arXiv subjects

Jian-gang Guo

Publications and source records attributed to Jian-gang Guo.

18 recordsLinked to original sources

arXiv2026

Bragg-Williams order competes with superconductivity

Orderings in charge and spin have been extensively studied to unravel their correlation to emergent superconductivity over the past decades. Bragg-Williams order (BWO), a classical structural order parameter describing site occupancy in alloys, has long been speculated to influence superconducting behavior. Yet, its role still remains ambiguous, largely due to the difficulty of isolating BWO from concomitant charge doping or competing electronic instabilities. Here, we establish In2/3PSe3 as a platform wherein indium vacancies are reversibly configurable between ordered and disordered states via thermal treatment. We show that the disordered phase undergoes a pressure-induced superconducting transition with a Tc of 11 K, significantly higher than the 7 K observed in its ordered counterpart. This constitutes a rare instance in which pure BWO variation drives a substantial shift in Tc. By combining a Ginzburg-Landau phenomenological analysis with a BCS-McMillan microscopic description, we demonstrate that BWO naturally suppresses superconductivity through electron-phonon interactions, a mechanism supported by ultra-low-wavenumber Raman measurements. Our findings support BWO as an independent order parameter that competes directly with superconductivity, extending the concept of competing orders beyond conventional electronic and magnetic degrees of freedom.

cond-mat.supr-con↗

arXiv2026

Intrinsically ultralow thermal conductivity in all-inorganic superatomic bulk crystals

Superatomic compounds, composed of atomic clusters interwoven by weak chemical bonds exhibit large anharmonicity vibrations, are excellent candidates for ultralow thermal conductivity (\k{appa}) materials. However, growing bulk superatomic single crystals is challenging due to complex chemical composition and chemical bonds, and studies on their intrinsic thermal property are scarce. Here, we grew high-quality superatomic single crystals of Re6Se8Te7 and Re6Te15, both of which are narrow band gap semiconductors that change into metals under external physical pressure. At room-temperature, the \k{appa} are 0.32 W m-1 K-1 and 0.53 W m-1 K-1 in Re6Se8Te7 and Re6Te15, respectively, ranking among the lowest value reported in all-inorganic bulk crystals. It is mainly attributed to the large Grüneisen parameter (1.93) and low average sound speed (< 1482 m/s), which are due to soft Te7 nets weakly embedded among the rigid Re6Se8 (Re6Te8) quasi-cubic clusters. The appearance of boson peak, i. e., hump of C(T)/T3, verifies the existence of disordered phonon transports. Besides, the temperature dependence of \k{appa} can be described by classic Debye-Callaway model. Notably, above 350 K, the \k{appa} values of Re6Se8Te7 and Re6Te15 are remarkably close to the upper limit derived from glassy-like diffusion model. This finding sets the superatomic compounds as a promising family for searching ultralow-\k{appa} and energy management materials.

cond-mat.mtrl-sci↗

arXiv2026

On estimating superconducting shielding volume fraction from susceptibility in pressurized Ruddlesden-Popper nickelates: Response to arXiv:2602.19282

In a recent preprint (arXiv:2602.19282) [1], the authors questioned the procedure we used to evaluate the demagnetization-corrected superconducting shielding volume fraction in pressurized Ruddlesden-Popper nickelates [2-5]. They further claimed that this methodology has neither been derived nor used previously, and they proposed an alternative normalization scheme. Here we clarify that our evaluation follows directly from the standard magnetostatic self-consistency relation for finite samples and has been widely adopted in the superconductivity literature for decades. We also demonstrate that the discrepancies claimed in Ref. [1] stem from a fundamental flaw in their approach, namely, the assumption that the measured diamagnetic moment is linearly proportional to the superconducting shielding volume fraction in the presence of a finite demagnetization factor N. This assumption is not valid for strongly demagnetized, thin disk-like specimens, where the internal field and the measured moment are coupled self-consistently through the demagnetizing field.

cond-mat.supr-con↗

arXiv2026

Superconductivity of 30.4 K and its Reemergence under Pressure in Fe1.11Se Synthesized via Ion-exchange and De-intercalation Reaction

Binary stoichiometry FeSe (s-FeSe) is a well-known parent of high-temperature unconventional superconductors owing to its charge-neutral layer, highly tunable structure and electronic properties, and rich interplays among multiple electronic phases. Yet the s-FeSe synthesized via high-temperature equilibrium reactions bears the notorious interstitial Fe,where merely 3% of them is sufficient to kill the superconductivity. Here, we successfully synthesized a new non-stoichiometric Fe1.11Se single crystal with a superconducting onset temperature (Tconset) of 30.4 K through a hydrothermal ion-exchange and de-intercalation route. 11% interstitial Fe ions exceed the equilibrium phase diagram limit. Intriguingly, under physical pressure, the Tconset of exhibits a "V"-shaped evolution with a minimum at 2-2.6 GPa, and then upturning into a second superconducting region, reminiscent of the behaviors in FeSe intercalates. Furthermore, a pressure-induced possible magnetic order, previously only observed in pressurized s-FeSe, shows up. These results offer fresh insights into the role of interstitial Fe in governing superconducting and transport properties under non-equilibrium synthesis and tuning strategies.

cond-mat.supr-con↗

arXiv2024

Fourfold Anisotropic Magnetoresistance and Unconventional Critical Exponents in Twinned FePd$_2$Te$_2$

As a special material symmetry operation, crystal twins usually influence physical properties. Here, detailed electrical transport and magnetic measurements were performed to reveal twinning effect on properties of van der Waals ferromagnet FePd$_2$Te$_2$. Orthorhombic crystal domains were observed in polarized optical microscopy and fixed $π$/2 angle between adjacent domains suggests a phase transition origin of the twins. FePd$_2$Te$_2$ exhibits fourfold in-plane anisotropic magnetoresistance. It is attributed to antiferromagnetic coupling component near atomically flat twin boundary and pseudo four-fold symmetry from perpendicular Fe chains. Intense magnetic domain motion is suggested by Hopkinson effect observed in magnetic susceptibility. A set of unusual critical exponents $β$ = 0.866, $γ$ = 1.043, $δ$ = 2.20 cannot be classified in any universal class predicted by renormalized group. Deviation from standard model reflects the non-saturating magnetization and slow growth of spontaneous magnetization resulting from crystal domain walls. These results show that additional symmetry from twins and twin boundary have a significant effect on electrical transport and magnetic properties of FePd$_2$Te$_2$. There is much room to modulate physical properties of twinned van der Waals ferromagnets through twins.

cond-mat.mtrl-sci↗

arXiv2024

Realization of Kagome Kondo lattice

The Kondo lattice, describing a grid of the local magnetic moments coupling to itinerant electrons, is a fertile ground of strongly correlated states in condensed matter physics. While the Kagome lattice has long been predicted to host Kondo physics with exotic magnetism and nontrivial topology, no experimental realization has been achieved. Here, we report the discovery of CsCr6Sb6, a van der Waals-like Kagome Kondo lattice featuring extremely flat, isolated bands at the Fermi level (EF) that composed entirely of Cr-3d electrons. We observe heavy fermions with the effective mass over 100 times greater than those of its vanadium counterpart. We also observe Kondo insulating behavior in an ultra-low carrier density of 1019 cm-3 and dimensionality-induced Kondo breakdown. More interestingly, the frustrated magnetism observed in the bulk give way to a hidden A-type antiferromagnetic ordering in few layers, in sharp contrast to the common sense of weakened magnetism with thinning. The realization of Kondo physics in Kagome lattice opens avenues for exploring diverse quantum criticalities in a strongly-correlated frustrated system.

cond-mat.str-el↗

arXiv2024

Superconductivity in pressurized trilayer La$_4$Ni$_3$O$_{10-δ}$ single crystals

The pursuit of discovering new high-temperature superconductors that diverge from the copper-based paradigm1-3 carries profound implications for elucidating mechanisms behind superconductivity and may also enable new applications4-8. Here, our investigation reveals that application of pressure effectively suppresses the spin and charge order in trilayer nickelate La4Ni3O10-δ single crystals, leading to the emergence of superconductivity with a maximum critical temperature (Tc) of around 30 K at 69.0 GPa. The DC susceptibility measurements confirm a substantial diamagnetic response below Tc, indicating the presence of bulk superconductivity with a volume fraction exceeding 80%. In the normal state, we observe a "strange metal" behavior, characterized by a linear temperature-dependent resistance extending up to 300 K. Furthermore, the layer-dependent superconductivity observed hints at a unique interlayer coupling mechanism specific to nickelates, setting them apart from cuprates in this regard. Our findings provide crucial insights into the fundamental mechanisms underpinning superconductivity, while also introducing a new material platform to explore the intricate interplay between the spin/charge order, flat band structures, interlayer coupling, strange metal behavior and high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2023

Tightly-bound and room-temperature-stable excitons in van der Waals degenerate-semiconductor Bi4O4SeCl2 with high charge-carrier density

Excitons, which represent a type of quasi-particles consisting of electron-hole pairs bound by the mutual Coulomb interaction, were often observed in lowly-doped semiconductors or insulators. However, realizing excitons in the semiconductors or insulators with high charge carrier densities is a challenging task. Here, we perform infrared spectroscopy, electrical transport, ab initio calculation, and angle-resolved-photoemission spectroscopy studies of a van der Waals degenerate-semiconductor Bi4O4SeCl2. A peak-like feature (i.e., alpha peak) is present around ~ 125 meV in the optical conductivity spectra at low temperature T = 8 K and room temperature. After being excluded from the optical excitations of free carriers, interband transitions, localized states and polarons, the alpha peak is assigned as the exciton absorption. Moreover, assuming the existence of weakly-bound excitons--Wannier-type excitons in this material violates the Lyddane-Sachs-Teller relation. Besides, the exciton binding energy of ~ 375 meV, which is about an order of magnitude larger than those of conventional semiconductors, and the charge-carrier concentration of ~ 1.25 * 10^19 cm^-3, which is higher than the Mott density, further indicate that the excitons in this highly-doped system should be tightly bound. Our results pave the way for developing the optoelectronic devices based on the tightly-bound and room-temperature-stable excitons in highly-doped van der Waals degenerate semiconductors.

cond-mat.mtrl-sci↗

arXiv2023

High quality and wafer-scale cubic silicon carbide single crystals

Silicon carbide (SiC) is an important semiconductor material for fabricating power electronic devices that exhibit higher switch frequency, lower energy loss and substantial reduction both in size and weight in comparison with its Si-based counterparts1-4. Currently, most devices, such as metal-oxide-semiconductor field effect transistors, which are core devices used in electric vehicles, photovoltaic industry and other applications, are fabricated on a hexagonal polytype 4H-SiC because of its commercial availability5. Cubic silicon carbide (3C-SiC), the only cubic polytype, has a moderate band gap of 2.36 eV at room-temperature, but a superior mobility and thermal conduction than 4H-SiC4,6-11. Moreover, the much lower concentration of interfacial traps between insulating oxide gate and 3C-SiC helps fabricate reliable and long-life devices7-10,12-14. The growth of 3C-SiC crystals, however, has remained a challenge up to now despite of decades-long efforts by researchers because of its easy transformation into other polytypes during growth15-19, limiting the 3C-SiC based devices. Here, we report that 3C-SiC can be made thermodynamically favored from nucleation to growth on a 4H-SiC substrate by top-seeded solution growth technique(TSSG), beyond what's expected by classic nucleation theory. This enables the steady growth of quality and large sized 3C-SiC crystals (2~4-inch in diameter and 4.0~10.0 mm in thickness) sustainable. Our findings broaden the mechanism of hetero-seed crystal growth and provide a feasible route to mass production of 3C-SiC crystals,offering new opportunities to develop power electronic devices potentially with better performances than those based on 4H-SiC.

cond-mat.mtrl-sci↗

arXiv2023

Observation of non-superconducting phase changes in LuH$_{2\pm\text{x}}$N$_y$

The recent report of near-ambient superconductivity in nitrogen doped lutetium hydride has triggered a worldwide fanaticism and raised major questions about the latest claims. An intriguing phenomenon of color changes in pressurized samples from blue to pink to red was observed and correlated with the claimed superconducting transition, but the origin and underlying physics of these color changes have yet to be elucidated. Here we report synthesis and characterization of high-purity nitrogen doped lutetium hydride LuH$_{2\pm\text{x}}$N$_y$ with the same structure and composition as in the main phase of near-ambient superconductor1. We find a new purple phase of LuH$_{2\pm\text{x}}$N$_y$ between blue and pink phase, and reveal that the sample color changes likely stem from pressure-driven redistribution of nitrogen and its interaction with the LuH$_2$ framework. No superconducting transition is found in all blue, purple, pink and red phases at temperatures 1.8-300 K and pressures 0-30 GPa. Instead, we identify a notable temperature-induced resistance anomaly of structural and/or electronic origin in LuH$_{2\pm\text{x}}$N$_y$, which is most pronounced in the pink phase and may have been erroneously interpreted as a sign of superconducting transition. This work establishes key benchmarks for nitrogen doped lutetium hydrides, allowing an in-depth understanding of the novel pressure-induced phase changes.

cond-mat.supr-con↗

arXiv2023

Superconductivity in an Orbital-reoriented SnAs Square Lattice: a Case Study of Li0.6Sn2As2 and NaSnAs

Searching for functional square lattices in layered superconductor systems offers an explicit clue to modify the electron behavior and find exotic properties. The trigonal SnAs3 structural units in SnAs-based systems are relatively conformable to distortion, which provides the possibility to achieve structurally topological transformation and higher superconducting transition temperatures. In the present work, the functional As square lattice was realized and activated in Li0.6Sn2As2 and NaSnAs through a topotactic structural transformation of trigonal SnAs3 to square SnAs4 under pressure, resulting in a record-high Tc among all synthesized SnAs-based compounds. Meanwhile, the conductive channel transfers from the out-of-plane pz orbital to the in-plane px+py orbitals, facilitating electron hopping within the square 2D lattice and boosting the superconductivity. The reorientation of p-orbital following a directed local structure transformation provides an effective strategy to modify layered superconductors.

cond-mat.supr-con↗

arXiv2022

Pressure-induced superconductivity in charge-density-wave compound LaTe2-xSbx (x=0 and 0.4)

Here, we have grown single crystals of LaTe2-xSbx (x=0 and 0.4) with continuously adjustable CDW. High-pressure x-ray diffraction show LaTe2 does not undergo phase transition and keep robust below 40 GPa. In-situ high-pressure electrical measurements show LaTe2-xSbx undergo semiconductor-metal-superconductivity transition at 4.6 and 2.5 GPa, respectively. With the doping of Sb, the highest Tc increases from 4.6 to 6.5 K. Theoretical calculations reveal that the CDW has been completely suppressed and the calculated Tc is about 2.97 K at 4.5 GPa, consistent with the measured value. Then, the pressure-induced superconductivity in LaTe2-xSbx can be explained in the framework of the BCS theory.

cond-mat.supr-con↗

arXiv2022

Insulator-metal-superconductor transition in medium-entropy van der Waals compound MEPSe3 (ME=Fe, Mn, Cd, and In) under high pressures

MPX3 (M=metals, X=S or Se) represents a large family of van der Waals (vdW) materials featuring with P-P dimers of ~2.3 Å separation. Its electrical transport property and structure can hardly be tuned by the intentional chemical doping and ionic intercalation. Here, we employ an entropy-enhancement strategy to successfully obtain a series of medium-entropy compounds MEPSe3 (ME=Fe, Mn, Cd and In), in which the electrical and magnetic properties changed simultaneously. Lone-pair electrons of P emerge due to the dissociation of the dimers as evidenced by a 35% elongation in the P-P interatomic distance. The band gap widens from 0.1 eV to 0.7 eV by this dissociation. Under external physical pressure up to ~50 GPa, a giant collapse of up to 15% in the c-axis happens, which is in contrast to the in-plane shrinkage of their counterparts Fe/MnPSe3. It leads to the recombination of P3- with lone pair electrons into a P-P dimer and the smallest bulk modulus of 28 GPa in MPX3. The MEPSe3 transits from a spin-glass insulator to metal, and to superconductor, which is rarely observed in the MPX3. Our findings highlight the P-P dimer as an indicator to probe diverse electronic structure and the effectiveness of entropy-enhancement in materials science.

cond-mat.supr-con↗

arXiv2022

Evolution of Superatomic-Charge-density-wave and Superconductivity under Pressure in AuTe$_2$Se$_{4/3}$

Superatomic crystal is a class of hierarchical materials composed of atomically precise clusters assembled via van der Waals or covalent-like interactions. AuTe$_2$Se$_{4/3}$, an all-inorganic superatomic superconductor exhibiting superatomic-charge-density-wave (S-CDW), provides a first platform to study the response of their collectively quantum phenomenon to the external pressure in superatomic crystals. We reveal a competition between S-CDW and superconductivity using cutting-edge measurements on thin flakes at low pressures. Prominently, the pressure modulation of S-CDW ordering is 1$\sim$2 order of magnitudes (0.1 GPa) lower than that of conventional atomic superconductors. As pressure increases to 2.5 GPa, the $T_{\mathrm{CDW}}$ is suppressed and the superconducting transition temperature ($T_{\mathrm{c}}$) is firstly enhanced, and reaches the maximum then quenches with increasing pressure. Above 7.3 GPa, a second superconducting phase emerges, and then a three-fold enhancement in the transition temperature ($T_{\mathrm{c}}$) happens. Analyses of the crystal structure and theoretical calculations suggest a pressure-mediated switch of the conduction channel from the $a$- to the $b$-axis occur, followed by a dimensional crossover of conductivity and the Fermi surface from 2D to 3D.

cond-mat.supr-con↗

arXiv2021

Discovery of two families of VSb-based compounds with V-kagome lattice

We report the structure and physical properties of two newly-discovered compounds AV8Sb12 and AV6Sb6 (A = Cs, Rb), which have C2 (space group: Cmmm) and C3 (space group: R-3m) symmetry, respectively. The basic V-kagome unit is present in both compounds, but stacking differently. A V2Sb2 layer is sandwiched between two V3Sb5 layers in AV8Sb12, altering the V-kagome lattice and lowering the symmetry of kagome layer from hexagonal to orthorhombic. In AV6Sb6, the building block is a more complex slab made up of two half-V3Sb5 layers that are intercalated by Cs cations along the c-axis. Transport property measurements demonstrate that both compounds are nonmagnetic metals, with carrier concentrations at around 1021cm-3. No superconductivity has been observed in CsV8Sb12 above 0.3 K under in-situ pressure up to 46 GPa. In contrast to CsV3Sb5, theoretical calculations and angle-resolved photoemission spectroscopy (ARPES) reveal a quasi-two-dimensional electronic structure in CsV8Sb12 with C2 symmetry and no van Hove singularities near the Fermi level. Our findings will stimulate more research into V-based kagome quantum materials.

cond-mat.supr-con↗

arXiv2021

Spatially separated superconductivity and enhanced charge-density-wave ordering in IrTe2 nano-flake

The interplay among various collective electronic states such as superconductivity (SC) and charge density wave (CDW) is of tremendous significance in low-dimensional transition metal dichalcogenides. Thus far, a consensus on the relationship between SC and CDW has not been established in IrTe2, where either competing or collaboration pictures have been suggested in the bulk or thick flakes. Here, we use the state-of-art Au-assisted exfoliation method to overcome the obstacle of interlayer Te-Te bonding, cleaving the IrTe2 down to monolayer for the first time. A striking feature revealed by angle-resolved polarized Raman spectroscopy (ARPRS) and electrical transport measurements is the concurrence of phase separation in one single piece of nano-flake, i.e. the superconducting (P-3m1) and CDW (P-3) area. In the pure SC area, the dimensional fluctuations completely suppress the CDW ordering and induce SC at 3.5 K. Meanwhile, the pure CDW area with much enhanced TCDW at 605 K (compared to TCDW = 280 K in bulk) is always accompanied by a unique wrinkle pattern. Detailed analyses suggest the local strain-induced bond breaking of the Te-Te dimer facilitates the emergence of the CDW order. Our works provide compelling evidence of competition between SC and CDW, highlighting the importance of microstructure in determining the ground states of IrTe2.

cond-mat.supr-con↗

arXiv2021

Spin-flip-driven giant magneto-transport in A-type antiferromagnet NaCrTe2

For anisotropic magneto-resistance (AMR) effect, its value synergistically depends on the magnitudes of magneto-resistance (MR) and magneto-crystalline anisotropy energy (MAE) simultaneously. In a magnetic material, the concurrence of gigantic AMR and MR signals is rather difficult due to weak spin-lattice coupling and small MAE. Here we report the considerable magneto-transport effect in layered A-type antiferromagnetic (AFM) NaCrTe2 by realigning the spin configurations. By applying H, the antiparallel spins of adjacent layers are flipped to ferromagnetic (FM) coupling either Ising-type along c-axis or XY-type within ab-plane. Theoretical calculations reveal that the energy bandgap narrows from 0.39 eV to 0.11 eV, accompanying a transition from semiconductor (high-R state) and half-semiconductor (low-R state), respectively. Thus, gigantic negative MR ratio of -90% is obtained at 10 K. More importantly, the decrement of R along H//c is far quicker than that of H//ab because the MAE of Ising-FM state is 1017 μeV/Cr3+ lower than that of XY-FM. The distinct trends result in the AMR ratio of 732% at 10 K, which is the record value to our best knowledge. These findings unravel the intrinsic origin of magneto in NaCrTe2 and will stimulate us to exploring the H-sensitive transport property in more AFM materials.

cond-mat.mtrl-sci↗

arXiv2021

Highly-robust reentrant superconductivity in CsV3Sb5 under pressure

Here we present the superconducting property and structural stability of kagome CsV3Sb5 under in-situ high pressures. For the initial SC-I phase, its Tc is quickly enhanced from 3.5 K to 7.6 K and then totally suppressed at P~10 GPa. Further increasing the applied pressures, an SC-II phase emerges at P~15 GPa and persists up to 100 GPa. The Tc rapidly increases to the maximal value of 5.2 K at P=53.6 GPa and rather slowly decreases to 4.7 K at P=100 GPa. A two-dome-like variation of Tc in CsV3Sb5 is concluded here. The Raman measurements demonstrate that weakening of E2g model and strengthening of A1g model occur without phase transition as entering the SC-II phase, which is supported by the results of phonon spectra calculations. Electronic structure calculations reveal that exertion of pressure may bridge the gap of topological surface nontrivial states near EF, i. e. Z2 invariant. Meanwhile, it enlarges Fermi surface significantly, consistent with the increased carrier density. The findings here point out the change of electronic structure and strengthened electron-phonon coupling should be responsible for the pressure-induced reentrant SC.

cond-mat.supr-con↗