arXiv subjects

Jianfa Zhao

Publications and source records attributed to Jianfa Zhao.

At least 19 recordsLinked to original sources

arXiv2026

Eight-unit-cell electronic modulations in cuprates originating from local molecular orbitals

The pair density wave (PDW) state with eight-unit-cell (8a0) periodicity has been widely regarded as the primary order in cuprates, yet its existence and origin remain subjects of intense debate. Using spectroscopic imaging scanning tunneling microscopy, we observe spatial modulations of the electronic states with approximately 8a0 periodicity in both the superconducting and insulating regimes of hole-doped Ca2CuO2Cl2 cuprate. We find that the 8a0 spatial patterns are generated by the formation of molecular orbitals by doped holes, which organize into 4a0*4a0 plaquettes as the basic unit. Our results identify the 4a0 molecular orbital as the fundamental electronic building block in cuprates, while the 8a0 PDW represents a spatial subharmonic that emerges at sufficiently high doping.

cond-mat.supr-con↗

arXiv2026

Superconductivity of Tellurium Polyhydride with Tc above 90K

We report experimental diacovery of superconductivity (SC) in tellurium (Te) polyhydride. The compound was synthesized at high pressure and high temperature conditions using a diamond anvil cell combined with a laser heating system. Subsequent in situ transport measurements at high pressures, performed as a function of temperature and applied magnetic field, revealed a superconducting transition with a critical temperature Tc about 91 K at 263 GPa. The superconducting phase is assigned to TeH4 with characterized face shared TeH12 cage forming quasi molecular H2 units based on synchrotron x-ray diffraction experiments. Analysis of the SC behavior at magnetic fields yielded a Ginzburg Landau (GL) coherence length of approximately 54 angstroms. Tellurium polyhydride thus becomes another chalcogen polyhydride superconductor in addition to the landmark discovery of the first polyhydride high Tc SC SH3.

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arXiv2026

Superconducting Hydride Mg2RhH6 Experimentally Achieved at Lower Pressure

Although tremendous progress has been made in recent years in the field of polyhydride superconductors, the realization of high critical temperature superconductivity still relies on formidable high pressures. Searching for superconducting hydrides at lower pressures is of particular importance. Here we report the first experimental synthesis of the Mg2RhH6, which achieves superconductivity under a significantly reduced pressure of 30 GPa. The synthesis of Mg2RhH6 proceeds via a two step process (1) preparation of the Mg2RhH5 precursor containing hydrogen atoms stabilized by covalent bonds, followed by (2) hydrogen supplementation resulting in the filling of electrons into anti bonding orbitals above 30 GPa, which was accompanied by the structural transition from RhH5 square pyramid to RhH6 octahedron. Superconductivity is achieved at 30 GPa with a Tc of 24 K, which is further enhanced to 29 K at 53 GPa, evidenced by a sharp drop of resistivity to zero and characteristic suppression of Tc under applied magnetic fields. Our experiments prove the Mg2RhH6 superconductor to be thermodynamically stable above 30 GPa, making it the first case exhibiting a Tc of approximately 30 K at a readily accessible pressure. This study pioneers a highly promising pathway for the rational design and discovery of high temperature superconductors within phonon mediated BCS framework.

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arXiv2026

Outstanding TC Enhancement in 5d-3d Y2NiIrO6 by Compression

Understanding and predicting the properties of 5d compounds critically depend on the identification of the superexchange interactions from which their magnetism emerges. The study of pressure effects on double perovskite Y2NiIrO6 (YNIO) provides deep insight toward this goal. At ambient pressure, YNIO is a ferrimagnetic insulator with the Ir4+-5d Jeff = 1/2 Mott-insulating state. Under physical pressure up to 17 GPa, the compound exhibits concurrent compression on Ni/Ir-O bond lengths and Ni-O-Ir bond angles, leading to an increase of the Curie temperature from 192 to 243 K. On the contrary, external pressure increases distanced Ir-Ir interaction and in turn induces magnetic frustration in Sr2IrO4/Sr3Ir2O7 due to the extended 5d orbitals. In YNIO, the rock-salt ordered Ni-Ir naturally blocks extended superexchange beyond the nearest neighbor, and in turn suppresses such magnetic frustration. Moreover, the orthogonal Ni eg-Ir t2g pathway in YNIO is robust under lattice distortion, while the superexchange is weakened by bond bending in La2NiMnO6 with a similar half-filed eg-t2g configuration. Our findings establish a framework for elucidating the mechanism of 5d-3d superexchange and guide bond-engineered magnetism in iridate-related systems.

cond-mat.str-el↗

arXiv2025

Half-filled metal and molecular-orbital-mediated pairing in cuprate

The cuprates exhibit anomalous momentum-space structure with antinodal gap and nodal arc in the underdoped regime, which evolves into a complete hole-type Fermi surface with a large Luttinger volume in the overdoped regime. The real-space electronic structure is also quite complex, as characterized by microscopic inhomogeneities and intertwined density wave orders. Here we show that doped holes in cuprate form localized electronic molecules consisting of 4a0 plaquettes, and each plaquette contains approximately two holes. The effective local doping level is thus around 1/8, which is sufficient to destroy the underlying AF order and more importantly, recovers the half-filled metallic state of the original CuO2 plane. The restored Fermi surface, hosting one hole per unit cell, is consistent with experimental results and satisfies the Luttinger theorem. We then construct the momentum-space structure of the half-filled metal by considering the real-space configuration of electronic molecules. We show that the electronic potential with 4a0 periodicity imposed by the plaquettes and the quantum size effect of electronic molecules obliterate the nested antinodal Fermi surface sheets, leaving behind short arcs with coherent quasiparticles around the node. We propose that two doped holes in each plaquette occupy the shared molecular orbital and form a spin singlet, which can mediate the pairing of itinerant holes on the remnant Fermi surface of the half-filled metal. The electric dipole moment between the molecular orbitals and the dopant ions may also provide a novel attractive interaction between itinerant holes. This phenomenological model for pair formation between itinerant holes on the half-filled Fermi surface mediated by localized molecular orbitals resolves several core issues concerning the mechanism of superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2025

Ultrafast charge-transfer dynamics in Ca$_2$CuO$_2$Cl$_2$ from time-resolved optical reflectivity

We employ time-resolved optical reflectivity to investigate the ultrafast dynamics of the charge-transfer gap (CTG) in a parent cuprate compound Ca$_2$CuO$_2$Cl$_2$ (CCOC). We observe a persistent photoinduced red shift of the CTG that lasts up to 1000 ps. The red shift during the slow decay after 10 ps can be well modeled by the localized picture, whereas its maximum value at ~0.9 ps involves additional contribution from the renormalization of the Hubbard U due to screening effect from delocalized electrons. Furthermore, coupling between the mid-gap absorption and a slow acoustic phonon launches coherent oscillations below the CTG, observed as a ~20 GHz modulation with a dispersion independent of the pump fluence. These results demonstrate the tunning of the CTG by light, unveil complex interplay between multiple degrees of freedom, and contribute to a deeper understanding of superconductivity and correlated materials.

cond-mat.str-el↗

arXiv2025

Lattice Quantum Geometry Controlling 118 K Multigap Superconductivity in Heavily Overdoped CuBa2Ca3Cu4O10+d

Synchrotron X-ray diffraction has been used to study the thermal structure evolution in CuBa2Ca3Cu4O10+d (Cu1234), a superconductor which exhibits a high critical temperature (Tc 118 K), high critical current density and large upper critical magnetic field. The lattice geometry at nanoscale of this cuprate belongs to the class of natural heterostructures at atomic limit like the artificial high Tc superlattices made of interface space charge in Mott insulator units intercalated by metal units. Temperature-dependent lattice parameters reveal a distinct structural transition at TC characterized by a drop of the c-axis and in plane Cu-O negative thermal expansion below TC. These results provide clear evidence of lattice reorganization associated with the chemical potential changes due to the opening of multiple superconducting gaps. Additionally, evidence for oxygen defects rearrangement is observed at temperatures above 200 K. We construct a phase diagram correlating temperature, the c/a axis ratio, and in plane Cu-O strain, identifying regions associated with gaps opening and oxygen rearrangement. These findings provide new insights into how lattice geometry control superconductivity to inform the material design of advanced nanoscale superconducting artificial quantum heterostructures.

cond-mat.supr-con↗

arXiv2025

High Quality Single Crystal of Kitaev Spin Liquid Candidate Material RuBr3 Synthesized under High Pressure

Kitaev quantum spin liquids have attracted significant attention in condensed matter physics over the past decade. To understand their emergent quantum phenomena, high-quality single crystals of substantial size are essential. Here, we report the synthesis of single crystals of the Kitaev quantum spin liquid candidate RuBr3, achieving millimeter-sized crystals through a self-flux method under high pressure and high temperature conditions. The crystals exhibit well-defined cleavage planes with a lustrous appearance. Transport characterizations exhibit a narrow band-gap semiconducting behavior with 0.13 eV and 0.11 eV band-gap in ab plane and along c axis, respectively. Magnetic measurement shows a transition to antiferromagnetic (AFM) state at approximately 29 K both in ab plane and along c axis. Notably, the N\'eel temperature increases to 34 K with an applied magnetic field of up to 7 T in the ab plane, but without any change along c axis. The large size and high quality of RuBr3 single crystals provide a valuable platform for investigating various interactions, particularly the Kitaev interaction, and for elucidating the intrinsic physical properties of Kitaev quantum spin liquids.

cond-mat.str-el↗

arXiv2024

One Pot Synthesis of Cubic Gauche Polymeric Nitrogen

The long sought cubic gauche polymeric nitrogen (cg-N) consisting of N-N single bonds has been synthesized by a simple route using sodium azide as a precursor at ambient conditions. The recrystallization process was designed to expose crystal faces with low activation energy that facilitates initiating the polymeric reaction at ambient conditions. The azide was considered as a precursor due to the low energy barrier in transforming double bonded N=N to single bonded cg-N. Raman spectrum measurements detected the emerging vibron peaks at 635 cm-1 for the polymerized sodium azide samples, demonstrating the formation of cg-N with N-N single bonds. Different from traditional high pressure technique and recently developed plasma enhanced chemical vapor deposition method, the route achieves the quantitative synthesis of cg-N at ambient conditions. The simple method to synthesize cg-N offers potential for further scale up production as well as practical applications of polymeric nitrogen based materials as high energy density materials.

cond-mat.mtrl-sci↗

arXiv2024

A facile route to synthesize cubic gauche polymeric nitrogen

In this work, the long-sought cg-N with N-N single bond has been synthesized for the first time by a thermal-driven-only chemical route at ambient conditions. The successful synthesis of cg-N was achieved by first creating a solution of azides, which was then pretreated under vacuum conditions. Following the pretreatment, the resultant concentrated azide was heated at temperatures ranging from 260{\deg}C to 330{\deg}C for a reaction time of 3 hours, ultimately leading to the formation of cg-N. The emergent intense Raman peak characterized of cg-N provides solid evidence that the double bonded nitrogen-nitrogen transforms into a single bond form, which agrees well with cg-N structure. To date, this is the only work achieving the quantity of cg-N synthesized at ambient conditions by a facile route that can be further developed for the scalable synthesis and applications of polymerized nitrogen-based materials as high energy density materials.

cond-mat.mtrl-sci↗

arXiv2023

Superconductivity above 30 K achieved in dense scandium

Superconductivity is one of most intriguing quantum phenomena, and the quest for elemental superconductors with high critical temperature (Tc) is of great scientific significance due to their relatively simple material composition and the underlying mechanism. Here we report the experimental discovery of densely compressed scandium (Sc) becoming the first elemental superconductor with Tc breaking into 30 K range, which is comparable to the Tc values of the classic La-Ba-Cu-O or LaFeAsO superconductors. Our results show that Tconset of Sc increases from ~3 K at around 43 GPa to ~32 K at about 283 GPa (Tczero ~ 31 K), which is well above liquid neon temperature. Interestingly measured Tc shows no sign of saturation up to the maximum pressure achieved in our experiments, indicating that Tc might be even higher upon further compression.

cond-mat.supr-con↗

arXiv2023

Visualizing the Zhang-Rice singlet, molecular orbitals and pair formation in cuprate

The parent compound of cuprates is a charge-transfer-type Mott insulator with strong hybridization between the Cu $3d_{\mathrm x^2-y^2}$ and O $2p$ orbitals. A key question concerning the pairing mechanism is the behavior of doped holes in the antiferromagnetic (AF) Mott insulator background, which is a prototypical quantum many-body problem. It was proposed that doped hole on the O site tends to form a singlet, known as Zhang-Rice singlet (ZRS), with the unpaired Cu spin. But experimentally little is known about the properties of a single hole and the interplay between them that leads to superconductivity. Here we use scanning tunneling microscopy to visualize the electronic states in hole-doped $\mathrm{Ca_2CuO_2Cl_2}$, aiming to establish the atomic-scale local basis for pair formation. A single doped hole is shown to have an in-gap state and a clover-shaped spatial distribution that can be attributed to a localized ZRS. When the dopants are close enough, they develop delocalized molecular orbitals with characteristic stripe- and ladder-shaped patterns, accompanied by the opening of a small gap around the Fermi level ($E_{\mathrm F}$). With increasing doping, the molecular orbitals proliferate in space and gradually form densely packed plaquettes, but the stripe and ladder patterns remain nearly the same. The low-energy electronic states of the molecular orbitals are intimately related to the local pairing properties, thus play a vitally important role in the emergence of superconductivity. We propose that the Cooper pair is formed by two holes occupying the stripe-like molecular orbital, while the attractive interaction is mediated by the AF spin background.

cond-mat.supr-con↗

arXiv2023

Observation of Flat Band and Van Hove Singularity in Non-superconducting Nitrogen-doped Lutetium Hydride

Hydrogen-rich materials offer a compelling avenue towards room temperature superconductivity, albeit under ultra-high pressure. However, the experimental investigation of the electronic band structure remains elusive, due to the inherent instability of most of the hydrogen-rich materials upon pressure release. Very recently, nitrogen-doped lutetium hydride was claimed to host room temperature superconductivity under near ambient pressure but was disproven by following works. Upon decompression, nitrogen doped lutetium hydride manifests a stable metallic phase with dark blue color. Moreover, high temperature superconductivity has been reported in lutetium hydrides Lu4H23 (~71 K) under around 200 GPa. These properties engender an unprecedented opportunity, allowing for the experimental investigation of the electronic band structure intrinsic to hydrogen-rich material. In this work, using angle resolved photoemission spectroscopy to investigate the non-superconducting nitrogen doped lutetium hydride, we observed significant flat band and Van Hove singularity marginally below the Fermi level. These salient features, identified as critical elements, proffer potential amplifiers for the realization of heightened superconductivity, as evidenced by prior research. Our results not only unveil a confluence of potent strong correlation effects and anisotropy within the Lu-H-N compound, but also provide a prospect for engineering high temperature superconductivity through the strategic manipulation of flat band and the VHS, effectively tailoring their alignment with the Fermi energy.

cond-mat.supr-con↗

arXiv2023

Superconductivity above 70 K observed in lutetium polyhydrides

The binary polyhydrides of heavy rare earth lutetium that shares a similar valence electron configuration to lanthanum have been experimentally discovered to be superconductive. The lutetium polyhydrides were successfully synthesized at high pressure and high temperature conditions using a diamond anvil cell in combinations with the in-situ high pressure laser heating technique. The resistance measurements as a function of temperature were performed at the same pressure of synthesis in order to study the transitions of superconductivity (SC). The superconducting transition with a maximum onset temperature (Tc) 71 K was observed at pressure of 218 GPa in the experiments. The Tc decreased to 65 K when pressure was at 181 GPa. From the evolution of SC at applied magnetic fields, the upper critical field at zero temperature μ0Hc2(0) was obtained to be ~36 Tesla. The in-situ high pressure X-ray diffraction experiments imply that the high Tc SC should arise from the Lu4H23 phase with Pm-3n symmetry that forms a new type of hydrogen cage framework different from those reported for previous light rare earth polyhydride superconductors.

cond-mat.supr-con↗

arXiv2022

A ferrotoroidic candidate with well-separated spin chains

The search of novel quasi one-dimensional (1D) materials is one of the important aspects in the field of material science. Toroidal moment, the order parameter of ferrotoroidic order, can be generated by a head-to-tail configuration of magnetic moment. It has been theoretically proposed that one-dimensional (1D) dimerized and antiferromagnetic-like spin chain hosts ferrotoroidicity and has the toroidal moment composed of only two antiparallel spins. Here, we report a ferrotoroidic candidate of Ba6Cr2S10 with such a theoretical model of spin chain. The structure consists of unique dimerized face-sharing CrS6 octahedral chains along the c axis. An antiferromagnetic-like ordering at ~10 K breaks both space- and time-reversal symmetries and the magnetic point group of mm'2' allows three ferroic orders in Ba6Cr2S10: (anti)ferromagnetic, ferroelectric and ferrotoroidic orders. Our investigation reveals that Ba6Cr2S10 is a rare ferrotoroidic candidate with quasi 1D spin chain, which can be considered as a starting point for the further exploration of the physics and applications of ferrotoroidicity.

cond-mat.str-el↗

arXiv2021

Long-Time Magnetic Relaxation in Antiferromagnetic Topological Material EuCd$_2$As$_2$

Magnetic topological materials have attracted much attention due to the correlation between topology and magnetism. Recent studies suggest that EuCd$_2$As$_2$ is an antiferromagnetic topological material. Here by carrying out thorough magnetic, electrical and thermodynamic property measurements, we discover a long time relaxation of the magnetic susceptibility in EuCd$_2$As$_2$. The (001) in-plane magnetic susceptibility at 5 K is found to continuously increase up to $\sim$10% over the time of $\sim$14 hours. The magnetic relaxation is anisotropic and strongly depends on the temperature and the applied magnetic field. These results will stimulate further theoretical and experimental studies to understand the origin of the relaxation process and its effect on the electronic structure and physical properties of the magnetic topological materials.

cond-mat.mtrl-sci↗

arXiv2021

Momentum-Resolved Visualization of Electronic Evolution in Doping a Mott Insulator

High temperature superconductivity in cuprates arises from doping a parent Mott insulator by electrons or holes. A central issue is how the Mott gap evolves and the low-energy states emerge with doping. Here we report angle-resolved photoemission spectroscopy measurements on a cuprate parent compound by sequential in situ electron doping. The chemical potential jumps to the bottom of the upper Hubbard band upon a slight electron doping, making it possible to directly visualize the charge transfer band and the full Mott gap region. With increasing doping, the Mott gap rapidly collapses due to the spectral weight transfer from the charge transfer band to the gapped region and the induced low-energy states emerge in a wide energy range inside the Mott gap. These results provide key information on the electronic evolution in doping a Mott insulator and establish a basis for developing microscopic theories for cuprate superconductivity.

cond-mat.str-el↗

arXiv2021

Imaging the atomic-scale electronic states induced by a pair of hole dopants in Ca2CuO2Cl2 Mott insulator

We use scanning tunneling microscopy to visualize the atomic-scale electronic states induced by a pair of hole dopants in Ca2CuO2Cl2 parent Mott insulator of cuprates. We find that when the two dopants approach each other, the transfer of spectral weight from high energy Hubbard band to low energy in-gap state creates a broad peak and nearly V-shaped gap around the Fermi level. The peak position shows a sudden drop at distance around 4 a0 and then remains almost constant. The in-gap states exhibit peculiar spatial distributions depending on the configuration of the two dopants relative to the underlying Cu lattice. These results shed important new lights on the evolution of low energy electronic states when a few holes are doped into parent cuprates.

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