arXiv subjects

Z. -X. Shen

Publications and source records attributed to Z. -X. Shen.

At least 37 records · Page 2Linked to original sources

arXiv2017

3D nature of ZrTe$_5$ band structure measured by high-momentum-resolution photoemission spectroscopy

We have performed a systematic high-momentum-resolution photoemission study on ZrTe$_5$ using $6$ eV photon energy. We have measured the band structure near the $Γ$ point, and quantified the gap between the conduction and valence band as $18 \leq Δ\leq 29$ meV. We have also observed photon-energy-dependent behavior attributed to final-state effects and the 3D nature of the material's band structure. Our interpretation indicates the gap is intrinsic and reconciles discrepancies on the existence of a topological surface state reported by different studies. The existence of a gap suggests that ZrTe$_5$ is not a 3D strong topological insulator nor a 3D Dirac semimetal. Therefore, our experiment is consistent with ZrTe$_5$ being a 3D weak topological insulator.

cond-mat.mtrl-sci↗

arXiv2016

Unusual nodal behaviors of the superconducting gap in the iron-based superconductor Ba(Fe$_{0.65}$Ru$_{0.35}$)$_2$As$_2$: Effects of spin-orbit coupling

We have investigated the superconducting gap of optimally doped Ba(Fe$_{0.65}$Ru$_{0.35}$)$_2$As$_2$ by angle-resolved photoemission spectroscopy (APRES) using bulk-sensitive 7 eV laser and synchrotron radiation. It was found that the gap is isotropic in the $k_x$-$k_y$ plane both on the electron and hole Fermi surfaces (FSs). The gap magnitudes of two resolved hole FSs show similar $k_z$ dependences and decrease as $k_z$ approaches $\sim$ 2$π$/$c$ (i.e., around the Z point) unlike the other Fe-based superconductors reported so far, where the superconducting gap of only one hole FS shows a strong $k_z$ dependence. This unique gap structure can be understood in the scenario that the $d_{z^2}$ orbital character is mixed into both hole FSs due to the finite spin-orbit coupling between almost degenerate FSs and is reproduced by calculations within the random phase approximation including the spin-orbit coupling.

cond-mat.supr-con↗

arXiv2016

Origin of the low critical observing temperature of the quantum anomalous Hall effect in V-doped (Bi, Sb)2Te3 film

The experimental realization of the quantum anomalous Hall (QAH) effect in magnetically-doped (Bi, Sb)2Te3 films stands out as a landmark of modern condensed matter physics. However, ultra-low temperatures down to few tens of mK are needed to reach the quantization of Hall resistance, which is two orders of magnitude lower than the ferromagnetic phase transition temperature of the films. Here, we systematically study the band structure of V-doped (Bi, Sb)2Te3 thin films by angle-resolved photoemission spectroscopy (ARPES) and show unambiguously that the bulk valence band (BVB) maximum lies higher in energy than the surface state Dirac point. Our results demonstrate clear evidence that localization of BVB carriers plays an active role and can account for the temperature discrepancy.

cond-mat.mes-hall↗

arXiv2016

Distinct Electronic Structure for the Extreme Magnetoresistance in YSb

An extreme magnetoresistance (XMR) has recently been observed in several non-magnetic semimetals. Increasing experimental and theoretical evidence indicates that the XMR can be driven by either topological protection or electron-hole compensation. Here, by investigating the electronic structure of a XMR material, YSb, we present spectroscopic evidence for a special case which lacks topological protection and perfect electron-hole compensation. Further investigations reveal that a cooperative action of a substantial difference between electron and hole mobility and a moderate carrier compensation might contribute to the XMR in YSb.

cond-mat.str-el↗

arXiv2016

Ideal charge density wave order in the high-field state of superconducting YBCO

The existence of charge density wave (CDW) correlations in cuprate superconductors has now been established. However, the nature of the ground state order has remained uncertain because disorder and the presence of superconductivity typically limit the CDW correlation lengths to a dozen unit cells or less. Here we explore the CDW correlations in YBa2Cu3Ox (YBCO) ortho-II and ortho-VIII crystals, which belong to the cleanest available cuprate family, at magnetic fields in excess of the resistive upper critical field (Hc2) where the superconductivity is heavily suppressed. We find an incommensurate, unidirectional CDW with a well-defined onset at a critical field strength that is proportional to Hc2. It is related to but distinct from the short-range bidirectional CDW that exists at zero magnetic field. The unidirectional CDW possesses a long inplane correlation length as well as significant correlations between neighboring CuO2 planes, yielding a correlation volume that is at least 2 - 3 orders of magnitude larger than that of the zero-field CDW. This is by far the largest CDW correlation volume observed in any cuprate crystal and so is presumably representative of the high-field ground-state of an "ideal" disorder-free cuprate.

cond-mat.str-el↗

arXiv2016

Directly characterizing the relative strength and momentum dependence of electron-phonon coupling using resonant inelastic x-ray scattering

The coupling between lattice and charge degrees of freedom in condensed matter materials is ubiquitous and can often result in interesting properties and ordered phases, including conventional superconductivity, charge density wave order, and metal-insulator transitions. Angle-resolved photoemission spectroscopy and both neutron and non-resonant x-ray scattering serve as effective probes for determining the behavior of appropriate, individual degrees of freedom -- the electronic structure and lattice excitation, or phonon dispersion, respectively. However, each provides less direct information about the mutual coupling between the degrees of freedom, usual through self-energy effects, which tend to renormalize and broaden spectral features precisely where the coupling is strong, impacting ones ability to quantitively characterize the coupling. Here we demonstrate that resonant inelastic x-ray scattering, or RIXS, can be an effective tool to directly determine the relative strength and momentum dependence of the electron-phonon coupling in condensed matter systems. Using a diagrammatic approach for an 8-band model of copper oxides, we study the contributions from the lowest order diagrams to the full RIXS intensity for a realistic scattering geometry, accounting for matrix element effects in the scattering cross-section as well as the momentum dependence of the electron-phonon coupling vertex. A detailed examination of these maps offers a unique perspective into the characteristics of electron-phonon coupling, which complements both neutron and non-resonant x-ray scattering, as well as Raman and infrared conductivity.

cond-mat.str-el↗

arXiv2016

Observation of charge density wave order in 1D mirror twin boundaries of single-layer MoSe2

Properties of two-dimensional transition metal dichalcogenides are highly sensitive to the presence of defects in the crystal structure. A detailed understanding of defect structure may lead to control of material properties through defect engineering. Here we provide direct evidence for the existence of isolated, one-dimensional charge density waves at mirror twin boundaries in single-layer MoSe2. Our low-temperature scanning tunneling microscopy/spectroscopy measurements reveal a substantial bandgap of 60 - 140 meV opening at the Fermi level in the otherwise one dimensional metallic structure. We find an energy-dependent periodic modulation in the density of states along the mirror twin boundary, with a wavelength of approximately three lattice constants. The modulations in the density of states above and below the Fermi level are spatially out of phase, consistent with charge density wave order. In addition to the electronic characterization, we determine the atomic structure and bonding configuration of the one-dimensional mirror twin boundary by means of high-resolution non-contact atomic force microscopy. Density functional theory calculations reproduce both the gap opening and the modulations of the density of states.

cond-mat.mes-hall↗

arXiv2015

Superconducting gap anisotropy in monolayer FeSe thin film

Fermi surface topology and pairing symmetry are two pivotal characteristics of a superconductor. Superconductivity in one monolayer (1ML) FeSe thin film has attracted great interest recently due to its intriguing interfacial properties and possibly high superconducting transition temperature (Tc) over 77 K. Here, we report high-resolution measurements of the Fermi surface and superconducting gaps in 1ML FeSe using angle-resolved photoemission spectroscopy (ARPES). Two ellipse-like electron pockets are clearly resolved overlapping with each other at the Brillouin zone corner. The superconducting gap is nodeless but moderately anisotropic, which put strong constraints on determining the pairing symmetry. The gap maxima locate along the major axis of ellipse, which cannot be explained by a single d-wave, extended s-wave, or s$\pm$ gap function. Four gap minima are observed at the intersection of electron pockets suggesting the existence of either a sign change or orbital-dependent pairing in 1ML FeSe.

cond-mat.supr-con↗

arXiv2015

Bandwidth and Electron Correlation-Tuned Superconductivity in Rb$_{0.8}$Fe$_{2}$(Se$_{1-z}$S$_z$)$_2$

We present a systematic angle-resolved photoemission spectroscopy study of the substitution-dependence of the electronic structure of Rb$_{0.8}$Fe$_{2}$(Se$_{1-z}$S$_z$)$_2$ (z = 0, 0.5, 1), where superconductivity is continuously suppressed into a metallic phase. Going from the non-superconducting Rb$_{0.8}$Fe$_{2}$(Se$_{1-z}$S$_z$)$_2$ to superconducting Rb$_{0.8}$Fe$_{2}$Se$_2$, we observe little change of the Fermi surface topology, but a reduction of the overall bandwidth by a factor of 2 as well as an increase of the orbital-dependent renormalization in the $d_{xy}$ orbital. Hence for these heavily electron-doped iron chalcogenides, we have identified electron correlation as explicitly manifested in the quasiparticle bandwidth to be the important tuning parameter for superconductivity, and that moderate correlation is essential to achieving high $T_C$.

cond-mat.supr-con↗

arXiv2015

Simultaneous emergence of superconductivity, inter-pocket scattering and nematic fluctuation in potassium-coated FeSe superconductor

Superconductivity originates from pairing of electrons. Pairing channel on Fermi surface and pairing glue are thus two pivotal issues for understanding a superconductor. Recently, high-temperature superconductivity over 40 K was found in electron-doped FeSe superconductors including K$_x$Fe$_{2-y}$Se$_2$, Li$_{0.8}$Fe$_{0.2}$OHFeSe, and 1 monolayer FeSe thin film. However, their pairing mechanism remains controversial. Here, we studied the systematic evolution of electronic structure in potassium-coated FeSe single crystal. The doping level is controlled precisely by in situ evaporating potassium onto the sample surface. We found that the superconductivity emerges when the inter-pocket scattering between two electron pockets is turned on by a Lifshitz transition of Fermi surface. The nematic order suppresses remarkably at the same doping and strong nematic fluctuation remains in a wide doping range of the phase diagram. Our results suggest an underlying correlation among superconductivity, inter-pocket scattering, and nematic fluctuation in electron-doped FeSe superconductors.

cond-mat.supr-con↗

arXiv2015

Three-Dimensional Charge Density Wave Order in YBa2Cu3O6.67 at High Magnetic Fields

Charge density wave (CDW) correlations have recently been shown to universally exist in cuprate superconductors. However, their nature at high fields inferred from nuclear magnetic resonance is distinct from that measured by x-ray scattering at zero and low fields. Here we combine a pulsed magnet with an x-ray free electron laser to characterize the CDW in YBa2Cu3O6.67 via x-ray scattering in fields up to 28 Tesla. While the zero-field CDW order, which develops below T ~ 150 K, is essentially two-dimensional, at lower temperature and beyond 15 Tesla, another three-dimensionally ordered CDW emerges. The field-induced CDW onsets around the zero-field superconducting transition temperature, yet the incommensurate in-plane ordering vector is field-independent. This implies that the two forms of CDW and high-temperature superconductivity are intimately linked.

cond-mat.str-el↗

arXiv2015

Se content $x$ dependence of electron correlation strength in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$

The iron chalcogenide Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ on the Te-rich side is known to exhibit the strongest electron correlations among the Fe-based superconductors, and is non-superconducting for $x$ < 0.1. In order to understand the origin of such behaviors, we have performed ARPES studies of Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ ($x$ = 0, 0.1, 0.2, and 0.4). The obtained mass renormalization factors for different energy bands are qualitatively consistent with DFT + DMFT calculations. Our results provide evidence for strong orbital dependence of mass renormalization, and systematic data which help us to resolve inconsistencies with other experimental data. The unusually strong orbital dependence of mass renormalization in Te-rich Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ arises from the dominant contribution to the Fermi surface of the $d_{xy}$ band, which is the most strongly correlated and may contribute to the suppression of superconductivity.

cond-mat.supr-con↗

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

Interfacial mode coupling as the origin of the enhancement of Tc in FeSe films on SrTiO3

Single unit cell films of iron selenide (1UC FeSe) grown on SrTiO3 (STO) substrates have recently shown superconducting energy gaps opening at temperatures close to the boiling point of liquid nitrogen (77 K), a record for iron-based superconductors. Towards understanding why Cooper pairs form at such high temperatures, a primary question to address is the role, if any, of the STO substrate. Here, we report high resolution angle resolved photoemission spectroscopy (ARPES) results which reveal an unexpected and unique characteristic of the 1UC FeSe/STO system: shake-off bands suggesting the presence of bosonic modes, most likely oxygen optical phonons in STO, which couple to the FeSe electrons with only small momentum transfer. Such coupling has the unusual benefit of helping superconductivity in most channels, including those mediated by spin fluctuations. Our calculations suggest such coupling is responsible for raising the superconducting gap opening temperature in 1UC FeSe/STO. This discovery suggests a pathway to engineer high temperature superconductors.

cond-mat.supr-con↗

arXiv2015

Magnetic excitations and phonons simultaneously studied by resonant inelastic x-ray scattering in optimally doped Bi$_{1.5}$Pb$_{0.55}$Sr$_{1.6}$La$_{0.4}$CuO$_{6+δ}$

Magnetic excitations in the optimally doped high-$T_\mathrm{c}$ superconductor Bi$_{1.5}$Pb$_{0.55}$Sr$_{1.6}$La$_{0.4}$CuO$_{6+δ}$ (OP-Bi2201, $T_\mathrm{c}\simeq 34$ K) are investigated by Cu $L_3$ edge resonant inelastic x-ray scattering (RIXS), below and above the pseudogap opening temperature. At both temperatures the broad spectral distribution disperses along the (1,0) direction up to $\sim$350~meV at zone boundary, similarly to other hole-doped cuprates. However, above $\sim$0.22 reciprocal lattice units, we observe a concurrent intensity decrease for magnetic excitations and quasi-elastic signals with weak temperature dependence. This anomaly seems to indicate a coupling between magnetic, lattice and charge modes in this compound. We also compare the magnetic excitation spectra near the anti-nodal zone boundary in the single layer OP-Bi2201 and in the bi-layer optimally doped Bi$_{1.5}$Pb$_{0.6}$Sr$_{1.54}$CaCu$_2$O$_{8+δ}$ (OP-Bi2212, $T_\mathrm{c}\simeq96$ K). The strong similarities in the paramagnon dispersion and in their energy at zone boundary indicate that the strength of the super-exchange interaction and the short-range magnetic correlation cannot be directly related to $T_\mathrm{c}$, not even within the same family of cuprates.

cond-mat.supr-con↗

arXiv2015

Distinctive momentum dependence of the band reconstruction in the nematic state of FeSe thin film

Nematic state, where the system is translationally invariant but breaks the rotational symmetry, has drawn great attentions recently due to experimental observations of such a state in both cuprates and iron-based superconductors. The mechanism of nematicity that is likely tied to the pairing mechanism of high-Tc, however, still remains controversial. Here, we studied the electronic structure of multilayer FeSe film by angle-resolved photoemission spectroscopy (ARPES). We found that the FeSe film enters the nematic state around 125 K, while the electronic signature of long range magnetic order has not been observed down to 20K indicating the non-magnetic origin of the nematicity. The band reconstruction in the nematic state is characterized by the splitting of the dxz and dyz bands. More intriguingly, such energy splitting is strong momentum dependent with the largest band splitting of ~80meV at the zone corner. The simple on-site ferro-orbital ordering is insufficient to reproduce the nontrivial momentum dependence of the band reconstruction. Instead, our results suggest that the nearest-neighbor hopping of dxz and dyz is highly anisotropic in the nematic state, the origin of which holds the key in understanding the nematicity in iron-based superconductors.

cond-mat.supr-con↗

arXiv2014

Direct characterization of photo-induced lattice dynamics in BaFe2As2

Ultrafast light pulses can modify the electronic properties of quantum materials by perturbing the underlying, intertwined degrees of freedom. In particular, iron-based superconductors exhibit a strong coupling among electronic nematic fluctuations, spins, and the lattice, serving as a playground for ultrafast manipulation. Here we use time-resolved x-ray scattering to measure the lattice dynamics of photo-excited BaFe2As2. Upon optical excitation, no signature of an ultrafast change of the crystal symmetry is observed, but the lattice oscillates rapidly in time due to the coherent excitation of an A1g mode that modulates the Fe-As-Fe bond angle. We directly quantify the coherent lattice dynamics and show that even a small photo-induced lattice distortion can induce notable changes in the electronic and magnetic properties. Our analysis implies that transient structural modification can generally be an effective tool for manipulating the electronic properties of multi-orbital systems, where electronic instabilities are sensitive to the orbital character of bands near the Fermi level.

cond-mat.str-el↗

arXiv2014

Direct observation of bulk charge modulations in optimally-doped Bi$_{1.5}$Pb$_{0.6}$Sr$_{1.54}$CaCu$_{2}$O$_{8+δ}$

Bulk charge density modulations, recently observed in high critical-temperature ($T_\mathrm{c}$) cuprate superconductors, coexist with the so-called pseudogap and compete with superconductivity. However, its direct observation has been limited to a narrow doping region in the underdoped regime. Using energy-resolved resonant x-ray scattering we have found evidence for such bulk charge modulations, or soft collective charge modes (soft CCMs), in optimally doped Bi$_{1.5}$Pb$_{0.6}$Sr$_{1.54}$CaCu$_{2}$O$_{8+δ}$ (Pb-Bi2212) around the summit of the superconducting dome with momentum transfer $q_{\parallel}\sim0.28$ reciprocal lattice units (r.l.u.) along the Cu-O bond direction. The signal is stronger at $T\simeq T_\mathrm{c}$ than at lower temperatures, thereby confirming a competition between soft CCMs and superconductivity. These results demonstrate that soft CCMs are not constrained to the underdoped regime, suggesting that soft CCMs appear across a large part of the phase diagram of cuprates and are intimately entangled with high-$T_\mathrm{c}$ superconductivity.

cond-mat.supr-con↗