arXiv subjects

G. D. Gu

Publications and source records attributed to G. D. Gu.

At least 37 records · Page 2Linked to original sources

arXiv2023

Perpendicular in-plane negative magnetoresistance in ZrTe5

The unique band structure in topological materials frequently results in unusual magneto-transport phenomena, one of which is in-plane longitudinal negative magnetoresistance (NMR) with the magnetic field aligned parallel to the electrical current direction. This NMR is widely considered as a hallmark of chiral anomaly in topological materials. Here we report the observation of in-plane NMR in the topological material ZrTe5 when the in-plane magnetic field is both parallel and perpendicular to the current direction, revealing an unusual case of quantum transport beyond the chiral anomaly. We find that a general theoretical model, which considers the combined effect of Berry curvature and orbital moment, can quantitatively explain this in-plane NMR. Our results provide new insights into the understanding of in-plane NMR in topological materials.

cond-mat.mes-hall↗

arXiv2023

Spin canting and lattice symmetry in La$_2$CuO$_4$

While the dominant magnetic interaction in La$_2$CuO$_4$ is superexchange between nearest-neighbor Cu moments, the pinning of the spin direction depends on weak anisotropic effects associated with spin-orbit coupling. The symmetry of the octahedral tilt pattern allows an out-of-plane canting of the Cu spins, which is compensated by an opposite canting in nearest-neighbor layers. A strong magnetic field applied perpendicular to the planes can alter the spin canting pattern to induce a weak ferromagnetic phase. In light of recent evidence that the lattice symmetry is lower than originally assumed, we take a new look at the nature of the field-induced spin-rotation transition. Comparing low-temperature neutron diffraction intensities for several magnetic Bragg peaks measured in fields of 0 and 14 T, we find that a better fit is provided by a model in which spins rotate within both neighboring planes but by different amounts, resulting in a noncollinear configuration. This model allows a more consistent relationship between lattice symmetry and spin orientation at all Cu sites.

cond-mat.str-el↗

arXiv2023

Structure of Charge Density Waves in La$_{1.875}$Ba$_{0.125}$CuO$_4$

Although charge-density wave (CDW) correlations exist in several families of cuprate supercon-ductors, they exhibit substantial variation in CDW wavevector and correlation length, indicating a key role for CDW-lattice interactions. We investigated this interaction in La$_{1.875}$Ba$_{0.125}$CuO$_4$ using single crystal x-ray diffraction to collect a large number of CDW peak intensities, and determined the Cu and La/Ba atomic distortions induced by the formation of CDW order. Within the CuO$_2$ planes, the distortions involve a periodic modulation of the Cu-Cu spacing along the direction of the ordering wave vector. The charge ordering within the copper-oxygen layer induces an out-of-plane breathing modulation of the surrounding lanthanum layers, which leads to a related distortion on the adjacent copper-oxygen layer. Our result implies that the CDW-related structural distortions do not remain confined to a single layer but rather propagate an appreciable distance through the crystal. This leads to overlapping structural modulations, in which CuO$_2$ planes exhibit distortions arising from the orthogonal CDWs in adjacent layers as well as distortions from the CDW within the layer itself. We attribute this striking effect to the weak c-axis charge screening in cuprates and suggest this effect could help couple the CDW between adjacent planes in the crystal.

cond-mat.supr-con↗

arXiv2023

Designing the stripe-ordered cuprate phase diagram through uniaxial-stress

The ability to efficiently control charge and spin in the cuprate high-temperature superconductors is crucial for fundamental research and underpins technological development. Here, we explore the tunability of magnetism, superconductivity and crystal structure in the stripe phase of the cuprate La_2-xBa_xCuO_4, with x = 0.115 and 0.135, by employing temperature-dependent (down to 400 mK) muon-spin rotation and AC susceptibility, as well as X-ray scattering experiments under compressive uniaxial stress in the CuO_2 plane. A sixfold increase of the 3-dimensional (3D) superconducting critical temperature T_c and a full recovery of the 3D phase coherence is observed in both samples with the application of extremely low uniaxial stress of 0.1 GPa. This finding demonstrates the removal of the well-known 1/8-anomaly of cuprates by uniaxial stress. On the other hand, the spin-stripe order temperature as well as the magnetic fraction at 400 mK show only a modest decrease under stress. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. However, a substantial decrease of the magnetic volume fraction and a full suppression of the low-temperature tetragonal structure is found at elevated temperatures, which is a necessary condition for the development of the 3D superconducting phase with optimal T_c. Our results evidence a remarkable cooperation between the long-range static spin-stripe order and the underlying crystalline order with the three-dimensional fully coherent superconductivity. Overall, these results suggest that the stripe- and the SC order may have a common physical mechanism.

cond-mat.supr-con↗

arXiv2023

Low-energy quasi-circular electron correlations with charge order wavelength in $\textrm{Bi}_2\textrm{Sr}_2\textrm{Ca}\textrm{Cu}_2\textrm{O}_{8+δ}$

In the study of dynamic charge order correlations in the cuprates, most high energy-resolution resonant inelastic x-ray scattering (RIXS) measurements have focused on momenta along the high-symmetry directions of the copper oxide plane. However, electron scattering along other in-plane directions should not be neglected as they may contain information relevant, for example, to the origin of charge order correlations or to our understanding of the isotropic scattering responsible for strange metal behavior in cuprates. We report high-resolution resonant inelastic x-ray scattering (RIXS) experiments that reveal the presence of dynamic electron correlations over the $q_x$-$q_y$ scattering plane in underdoped $\textrm{Bi}_2\textrm{Sr}_2\textrm{Ca}\textrm{Cu}_2\textrm{O}_{8+δ}$ with $T_c=54$ K. We use the softening of the RIXS-measured bond stretching phonon line as a marker for the presence of charge-order-related dynamic electron correlations. The experiments show that these dynamic correlations exist at energies below approximately $70$ meV and are centered around a quasi-circular manifold in the $q_x$-$q_y$ scattering plane with radius equal to the magnitude of the charge order wave vector, $q_{CO}$. We also demonstrate how this phonon-tracking procedure provides the necessary experimental precision to rule out fluctuations of short-range directional charge order (i.e. centered around $[q_x=\pm q_{CO}, q_y=0]$ and $[q_x=0, q_y=\pm q_{CO}]$) as the origin of the observed correlations.

cond-mat.str-el↗

arXiv2023

Temperature dependence of the energy band gap in ZrTe$_5$: implications for the topological phase

Using Landau level spectroscopy, we determine the temperature dependence of the energy band gap in zirconium pentatelluride (ZrTe$_5$). We find that the band gap reaches $E_g=(5 \pm 1)$ meV at low temperatures and increases monotonously when the temperature is raised. This implies that ZrTe$_5$ is a weak topological insulator, with non-inverted ordering of electronic bands in the center of the Brillouin zone. Our magneto-transport experiments performed in parallel show that the resistivity anomaly in ZrTe$_5$ is not connected with the temperature dependence of the band gap.

cond-mat.mes-hall↗

arXiv2022

Strongly-overdoped La$_{2-x}$Sr$_x$CuO$_4$: Evidence for Josephson-coupled grains of strongly-correlated superconductor

The interpretation of how superconductivity disappears in cuprates at large hole doping has been controversial. To address this issue, we present an experimental study of single-crystal and thin film samples of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) with $x\ge0.25$. In particular, measurements of bulk susceptibility on LSCO crystals with $x=0.25$ indicate an onset of diamagnetism at $T_{c1}=38.5$ K, with a sharp transition to a phase with full bulk shielding at $T_{c2}=18$ K, independent of field direction. Strikingly, the in-plane resistivity only goes to zero at $T_{c2}$. Inelastic neutron scattering on $x=0.25$ crystals confirms the presence of low-energy incommensurate magnetic excitations with reduced strength compared to lower doping levels. The ratio of the spin gap to $T_{c2}$ is anomalously large. Our results are consistent with a theoretical prediction for strongly overdoped cuprates by Spivak, Oreto, and Kivelson, in which superconductivity initially develops within disconnected self-organized grains characterized by a reduced hole concentration, with bulk superconductivity occurring only after superconductivity is induced by proximity effect in the surrounding medium of higher hole concentration. Beyond the superconducting-to-metal transition, local differential conductance measurements on an LSCO thin film suggest that regions with pairing correlations survive, but are too dilute to support superconducting order. Future experiments will be needed to test the degree to which these results apply to overdoped cuprates in general.

cond-mat.supr-con↗

arXiv2022

Strongly correlated doped hole carriers in the superconducting nickelates: Their location, local many-body state, and low-energy effective Hamiltonian

The families of high-temperature superconductors recently welcomed a new member: hole doped nickelate Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ with a $\sim$15K transition temperature. To understand its emergent low-energy behaviors and experimental properties, an immediate key question is whether the superconducting hole carriers reside in oxygen as in the cuprates, or in nickel as in most nickelates. We answer this crucial question via a ``(LDA+$U$)+ED'' scheme: deriving an effective interacting Hamiltonian of the hole carriers from density functional LDA+$U$ calculation, and studying its local many-body states via exact diagonalization. Surprisingly, distinct from the expected Ni$^{2+}$ spin-triplet state found in most nickelates, the local ground state of two holes is actually a Ni-O spin-singlet state with second hole greatly residing in oxygen. The emerged eV-scale model therefore resembles that of the cuprates, advocating further systematic experimental comparisons. Tracing the microscopic origin of this unexpected result to the lack of apical oxygen in this material, we proposed a route to increase superconducting temperature, and a possible quantum phase transition absent in the cuprates.

cond-mat.supr-con↗

arXiv2022

Spin singlet and quasiparticles excitations in cuprate superconductors

We followed step by step the transition from an antiferromagnetic (AF) Mott insulator to a superconducting (SC) metal in the Bi$_2$Sr$_2$CaCu$_{2}$O$_{8+δ}$ (Bi-2212) cuprate using the electronic Raman scattering spectroscopy. This was achieved by tracking the doping dependence of the spin singlet excitation originate from the AF Mott insulator, the normal state quasiparticles excitation related to the mobile charge carriers and the Bogoliubov quasiparticles related to the SC gap. We show that the signature of the pseudogap phase which develops during this transition, can be interpreted as the blocking of charge carriers by the enhancement of the antiferromagnetic correlations as the temperature drops. We find that the energy scale of the pseudogap, $Δ_{\textrm{pg}}(p)$, closely follows the one of the spin singlet excitation, $Δ_{\textrm{sse}}(p)$, with doping $p$. The quasiparticles lifetime considerably increases with doping when the pseudogap collapses. We reveal that the maximum amplitude of the SC gap, $Δ_{\textrm{sc}}^{\textrm{max}}$ and the SC transition temperature \Tc are linked in an extended range of doping such as $Δ_{\textrm{sc}}^{\textrm{max}}(p) \propto Δ_{\textrm{sse}}(p)\, T_c(p)$. This relation suggests that the AF correlations play a key role in the mechanism of superconductivity.

cond-mat.supr-con↗

arXiv2022

Coherent Emission from Surface Josephson Plasmons in Striped Cuprates

The interplay between charge order and superconductivity remains one of the central themes of research in quantum materials. In the case of cuprates, the coupling between striped charge fluctuations and local electromagnetic fields is especially important, as it affects transport properties, coherence and dimensionality of superconducting correlations. Here, we study the emission of coherent terahertz radiation in single-layer cuprates of the La$_{2-x}$Ba$_x$CuO$_4$ family, for which this effect is expected to be forbidden by symmetry. We find that emission vanishes for compounds in which the stripes are quasi-static, but is activated when $c$-axis inversion symmetry is broken by incommensurate or fluctuating charge stripes, such as in La$_{1.905}$Ba$_{0.095}$CuO$_4$ and in La$_{1.845}$Ba$_{0.155}$CuO$_4$. In this case, terahertz radiation is emitted by surface Josephson plasmons, which are generally dark modes, but couple to free space electromagnetic radiation because of the stripe modulation.

cond-mat.supr-con↗

arXiv2022

Spin-selective tunneling from nanowires of the candidate topological Kondo insulator SmB6

Incorporating relativistic physics into quantum tunneling can lead to exotic behavior such as perfect transmission via Klein tunneling. Here, we probe the tunneling properties of spin-momentum locked relativistic fermions by designing and implementing a tunneling geometry that utilizes nanowires of the topological Kondo insulator candidate, SmB6. The nanowires are attached to the end of scanning tunneling microscope tips, and used to image the bicollinear stripe spin-order in the antiferromagnet Fe1.03Te with a Neel temperature of ~50 K. The antiferromagnetic stripes become invisible above 10 K concomitant with the suppression of the topological surface states. We further demonstrate that the direction of spin-polarization is tied to the tunneling direction. Our technique establishes SmB6 nanowires as ideal conduits for spin-polarized currents.

cond-mat.str-el↗

arXiv2022

Multimodal synchrotron X-ray diffraction across the superconducting transition of Sr$_{0.1}$Bi$_2$Se$_3$

In the doped topological insulator Sr$_x$Bi$_2$Se$_3$, a pronounced in-plane two-fold symmetry is observed in electronic properties below the superconducting transition temperature $T_c \sim$ 3 K, despite the three-fold symmetry of the observed $R\bar{3}m$ space group. The axis of two-fold symmetry is nominally pinned to one of three rotational equivalent directions and crystallographic strain has been proposed to be the origin of this pinning. We carried out multimodal synchrotron diffraction and resistivity measurements down to $\sim$0.68 K and in magnetic fields up to 45 kG on a single crystal of Sr$_{0.1}$Bi$_2$Se$_3$ to probe the effect of superconductivity on the crystallographic distortion. Our results indicate that there is no in-plane crystallographic distortion at the level of $1x10^{-5}$ associated with the superconducting transition. These results further support the model that the large two-fold in-plane anisotropy of superconducting properties of Sr$_x$Bi$_2$Sr$_3$ is not structural in origin but electronic, namely it is caused by a nematic superconducting order parameter of Eu symmetry.

cond-mat.supr-con↗

arXiv2022

Testing for pair-density-wave order in La$_{1.875}$Ba$_{0.125}$CuO$_4$

Charge order is commonly believed to compete with superconducting order. An intertwined form of superconducting wave function, known as pair-density-wave (PDW) order, has been proposed; however, there has been no direct evidence, theoretical or experimental, that it forms the ground state of any cuprate superconductor. As a test case, we consider \lbco\ with $x=1/8$, where charge and spin stripe orders within the CuO$_2$ planes compete with three-dimensional superconducting order. We report measurements of the superconducting critical current perpendicular to the planes in the presence of an in-plane magnetic field. The variation of the critical current with orientation of the field is inconsistent with a theoretical prediction specific to the PDW model. It appears, instead, that the orientation dependence of the critical-current density might be determined by a minority phase of $d$-wave superconductivity that is present as a consequence of doped-charge inhomogeneity.

cond-mat.supr-con↗

arXiv2022

Signatures of a magnetic-field-induced Lifshitz transition in the ultra-quantum limit of the topological semimetal ZrTe$_5$

The quantum limit (QL) of an electron liquid, realised at strong magnetic fields, has long been proposed to host a wealth of strongly correlated states of matter. Electronic states in the QL are, for example, quasi-one dimensional (1D), which implies perfectly nested Fermi surfaces prone to instabilities. Whereas the QL typically requires unreachably strong magnetic fields, the topological semimetal ZrTe$_5$ has been shown to reach the QL at fields of only a few Tesla. Here, we characterize the QL of ZrTe$_5$ at fields up to 64 T by a combination of electrical-transport and ultrasound measurements. We find that the Zeeman effect in ZrTe$_5$ enables an efficient tuning of the 1D Landau band structure with magnetic field. This results in a Lifshitz transition to a 1D Weyl regime in which perfect charge neutrality can be achieved. Since no instability-driven phase transitions destabilise the 1D electron liquid for the investigated field strengths and temperatures, our analysis establishes ZrTe$_5$ as a thoroughly understood platform for potentially inducing more exotic interaction-driven phases at lower temperatures.

cond-mat.str-el↗

arXiv2022

Evolution of magnetic stripes under uniaxial stress in La$_{1.885}$Ba$_{0.115}$CuO$_4$ studied by neutron scattering

Here we present the effect of uniaxial stress on the magnetic stripes in the cuprate system La$_{2-x}$Ba$_{x}$CuO$_4$ with $x = 0.115$, previously found to have a stress-induced enhancement in the superconducting transition temperature. By means of neutron scattering, we show that the static stripes are suppressed by stress, pointing towards a trade-off between superconductivity and static magnetism, in direct agreement with previously reported $μ$SR measurements. Additionally, we show that some of the reduced weight in the elastic channel appears to have moved to the inelastic channel. Moreover, a stress-induced momentum shift of the fluctuations towards the typical 1/8 value of commensurability is observed. These results impose a strong constraint on the theoretical interpretation of stress-enhanced superconductivity in cuprate systems.

cond-mat.supr-con↗

arXiv2022

Terahertz phase slips in striped La$_{2-x}$Ba$_x$CuO$_4$

Interlayer transport in high-$T_C$ cuprates is mediated by superconducting tunneling across the CuO$_2$ planes. For this reason, the terahertz frequency optical response is dominated by one or more Josephson plasma resonances and becomes highly nonlinear at fields for which the tunneling supercurrents approach their critical value, $I_C$. These large terahertz nonlinearities are in fact a hallmark of superconducting transport. Surprisingly, however, they have been documented in La$_{2-x}$Ba$_x$CuO$_4$ also above $T_C$ for doping values near $x=1/8$, and interpreted as an indication of superfluidity in the stripe phase. Here, Electric Field Induced Second Harmonic (EFISH) is used to study the dynamics of time-dependent interlayer voltages when La$_{2-x}$Ba$_x$CuO$_4$ is driven with large-amplitude terahertz pulses, in search of other characteristic signatures of Josephson tunnelling in the normal state. We show that this method is sensitive to the voltage anomalies associated with 2$π$ Josephson phase slips, which near $x=1/8$ are observed both below and above $T_C$. These results document a new regime of nonlinear transport that shares features of fluctuating stripes and superconducting phase dynamics.

cond-mat.supr-con↗

arXiv2021

Oscillating paramagnetic Meissner effect and Berezinskii-Kosterlitz-Thouless transition in $Bi_2Sr_2CaCu_2O_{8+δ}$ monolayer

Monolayers of a prototypical cuprate high transition-temperature ($T_C$) superconductor $Bi_2Sr_2CaCu_2O_{8+δ}$ (Bi2212) was recently found to show $T_C$ and other electronic properties similar to those of the bulk. The robustness of superconductivity in an ideal two-dimensional (2D) system was an intriguing fact that defied the Mermin-Wagner theorem. Here, we took advantage of the high sensitivity of scanning SQUID susceptometry to image the phase stiffness throughout the phase transition of Bi2212 in the 2D limit. We found susceptibility oscillated with flux between diamagnetism and paramagnetism in a Fraunhofer-like pattern up till $T_C$. The temperature and sample size-dependence of the modulation period agreed well with our Coulomb gas analogy of a finite 2D system based on Berezinskii-Kosterlitz-Thouless (BKT) transition. In the multilayers, the susceptibility oscillation differed in a small temperature regime below $T_C$ in consistent with a dimensional-crossover led by interlayer coupling. Serving as strong evidence of BKT transition in the bulk, there appeared a sharp superfluid density jump at zero-field and paramagnetism at small fields just below $T_C$. These results unified the phase transitions from the monolayer Bi2212 to the bulk as BKT transition with finite interlayer coupling. This elucidating picture favored the pre-formed pairs scenario for the underdoped cuprates regardless of lattice dimensionality.

cond-mat.supr-con↗

arXiv2021

Crossover behavior in the magnetoresistance of thin flakes of the topological material ZrTe5

ZrTe5 is a layered material that exhibits intricate topological effects. Intensive theoretically and experimental efforts have been devoted to try to understand the physics in this materials. In this paper the temperature dependent magneto-transport properties of ZrTe5 thin flakes are investigated. A characteristic temperature T* is observed in the temperature dependence of three different types of magnetoresistance simultaneously, which are the saturated Hall anomaly, the chiral anomaly and the longitudinal magnetoresistance. Furthermore, the value of T* decreases monotonically from 200K to 160K with increasing thickness of the ZrTe5 thin flakes from 42nm to 89nm. Temperature induced topological phase transitions are attributed to the cause of such anomaly in the three types of magnetoresistance at T*. Our findings provide a multi-parameter indicator for the emergence of topological phase transition in ZrTe5 and could be extended to the study of other topological materials. The temperature dependence of the three types of magnetoresistance also shed light on the role of anomalous Hall Effect in the transport properties of ZrTe5.

cond-mat.mtrl-sci↗