arXiv subjects

Kwing To Lai

Publications and source records attributed to Kwing To Lai.

At least 19 recordsLinked to original sources

arXiv2026

Thickness-driven crossover from conventional to chiral nonreciprocal superconductivity in kagome metal CsV3Sb5

Superconductivity and its potential applications are governed by the symmetry of the superconducting order parameter. In the kagome metal CsV3Sb5, most bulk studies indicate conventional s-wave pairing. However, ultrathin flakes exhibit nonreciprocal transport, in particular a zero-field superconducting diode effect, which requires broken inversion and time-reversal symmetries. Here, using thickness dependent transport measurements, we observe the emergence of non-reciprocal second-harmonic magnetotransport signals and a zero-field superconducting diode effect, accompanied by a pronounced reduction of the out-of-plane coherence length with decreasing thickness. Upper critical field measurements further reveal a dimensional crossover from three-dimensional superconductivity in bulk to two-dimensional superconductivity in thin flakes. These findings indicate a thickness-induced chiral superconducting phase that breaks both inversion and time-reversal symmetries in the two-dimensional limit. Our work not only clarifies long-standing controversies regarding the pairing symmetry in CsV3Sb5, but also establishes thin-flake kagome superconductors as a versatile platform for engineering nonreciprocal quantum devices and exploring emergent topological phases.

cond-mat.supr-con↗

arXiv2025

Giant critical current peak induced by pressure in kagome superconductor RbV$_{3}$Sb$_{5}$

Superconductivity can coexist or compete with other orders such as magnetism or density waves. Optimizing superconductivity requires identifying competing orders that may disrupt Cooper pair coherence. Here, we use the self-field critical current ($I_{\rm c,sf}$) to probe pressure-tuned superconductivity in the kagome superconductor RbV$_3$Sb$_5$. As pressure destabilizes the charge-density wave (CDW) state, $I_{\rm c,sf}$ drastically enhances, peaking near the critical pressure where the CDW state is completely suppressed at zero temperature. Surprisingly, a weaker $I_{\rm c,sf}$ peak emerges within the CDW phase. Near the pressure of the weaker peak, the superconducting phase transition temperature shifts from an increasing trend with pressure to a near plateau. Our analysis suggests the possibility of a sudden change in the CDW pattern or a Lifshitz transition, highlighting the need for microscopic examinations of the CDW state for understanding the pressure evolution of superconductivity in RbV$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2025

Superconducting critical temperature and dimensionality tuning of RbV$_3$Sb$_5$ via biaxial strain

Kagome metal AV$_3$Sb$_5$ (A=K, Rb, Cs) has emerged as an intriguing platform for exploring the interplay between superconductivity and other quantum states. Among the three compounds, RbV$_3$Sb$_5$ has a notably lower superconducting critical temperature ($T_c$) at ambient pressure, posing challenges in exploring the superconducting state. For instance, the upper critical field ($H_{c2}$) is small and thus difficult to measure accurately against other control parameters. Hence, enhancing superconductivity would facilitate $H_{c2}$ measurements, providing insights into key superconducting properties such as the dimensionality. In this letter, we report the tuning of the $T_c$ in RbV$_3$Sb$_5$ through the application of biaxial strain. Utilizing a negative thermal expansion material ZrW$_2$O$_8$ as a substrate, we achieve a substantial biaxial strain of $ε=1.50\%$, resulting in a remarkable 75\% enhancement in $T_c$. We investigate the $H_{c2}$ as a function of temperature, revealing a transition from multi-band to single-band superconductivity with increasing tensile strain. Additionally, we study the $H_{c2}$ as a function of field angle, revealing a plausible correlation between the $T_c$ enhancement and the change in dimensionality of the superconductivity under tensile strain. Further analysis quantitatively illustrates a transition towards two-dimensional superconductivity in RbV$_3$Sb$_5$ when subjected to tensile strain. Our work demonstrates that the application of biaxial strain allows for the tuning of both the $T_c$ and superconducting dimensionality in RbV$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2025

Discovery of a new phase in thin flakes of KV$_{3}$Sb$_{5}$ under pressure

We report results of magnetotransport measurements on KV$_3$Sb$_5$ thin flakes under pressure. Our zero-field electrical resistance reveals an additional anomaly emerging under pressure ($p$), marking a previously unidentified phase boundary $T^{\rm \ast}$($p$). Together with the established $T_{\rm CDW}(p)$ and $T_c(p)$, denoting the charge-density-wave transition and a superconducting transition, respectively, the temperature-pressure phase diagram of KV$_3$Sb$_5$ features a rich interplay among multiple phases. The Hall coefficient evolves reasonably smoothly when crossing the $T^{\rm \ast}$ phase boundary compared with the variation when crossing $T_{\rm CDW}$, indicating the preservation of the pristine electronic structure. The mobility spectrum analysis provides further insights into distinguishing different phases. Finally, our high-pressure quantum oscillation studies up to 31 T combined with density functional theory calculations further demonstrate that the new phase does not reconstruct the Fermi surface, confirming that the translational symmetry of the pristine metallic state is preserved.

cond-mat.supr-con↗

arXiv2024

Quantum Phase Transition as a Promising Route to Enhance the Critical Current in Kagome Superconductor CsV$_{3}$Sb$_{5}$

Developing strategies to systematically increase the critical current, the threshold current below which the superconductivity exists, is an important goal of materials science. Here, the concept of quantum phase transition is employed to enhance the critical current of a kagome superconductor CsV$_3$Sb$_5$, which exhibits a charge density wave (CDW) and superconductivity that are both affected by hydrostatic pressure. As the CDW phase is rapidly suppressed under pressure, a large enhancement in the self-field critical current ($I_{\rm c,sf}$) is recorded. The observation of a peak-like enhancement of $I_{\rm c,sf}$ at the zero-temperature limit ($I_{\rm c,sf}(0)$) centred at $p^*\approx 20$~kbar, the same pressure where the CDW phase transition vanishes, further provides strong evidence of a zero-temperature quantum anomaly in this class of pressure-tuned superconductor. Such a peak in $I_{\rm c,sf}(0)$ resembles the findings in other well-established quantum-critical superconductors, hinting at the presence of enhanced quantum fluctuations associated with the CDW phase in CsV$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2024

Studying Critical Parameters of Superconductor via Diamond Quantum Sensors

Critical parameters are the key to superconductivity research, and reliable instrumentations can facilitate the study. Traditionally, one has to use several different measurement techniques to measure critical parameters separately. In this work, we develop the use of a single species of quantum sensor to determine and estimate several critical parameters with the help of independent simulation data. We utilize the nitrogen-vacancy (NV) center in the diamond, which recently emerged as a promising candidate for probing exotic features in condensed matter physics. The non-invasive and highly stable nature provides extraordinary opportunities to solve scientific problems in various systems. Using a high-quality single-crystalline YBa$_{2}$Cu$_{4}$O$_{8}$ (YBCO) as a platform, we demonstrate the use of diamond particles and a bulk diamond to probe the Meissner effect. The evolution of the vector magnetic field, the $H-T$ phase diagram, and the map of fluorescence contour are studied via NV sensing. Our results reveal different critical parameters, including lower critical field $H_{c1}$, upper critical field $H_{c2}$, and critical current density $j_{c}$, as well as verifying the unconventional nature of this high-temperature superconductor YBCO. Therefore, NV-based quantum sensing techniques have huge potential in condensed matter research.

cond-mat.mes-hall↗

arXiv2024

Large Fermi surface in pristine kagome metal CsV$_3$Sb$_5$ and enhanced quasiparticle effective masses

The kagome metal CsV$_3$Sb$_5$ is an ideal platform to study the interplay between topology and electron correlation. To understand the fermiology of CsV$_3$Sb$_5$, intensive quantum oscillation (QO) studies at ambient pressure have been conducted. However, due to the Fermi surface reconstruction by the complicated charge density wave (CDW) order, the QO spectrum is exceedingly complex, hindering a complete understanding of the fermiology. Here, we directly map the Fermi surface of the pristine CsV$_3$Sb$_5$ by measuring Shubnikov-de Haas QOs up to 29 T under pressure, where the CDW order is completely suppressed. The QO spectrum of the pristine CsV$_3$Sb$_5$ is significantly simpler than the one in the CDW phase, and the detected oscillation frequencies agree well with our density functional theory calculations. In particular, a frequency as large as 8,200 T is detected. Pressure-dependent QO studies further reveal a weak but noticeable enhancement of the quasiparticle effective masses on approaching the critical pressure where the CDW order disappears, hinting at the presence of quantum fluctuations. Our high-pressure QO results reveal the large, unreconstructed Fermi surface of CsV$_3$Sb$_5$, paving the way to understanding the parent state of this intriguing metal in which the electrons can be organized into different ordered states.

cond-mat.supr-con↗

arXiv2024

Shubnikov-de Haas oscillations of biaxial-strain-tuned superconductors in pulsed magnetic field up to 60 T

Two-dimensional (2D) materials have gained increasing prominence not only in fundamental research but also in daily applications. However, to fully harness their potential, it is crucial to optimize their properties with an external parameter and track the electronic structure simultaneously. Magnetotransport over a wide magnetic field range is a powerful method to probe the electronic structure and, for metallic 2D materials, quantum oscillations superimposed on the transport signals encode Fermi surface parameters. In this manuscript, we utilize biaxial strain as an external tuning parameter and investigate the effects of strain on the electronic properties of two quasi-2D superconductors, MoTe$_2$ and RbV$_3$Sb$_5$, by measuring their magnetoresistance in pulsed magnetic fields up to 60 T. With a careful selection of insulating substrates, we demonstrate the possibility of both the compressive and tensile biaxial strain, imposed on MoTe$_2$ and RbV$_3$Sb$_5$, respectively. For both systems, the applied strain has led to superconducting critical temperature enhancement compared to their free-standing counterparts, proving the effectiveness of this biaxial strain method at cryogenic temperatures. Clear quantum oscillations in the magnetoresistance -- the Shubnikov-de Haas (SdH) effect -- are obtained in both samples. In strained MoTe$_2$, the magnetoresistance exhibits a nearly quadratic dependence on the magnetic field and remains non-saturating even at the highest field. Whereas in strained RbV$_3$Sb$_5$, two SdH frequencies showed a substantial enhancement in effective mass values, hinting at a possible enhancement of charge fluctuations. Our results demonstrate that combining biaxial strain and pulsed magnetic field paves the way for studying 2D materials under unprecedented conditions.

cond-mat.supr-con↗

arXiv2023

Tunable non-Lifshitz-Kosevich temperature dependence of Shubnikov-de Haas oscillation amplitudes in SmSb

The Lifshitz-Kosevich (LK) theory is the pillar of magnetic quantum oscillations, which have been extensively applied to characterize a wide range of metallic states. In this study, we focus on the Shubnikov-de Haas (SdH) effect observed in SmSb, a rare-earth monopnictide. We observed a significant departure from the expected LK theory near $T_N=2.4$~K: both a peak-like anomaly and an enhancement in the temperature dependence of quantum oscillation amplitude are seen in SmSb. Moreover, we discovered a remarkable sensitivity of the SdH amplitudes to sample purity. By adjusting the sample purity, we were able to tune the temperature dependence of the $α$ band's SdH amplitudes from a peak-like anomalous behavior to an enhancement. Therefore, SdH oscillations from the $α$ band connect the two well-known non-LK behaviours, controllable through varying the sample purity, paving the way for developing further understanding of the mechanism leading to the anomalous quantum oscillations.

cond-mat.mtrl-sci↗

arXiv2023

Suppression of both superconductivity and structural transition in hole-doped MoTe$_2$ induced by Ta substitution

Type-II Weyl semimetal MoTe$_2$ exhibits a first-order structural transition at $T_s$ $\sim$250~K and superconducts at $T_c$ $\sim$0.1~K at ambient pressure. Both $T_s$ and $T_c$ can be manipulated by several tuning parameters, such as hydrostatic pressure and chemical substitution. It is often reported that suppressing $T_s$ enhances $T_c$, but our study shows a different behaviour when MoTe$_2$ is hole-doped by Ta. When $T_s$ is suppressed by Ta doping, $T_c$ is also suppressed. Our findings suggest that the suppression of $T_s$ does not necessarily enhance superconductivity in MoTe$_2$. By connecting with the findings of electron-doped MoTe$_2$, we argue that varying electron carrier concentration can effectively tune $T_c$. In addition, the Hall coefficient is enhanced around the doping region, where $T_s$ is completely suppressed, suggesting that the critical scattering around the structural transition may also play a role in suppressing $T_c$.

cond-mat.supr-con↗

arXiv2023

Similarities and Differences in the Fermiology of Kagome Metals AV$_{3}$Sb$_{5}$ (A=K, Rb, Cs) Revealed by Shubnikov-de Haas Oscillations

Materials with AV$_3$Sb$_5$ (A=K, Rb, Cs) stoichiometry are recently discovered kagome superconductors with the electronic structure featuring a Dirac band, van Hove singularities and flat bands. These systems undergo anomalous charge-density-wave (CDW) transitions at $T_{\rm CDW}$~80-100 K, resulting in the reconstruction of the Fermi surface from the pristine phase. Although comprehensive investigations of the electronic structure via quantum oscillations (QOs) have been performed on the sister compounds CsV$_3$Sb$_5$ and RbV$_3$Sb$_5$, a detailed QO study of KV$_3$Sb$_5$ is so far absent. Here, we report the Shubnikov-de Haas QO study in KV$_3$Sb$_5$. We resolve a large number of new frequencies with the highest frequency of 2202 T (occupying ~54% of the Brillouin zone area in the $k_x$-$k_y$ plane). The Lifshitz-Kosevich analysis further gives relatively small cyclotron effective masses, and the angular dependence study reveals the two-dimensional nature of the frequencies with a sufficient signal-to-noise ratio. Finally, we compare the QO spectra for all three AV$_3$Sb$_5$ compounds collected under the same conditions, enabling us to point out the similarities and differences across these systems. Our results fill in the gap of the QO study in KV$_3$Sb$_5$ and provide valuable data to understand the band structure of all three members of AV$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2023

Anomalous Hall effect and two-dimensional Fermi surfaces in the charge-density-wave state of kagome metal RbV$_3$Sb$_5$

AV$_3$Sb$_5$ (A=Cs, K, Rb) are recently discovered superconducting systems ($T_{\rm c}\sim0.9-2.5$ K) in which the vanadium atoms adopt the kagome structure. Intriguingly, these systems enter a charge-density-wave (CDW) phase ($T_{\rm CDW}\sim80-100$ K), and further evidence shows that the time-reversal symmetry is broken in the CDW phase. Concurrently, the anomalous Hall effect has been observed in KV$_3$Sb$_5$ and CsV$_3$Sb$_5$ inside the novel CDW phase. Here, we report a comprehensive study of a high-quality RbV$_3$Sb$_5$ single crystal with magnetotransport measurements. Our data demonstrate the emergence of anomalous Hall effect in RbV$_3$Sb$_5$ when the charge-density-wave state develops. The magnitude of anomalous Hall resistivity at the low temperature limit is comparable to the reported values in KV$_3$Sb$_5$ and CsV$_3$Sb$_5$. The magnetoresistance channel further reveals a rich spectrum of quantum oscillation frequencies, many of which have not been reported before. In particular, a large quantum oscillation frequency (2235 T), which occupies $\sim$56% of the Brillouin zone area, has been recorded. For the quantum oscillation frequencies with sufficient signal-to-noise ratio, we further perform field-angle dependent measurements and our data indicate two-dimensional Fermi surfaces in RbV$_3$Sb$_5$. Our results provide indispensable information for understanding the anomalous Hall effect and band structure in kagome metals AV$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2023

Drastic enhancement of the superconducting temperature in type-II Weyl semimetal candidate MoTe$_2$ via biaxial strain

Type-II Weyl semimetal candidate MoTe$_2$, which superconducts at T_c~0.1 K, is one of the promising candidates for realizing topological superconductivity. However, the exceedingly low $T_c$ is associated with a small upper critical field ($H_{c2}$), implying a fragile superconducting phase that only exists on a small region of the $H$-$T$ phase diagram. Here, we describe a simple and versatile approach based on the differential thermal expansion between dissimilar materials to subject a thin single crystalline MoTe$_2$ to biaxial strain. With this approach, we successfully enhance the $T_c$ of MoTe$_2$ five-fold and consequently expand the superconducting region on the $H$-$T$ phase diagram significantly. To demonstrate the relative ease of studying the superconductivity in the biaxially strained MoTe$_2$, we further present the magnetotransport data, enabling the study of the temperature-dependent $H_{c2}$ and the anisotropy of the superconducting state which would otherwise be difficult to obtain in a free-standing MoTe$_2$. Our work shows that biaxial strain is an effective knob to tune the electronic properties of MoTe$_2$. Due to the simplicity of our methodology to apply biaxial strain, we anticipate its direct applicability to a wider class of quantum materials.

cond-mat.supr-con↗

arXiv2023

Nodeless superconductivity in kagome metal CsV$_{3}$Sb$_{5}$ with and without time reversal symmetry breaking

The kagome metal CsV$_{3}$Sb$_{5}$ features an unusual competition between the charge-density-wave (CDW) order and superconductivity. Evidence for time-reversal symmetry breaking (TRSB) inside the CDW phase has been accumulating. Hence, the superconductivity in CsV$_{3}$Sb$_{5}$ emerges from a TRSB normal state, potentially resulting in an exotic superconducting state. To reveal the pairing symmetry, we first investigate the effect of nonmagnetic impurity. Our results show that the superconducting critical temperature is insensitive to disorder, pointing to conventional $s$-wave superconductivity. Moreover, our measurements of the self-field critical current ($I_{c,sf}$), which is related to the London penetration depth, also confirm conventional $s$-wave superconductivity with strong coupling. Finally, we measure $I_{c,sf}$ where the CDW order is removed by pressure and superconductivity emerges from the pristine normal state. Our results show that $s$-wave gap symmetry is retained, providing strong evidence for the presence of conventional $s$-wave superconductivity in CsV$_{3}$Sb$_{5}$ irrespective of the presence of the TRSB

cond-mat.supr-con↗

arXiv2022

Emergence of large quantum oscillation frequencies in thin flakes of the kagome superconductor CsV$_{3}$Sb$_{5}$

Kagome metals AV$_3$Sb$_5$~(A = K, Rb, Cs) are recently discovered platforms featuring an unusual charge-density-wave (CDW) order and superconductivity. The electronic band structure of a kagome lattice can host both flat bands as well as Dirac-like bands, offering the possibility to stabilize various quantum states. Here, we probe the band structure of CsV$_3$Sb$_5$ via Shubnikov-de Haas quantum oscillations on both bulk single crystals and thin flakes. Although our frequency spectra are broadly consistent with the published data, we unambiguously reveal the existence of new frequencies with large frequencies ranging from $\sim$2085~T to $\sim$2717~T in thin flakes when the magnetic field is along the $c$-axis. These quasi-two-dimensional frequencies correspond to $\sim$52\% to 67\% of the CDW-distorted Brillouin zone volume. The Lifshitz-Kosevich analysis further uncovers surprisingly small cyclotron effective masses, of the order of $\sim$0.1~$m_e$, for these frequencies. Consequently, a large number of high-velocity carriers exists in the thin flake of CsV$_3$Sb$_5$. Comparing with our band structure calculations, we argue that an orbital-selective modification of the band structure is active. Our results provide indispensable information for understanding the fermiology of CsV$_3$Sb$_5$, paving a way for understanding how an orbital-selective mechanism can become an effective means to tune its electronic properties.

cond-mat.supr-con↗

arXiv2022

Peak in the critical current density in (Ca$_{x}$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$ tuned towards the structural quantum critical point

(Ca$_{x}$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$ is a rare system that has been shown to display an interesting interplay between structural quantum criticality and superconductivity. A putative structural quantum critical point, which is hidden beneath a broad superconducting dome, is believed to give rise to optimized superconducting properties in (Ca$_{x}$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$. However, the presence of the superconducting dome itself hinders the examination of the quantum critical point through electrical transport, as the transport coefficients vanish in the superconducting state. Here, we use critical current density to explore within the superconducting dome. Our measurements reveal a large enhancement of the critical current density at the zero-temperature limit when the system is tuned towards the structural quantum critical point.

cond-mat.supr-con↗

arXiv2022

Microscopic study of optically-stable, coherent color centers in diamond generated by high-temperature annealing

Single color centers in solid have emerged as promising physical platforms for quantum information science. Creating these centers with excellent quantum properties is a key foundation for further technological developments. In particular, the microscopic understanding of the spin bath environments is the key to engineer color centers for quantum control. In this work, we propose and demonstrate a distinct high-temperature annealing (HTA) approach for creating high-quality nitrogen vacancy (NV) centers in implantation-free diamonds. Simultaneously using the created NV centers as probes for their local environment we verify that no damage was microscopically induced by the HTA. Nearly all single NV centers created in ultra-low-nitrogen-concentration membranes possess stable and Fourier-transform-limited optical spectra. Furthermore, HTA strongly reduces noise sources naturally grown in ensemble samples, and leads to more than three-fold improvements of decoherence time and sensitivity. We also verify that the vacancy activation and defect reformation, especially H3 and P1 centers, can explain the reconfiguration between spin baths and color centers. This novel approach will become a powerful tool in vacancy-based quantum technology.

cond-mat.mes-hall↗

arXiv2021

Current-induced superconducting anisotropy of Sr$\mathsf{_2}$RuO$\mathsf{_4}$

In the unconventional superconductor Sr$\mathsf{_2}$RuO$\mathsf{_4}$, unusual first-order superconducting transition has been observed in the low-temperature and high-field region, accompanied by a four-fold anisotropy of the in-plane upper critical magnetic field $H_{c2}$. The origin of such unusual $H_{c2}$ behavior should be closely linked to the debated superconducting symmetry of this oxide. Here, toward clarification of the unusual $H_{c2}$ behavior, we performed the resistivity measurements capable of switching in-plane current directions as well as precisely controlling the field directions. Our results reveal that resistive $H_{c2}$ under the in-plane current exhibits an additional two-fold anisotropy. By systematically analyzing $H_{c2}$ data taken under various current directions, we succeeded in separating the two-fold $H_{c2}$ component into the one originating from applied current and the other originating from certain imperfection in the sample. The former component, attributable to vortex flow effect, is weakened at low temperatures where $H_{c2}$ is substantially suppressed. The latter component is enhanced in the first order transition region, possibly reflecting a change in the nature of the superconducting state under high magnetic field.

cond-mat.supr-con↗