arXiv subjects

Zhuan Xu

Publications and source records attributed to Zhuan Xu.

At least 19 recordsLinked to original sources

arXiv2026

Supercurrent effect in a charge density wave intertwined superconductor

The energy-momentum (E-k) dispersion of quasiparticles constitutes a fundamental concept in condensed matter systems. The ability to modify the E-k dispersion, exemplified by supercurrent-induced Doppler shifts of Bogoliubov quasiparticle spectra in superconductors, enables manipulation of various emergent quantum properties. However, investigations into the supercurrent effect on superconductors intertwined with charge orders remain scarce. Here, we report that the Meissner current, generated by the diamagnetic response to an applied in-plane magnetic field, can tailor Bogoliubov quasiparticle excitations at the precursor charge density wave (CDW) vectors. Our scanning tunneling spectroscopic imaging reveals a field-driven symmetry breaking of CDW modulations, specifically a C3v-to-Cs transition, in superconducting NbSe2. Model calculations suggest that the observed anisotropy originates from a selective Doppler-shift-induced E-k dispersion reconstruction. Furthermore, altering the field direction enables on-demand tuning of anisotropic CDW modulations and visualization of their momentum-space distribution. These results highlight a novel mechanism for controlling emergent electronic phases through momentum-space engineering.

cond-mat.supr-con↗

arXiv2026

Dimensionality-Dependent Exciton Dispersion in a Single-Band Mott Insulator

Excitonic band structure is critical for investigating exciton dynamics. Theoretically, quantum effects from exchange scattering between electron-hole pairs significantly modulate exciton dispersion. Here, we report the direct observation of dimensionality-dependent exciton dispersion in a single-band Mott insulator Nb3Cl8 through high-resolution electron energy loss spectroscopy. In the high-temperature phase, the exciton in Nb3Cl8 hosts an exceptionally large binding energy, and exhibits clear quasi-two-dimensional massless linear dispersion. In contrast, in the low-temperature phase, the exciton splits into two bands, both displaying three-dimensional parabolic dispersion. These dramatic changes in the exciton dispersion stem from the dimensional mutation driven by a substantial enhancement of interlayer coupling across the phase transition. This Letter provides a clear and typical example of how exciton behavior evolves with dimensionality.

cond-mat.str-el↗

arXiv2023

Third order nonlinear transport properties in topological chiral antiferromagnetic semimetal CoNb3S6

The topology between Bloch states in reciprocal space has attracted tremendous attention in recent years. The quantum geometry of the band structure is composed of quantum metric as real part and berry curvature as imaginary part. While the Berry curvature, the Berry curvature dipole and Berry connection polarizability have been recently revealed by the first order anomalous hall, second order and third order nonlinear Hall effect respectively, the quantum metric induced second order nonlinear transverse and longitudinal response in topological antiferromagnetic material MnBi2Te4 was only very recently reported. Here we demonstrate the similar third order nonlinear transport properties in the topological antiferromagnetic CoNb3S6. We observed that the third order nonlinear longitudinal V3ω xx increase significantly at the antiferromagnetic transition temperature TN ~ 29 K, which was probably induced by the quantum metric without time-reversal symmetry or inversion symmetry. Besides, temperature-dependent nonlinear behaviour was observed in the first order I-V curve below the Neel temperature TN, which was not reported in MnBi2Te4 and FeSn. Such nonlinear I-V behaviour hints for the possible existence of Charge Density Wave (CDW) state, which has been discovered in its sister material FeNb3S6. Simultaneously, two plateaus in the third order nonlinear longitudinal V3ω xx~ I^ω curve are observed, which is also speculated to be related with the possible CDW state. However, the genuine mechanism for the first order nonlinear I-V and its relation with the third order nonlinear transport call for more experimental investigations and theoretical interpretation. Our work provides a way to explore third harmonic nonlinear transport and interaction with magnetic order and CDW.

cond-mat.mtrl-sci↗

arXiv2023

The Influence of Dimensionality on the Charge Density Wave Transition and Its Application on Mid-infrared Photodetection

Two-dimensional charge density wave (CDW) materials received much attention for high responsivity and broadband photodetection in recent years, due to their collective electron transport and narrow bandgap. However, the high dark current density problem hinders their real application. Here we report a sharp CDW transition in quasi-1D (TaSe4)2I, and apply it for broadband photodetection. Especially at mid-infrared region, the device shows both high photo responsivity of 1.18e3 A/W and large light on-off ratio of 80, which is superior than 2D CDW TaS2 and most reported low-dimensional materials. The fact for such high performance lies on two aspects. One is the much lower dark current density resulted from the pseudo gap associated with 1D Luttinger liquid state, which is supported by finite size scaling of nonlinear I-V at variable temperatures and occurrence of 1D structural phase transition consolidated by In-situ Raman spectroscopy. The other is the high photocurrent associated with the Frohlich superconductivity state, manifested by an ultrasensitive switching, which can be only accessible in 1D CDW materials, in agreement with our density functional theory calculation. Our work thus reveals the pivotal role of dimensionality in CDW phase transition, and paves a way for implementing highly sensitive broadband photodetector.

cond-mat.mtrl-sci↗

arXiv2022

Sign-reversed anomalous Nernst effect in the ferromagnetic Weyl-semimetal Fe$_{3-x}$GeTe$_2$: the role of Fe vacancies

Anomalous Nernst effect, as a thermal partner of anomalous Hall effect, is particularly sensitive to the Berry curvature anomaly near the Fermi level, and has been used to probe the topological nature of quantum materials. In this work, we report the observation of both effects in the ferromagnetic Weyl-semimetal Fe$_{3-x}$GeTe$_2$ with tunable Fe vacancies. With decreasing Fe vacancies, the anomalous Hall conductivity evolves as a function of the longitudinal conductivity from the hopping region to the region where the intrinsic Berry curvature contribution dominates. Concomitant evolutions in the anomalous Nernst signal and the anomalous off-diagonal thermoelectric coefficient are observed below the Curie temperature, displaying a unique sign change caused by the Fe vacancies. Combining these results with first-principles calculations, we argue that the Fe-vacancy concentration plays a unique role in simultaneously tuning the chemical potential and ferromagnetism, which in turn controls the Berry curvature contribution in this family of ferromagnetic topological semimetals.

cond-mat.mtrl-sci↗

arXiv2019

Anomalous quantum metal in a 2D crystalline superconductor with intrinsic electronic non-uniformity

The details of the superconducting to quantum metal transition (SQMT) at T=0 are an open problem that invokes much interest in the nature of this exotic and unexpected ground state1-3. However, the SQMT was not yet investigated in a crystalline 2D superconductor with coexisting and fluctuating quantum orders. Here, we report the observation of a SQMT in 2D ion-gel gated 1T-TiSe24, driven by magnetic field. A field-induced crossover between Bose quantum metal and vortex quantum creeping with increasing field is observed. We discuss the interplay between superconducting and CDW fluctuations (discommensurations) and their relation to the anomalous quantum metal (AQM) phase. From our findings, gate-tunable 1T-TiSe2 emerges as a privileged platform to scrutinize, in a controlled way, the details of the SQMT, the role of coexisting fluctuating orders and, ultimately, obtain a deeper understanding of the fate of superconductivity in strictly two-dimensional crystals near zero temperature.

cond-mat.supr-con↗

arXiv2017

Temperature and Angular Dependence of the Upper Critical Field in K$_{2}$Cr$_{3}$As$_{3}$

We report measurements of the upper critical field $H_{\mathrm{c2}}$ as functions of temperature $T$, polar angle $θ$ (of the field direction with respect to the crystallographic $c$ axis), and azimuthal angle $ϕ$ (of the field direction relative to the $a$ axis within the $ab$ plane) for the Cr-based superconductor K$_{2}$Cr$_{3}$As$_{3}$ with a quasi-one-dimensional and non-centrosymmetric crystal structure. We confirm that the anisotropy in $H_{\mathrm{c2}}(T)$ becomes inverse with decreasing temperature. At low temperatures, $H_{\mathrm{c2}}(θ)$ data are featured by two maxima at $θ$ = 0 ($\mathbf{H}\parallel c$) and $π/2$ ($\mathbf{H}\bot c$), which can be quantitatively understood only if uniaxial effective-mass anisotropy and absence of Pauli paramagnetic effect for $\mathbf{H}\bot c$ are taken simultaneously into consideration. The in-plane $H_{\mathrm{c2}}(ϕ)$ profile shows a unique threefold modulation especially at low temperatures. Overall, the characteristic of the $H_{\mathrm{c2}}(θ, ϕ, T)$ data mostly resemble those of the heavy-fermion superconductor UPt$_3$, and we argue in favor of a dominant spin-triplet superconductivity with odd parity in K$_{2}$Cr$_{3}$As$_{3}$.

cond-mat.supr-con↗

arXiv2016

Antiferromagnetic Kondo lattice compound CePt$_{3}$P

A new ternary platinum phosphide CePt$_{3}$P was synthesized and characterized by means of magnetic, thermodynamic and transport measurements. The compound crystallizes in an antiperovskite tetragonal structure similar to that in the canonical family of platinum-based superconductors $A$Pt$_{3}$P ($A$ = Sr, Ca, La) and closely related to the noncentrosymmetric heavy fermion superconductor CePt$_{3}$Si. In contrast to all the superconducting counterparts, however, no superconductivity is observed in CePt$_{3}$P down to 0.5 K. Instead, CePt$_{3}$P displays a coexistence of antiferromagnetic ordering, Kondo effect and crystalline electric field effect. A field-induced spin-flop transition is observed below the magnetic ordering temperature $T_{N1}$ of 3.0 K while the Kondo temperature is of similar magnitude as $T_{N1}$. The obtained Sommerfeld coefficient of electronic specific heat is $γ_{Ce}$ = 86 mJ/molK$^{2}$ indicating that CePt$_{3}$P is a moderately correlated antiferromagnetic Kondo lattice compound.

cond-mat.str-el↗

arXiv2016

Topological phase transition induced extreme magnetoresistance in TaSb$_{2}$

We report extremely large positive magnetoresistance of 1.72 million percent in single crystal TaSb$_{2}$ at moderate conditions of 1.5 K and 15 T. The quadratic growth of magnetoresistance (MR $\propto\,B^{1.96}$) is not saturating up to 15 T, a manifestation of nearly perfect compensation with $<0.1\%$ mismatch between electron and hole pockets in this semimetal. The compensation mechanism is confirmed by temperature-dependent MR, Hall and thermoelectric coefficients of Nernst and Seebeck, revealing two pronounced Fermi surface reconstruction processes without spontaneous symmetry breaking, \textit{i.e.} Lifshitz transitions, at around 20 K and 60 K, respectively. Using quantum oscillations of magnetoresistance and magnetic susceptibility, supported by density-functional theory calculations, we determined that the main hole Fermi surface of TaSb$_{2}$ forms a unique shoulder structure along the $F-L$ line. The flat band top of this shoulder pocket is just a few meV above the Fermi level, leading to the observed topological phase transition at 20 K when the shoulder pocket disappears. Further increase in temperature pushes the Fermi level to the band top of the main hole pocket, induced the second Lifshitz transition at 60 K when hole pocket vanishes completely.

cond-mat.str-el↗

arXiv2016

Vortex crossing, trapping and pinning in superconducting nanowires of a NbSe$_2$ two-dimensional crystal

Nanowires of two-dimensional (2D) crystals of type-II superconductor NbSe$_2$ prepared by electron-beam lithography were studied, focusing on the effect of the motion of Abrikosov vortices. We present magnetoresistance measurements on these nanowires and show features related to vortex crossing, trapping, and pinning. The vortex crossing rate was found to vary non-monotonically with the applied field, which results in non-monotonic magnetoresistance variations in agreement with theoretical calculations in the London approximation. Above the lower critical field, $H_{c1}$, the crossing rate is also influenced by vortices trapped by sample boundaries or pinning centers, leading to sample-specific magnetoresistance patterns. We show that the local pinning potential can be modified by intentionally introducing surface adsorbates, making the magnetoresistance pattern a "magneto fingerprint" of the sample-specific configuration of vortex pinning centers in a 2D crystal superconducting nanowire.

cond-mat.supr-con↗

arXiv2015

Vortex crossing and trapping in doubly connected mesoscopic loops of a single-crystal type II superconductor

Numerical calculations on a mesoscopic ring of a type II superconductor in the London limit suggest that an Abrikosov vortex can be trapped in such a structure above a critical magnetic field and generate a phase shift in the magnetoresistance oscillations. We prepared submicron-sized superconducting loops of single-crystal, type II superconductor NbSe$_2$ and measured magnetoresistance oscillations resulting from vortices crossing the loops. The free energy barrier for vortex crossing determines the crossing rate and is periodically modulated by the external magnetic flux threading the loop. We demonstrated experimentally that the crossing of vortices can be directed at a pair of constrictions in the loop, leading to more pronounced magnetoresistance oscillations than those in a uniform ring. The vortex trapping in both a simple ring and a ring featuring two constrictions was found to result in a phase shift in the magnetoresistance oscillations as predicted in the numerical calculations. The controlled crossing and trapping of vortices demonstrated in our NbSe$_2$ devices provide a starting point for the manipulation of individual Abrikosov vortices, which is useful for future technologies.

cond-mat.supr-con↗

arXiv2015

Local noncentrosymmetricity and possible spin-momentum locking in Sr$_3$Ru$_2$O$_7$

Strong spin-orbital coupling (SOC) was found previously to lead to dramatic effects in quantum materials, such as those found in topological insulators. It was shown theoretically that local noncentrosymmetricity resulting from the rotation of RuO$_6$ octahedral in Sr$_3$Ru$_2$O$_7$ will also give rise to an effective SOC\cite{SocSr327,MicroscopicnematicSr327}. In the presence of a magnetic field applied along a specific in-plane direction, the Fermi surface was predicted to undergo a reconstruction. Here we report results of our in-plane magnetoresistivity and magnetothermopower measurements on single crystals of Sr$_3$Ru$_2$O$_7$ with an electrical or a thermal current applied along specific crystalline directions and a magnetic field rotating in the $ab$ plane (Fig. 1a), showing a minimal value for field directions predicted by the local noncentrosymmetricity theory. Furthermore, the thermopower, and therefore, the electron entropy, were found to be suppressed as the field was applied perpendicular to the thermal current, which suggests that the spin and the momentum in Sr$_3$Ru$_2$O$_7$ are locked over substantial parts of the Fermi surface, likely originating from local noncentrosymmetricity as well.

cond-mat.str-el↗

arXiv2015

Coexistence of superconductivity and ferromagnetism in Sr0.5Ce0.5FBiS2

Through the combination of X-ray diffraction, electrical transport, magnetic susceptibility, and the heat capacity measurements, we studied the effect of Ce doping in the newly discovered SrFBiS${2}$ system. It is found that Sr${0.5}$Ce${0.5}$FBiS${2}$ undergoes a second-order transition below $7.5 K, followed by a superconducting transition with the critical temperature Tc=2.8 K. Our transport, specific heat and DC-magnetization results suggest the presence of bulk ferromagneticcorrelation of Ce ion below 7.5 K that coexist with superconductivity when the temperature is further lowered below 2.8 K.

cond-mat.supr-con↗

arXiv2014

Electronic phase diagram in the new BiS2-based Sr1-xLaxFBiS2 system

In this paper, we systematically study the electron doping effect in a new BiS2-based system Sr1-xLaxFBiS2(0 0.3, reaching its maximal superconducting transition temperature Tc=3.5 K at x=0.55. In the normal state of superconducting samples, it is clearly seen that there exists a crossover from metallic to semiconducting state below a temperature Tmin, which shifts to lower temperatures with increasing La content. Based on these measurements, the associated electronic phase diagram of Sr1-xLaxFBiS2 system has thus been established.

cond-mat.supr-con↗

arXiv2013

Single-crystal superconducting nanowires of NbSe$_2$ fabricated by reactive plasma etching

We present the preparation and measurements of nanowires of single-crystal NbSe$_2$. These nanowires were prepared on ultrathin ($\lesssim10\text{ nm}$) flakes of NbSe$_2$ mechanically exfoliated from a bulk single crystal using a process combining electron beam lithography and reactive plasma etching. The electrical contacts to the nanowires were prepared using Ti/Au. Our technique, which overcomes several limitations of methods developed previously for fabricating superconducting nanowires, also allows for the preparation of complex superconducting nanostructures with a desired geometry. Current-voltage characteristics of individual superconducting single-crystal nanowires with widths down to 30~nm and cross-sectional areas as low as 270 nm$^2$ were measured for the first time.

cond-mat.supr-con↗

arXiv2013

Electronic structure of relativistic Mott insulator Li$_2$RhO$_3$

Motivated by studies of coexisting electron correlation and spin-orbit coupling effect in Na$_2$IrO$_3$ and a recent experiment of its 4d analogue Li$_2$RhO$_3$, we performed first-principles calculations of the rhodium oxide compound. The experimentally observed ground state of Li$_2$RhO$_3$ can be recovered only if both spin-orbit coupling and on-site Coulomb interaction are taken into consideration. Within the proper $U$ range for 4d-orbitals ($2\leqslant U\leqslant 4$ eV), the ground state of Li$_2$RhO$_3$ could be either zigzag-AFM or stripy-AFM, both yielding energy gap close to experimental observation. Furthermore, the total energy differences between the competing magnetic phases are $\leqslant 3$ meV/Rh within $2\leqslant U\leqslant 4$ eV, manifesting strong magnetic frustration in the compound. Finally, the phase energy of Li$_2$RhO$_3$ cannot be fitted with the two-dimensional Heisenberg-Kitaev model involving only the nearest neighbor interactions, and we propose that inter-layer interactions may be responsible for the discrepancy.

cond-mat.str-el↗

arXiv2013

Superconductivity induced by La doping in Sr1-xLaxFBiS2 system

Through a combination of X-ray diffraction, electrical transport, magnetic susceptibility, and heat capacity measurements, we report the effect of La doping on Sr in the newly discovered SrFBiS2 system. Superconducting transition with critical temperature Tc of 2.8 K, developed from a semiconducting-like normal state, was found in Sr0.5La0.5FBiS2. A strong diamagnetic signal and a clear specific heat anomaly associated with this transition were observed, confirming bulk superconductivity. The upper critical field Hc2(0) was estimated to be 1 Tesla by using the Ginzburg-Landau approach. Our experiments therefore demonstrate that bulk superconductivity can be achieved by electron doping in the SrFBiS2 system.

cond-mat.supr-con↗

arXiv2010

Suppression of $T_c$ by Zn impurity in the electron-type LaFe$_{0.925-y}$Co$_{0.075}$Zn$_y$AsO system

The effect of non-magnetic Zn impurity on superconductivity in electron-type pnictide superconductor LaFe$_{0.925-y}$Co$_{0.075}$Zn$_y$AsO is studied systematically. The optimally doped LaFe$_{0.925}$Co$_{0.075}$AsO without Zn impurity exhibits superconductivity at $T_c^{mid}$ of 13.2 K, where $T_c^{mid}$ is defiend as the mid-point in the resistive transition. In the presence of Zn impurity, the superconducting transition temperature, $T_c^{mid}$, is severely suppressed. The result is consistent with the theoretic prediction on the effect of non-magnetic impurity in the scenario of $s_{\pm}$ pairing, but it is in sharp contrast to the previous report on the effect of Zn impurity in the F-doped systems. The possible interpretation of the different effects of Zn impurity on superconductivity in different systems is discussed.

cond-mat.supr-con↗