe(Se,Te)

Maximizing the sustainable supercurrent density, Jc, is crucial to high current applications of superconductivity and, to achieve this, preventing dissipative motion of quantized vortices is key. Irradiation of superconductors with high-energy heavy ions can be used to create nanoscale defects that act as deep pinning potentials for vortices. This approach holds unique promise for high current applications of iron-based superconductors because Jc amplification persists to much higher radiation doses than in cuprate superconductors without significantly altering the superconducting critical temperature. However, for these compounds virtually nothing is known about the atomic scale interplay of the crystal damage from the high-energy ions, the superconducting order parameter, and the vortex pinning processes. Here, we visualize the atomic-scale effects of irradiating FeSexTe1-x with 249 MeV Au ions and find two distinct effects: compact nanometer-sized regions of crystal disruption or 'columnar defects', plus a higher density of single atomic-site 'point' defects probably from secondary scattering. We show directly that the superconducting order is virtually annihilated within the former while suppressed by the latter. Simultaneous atomically-resolved images of the columnar crystal defects, the superconductivity, and the vortex configurations, then reveal how a mixed pinning landscape is created, with the strongest pinning occurring at metallic-core columnar defects and secondary pinning at clusters of pointlike defects, followed by collective pinning at higher fields.

cond-mat.supr-con↗

arXiv2015

Strong interaction between electrons and collective excitations in multiband superconductor MgB2

We use a tunable laser ARPES to study the electronic properties of the prototypical multiband BCS superconductor MgB2. Our data reveal a strong renormalization of the dispersion (kink) at ~65 meV, which is caused by coupling of electrons to the E2g phonon mode. In contrast to cuprates, the 65 meV kink in MgB2 does not change significantly across Tc. More interestingly, we observe strong coupling to a second, lower energy collective mode at binding energy of 10 meV. This excitation vanishes above Tc and is likely a signature of the elusive Leggett mode.

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

Reconciliation of local and long range tilt correlations in underdoped La2-xBaxCuO4

A long standing puzzle regarding the disparity of local and long range CuO6 octahedral tilt correlations in the underdoped regime of La2-xBaxCuO4 is addressed by utilizing complementary neutron powder diffraction and inelastic neutron scattering (INS) approaches. Long-range and static CuO6 tilt order with orthogonally inequivalent Cu-O bonds in the CuO2 planes in the low temperature tetragonal (LTT) phase is succeeded on warming through the low-temperature transition by one with orthogonally equivalent bonds in the low temperature orthorhombic (LTO) phase. In contrast, the signatures of LTT-type tilts in the instantaneous local atomic structure persist on heating throughout the LTO crystallographic phase on the nanoscale, although becoming weaker as temperature increases. Analysis of the INS spectra for the x = 1/8 composition reveals the dynamic nature of the LTT-like tilt fluctuations within the LTO phase and their 3D character. The doping dependence of relevant structural parameters indicates that the magnitude of the Cu-O bond anisotropy has a maximum at x = 1/8 doping where bulk superconductivity is most strongly suppressed, suggesting that the structural anisotropy might be influenced by electron-phonon coupling and the particular stability of the stripe-ordered phase at this composition. The bond-length modulation that pins stripe order is found to be remarkably subtle, with no anomalous bond length disorder at low temperature, placing an upper limit on any in-plane Cu-O bondlength anisotropy. The results further reveal that although appreciable octahedral tilts persist through the high-temperature transition and into the high temperature tetragonal (HTT) phase, there is no significant preference between different tilt directions in the HTT regime.

cond-mat.supr-con↗

arXiv2014

Nanoscale interplay of strain and doping in a high-temperature superconductor

The highest temperature superconductors are electronically inhomogeneous at the nanoscale, suggesting the existence of a local variable which could be harnessed to enhance the superconducting pairing. Here we report the relationship between local doping and local strain in the cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$. We use scanning tunneling microscopy to discover that the crucial oxygen dopants are periodically distributed, in correlation with local strain. Our picoscale investigation of the intra-unit-cell positions of all oxygen dopants provides essential structural input for a complete microscopic theory.

cond-mat.str-el↗

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↗