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The research on van der Waals (vdW) layered ferromagnets have promoted the development of nanoscale spintronics and applications. However, low-temperature ferromagnetic properties of these materials greatly hinder their applications. Here, we report pressure-enhanced ferromagnetic behaviours in layered CrSiTe3 flakes revealed by high-pressure magnetic circular dichroism (MCD) measurement. At ambient pressure, CrSiTe3 undergoes a paramagnetic-to-ferromagnetic phase transition at 32.8 K, with a negligible hysteresis loop, indicating a soft ferromagnetic behaviour. Under 4.6 GPa pressure, the soft ferromagnet changes into hard one, signalled by a rectangular hysteretic loop with remnant magnetization at zero field. Interestingly, with further increasing pressure, the coercive field (H_c) dramatically increases from 0.02 T at 4.6 GPa to 0.17 T at 7.8 GPa, and the Curie temperature (T_c^h: the temperature for closing the hysteresis loop) also increases from ~36 K at 4.6 GPa to ~138 K at 7.8 GPa. The influences of pressure on exchange interactions are further investigated by density functional theory calculations, which reveal that the in-plane nearest-neighbor exchange interaction and magneto-crystalline anisotropy increase simultaneously as pressure increases, leading to increased H_c and T_c^h in experiments. The effective interaction between magnetic couplings and external pressure offers new opportunities for both searching room-temperature layered ferromagnets and designing pressure-sensitive magnetic functional devices.

Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus far, the most studied Kitaev systems are 4d/5d-based honeycomb magnets. Recent theoretical studies predicted that 3d-based honeycomb magnets, including Na2Co2TeO6 (NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTO's quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T < B (vertical to b-axis) < 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3d-magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3d compounds, promoting further interests on the spin-orbital effect in quantum magnets., 19 pages and 5 figures for main text. 16 pages and 6 figures for Supplemental Material

Layered ferromagnets with high coercivity have special applications in nanoscale memory elements in electronic circuits, such as data storage. Therefore, searching for new hard ferromagnets and effectively tuning or enhancing the coercivity are the hottest topics in layered magnets today. Here, we report a strain tunability of perpendicular magnetic anisotropy in van der Waals (vdW) ferromagnets VI3 using magnetic circular dichroism measurements. For an unstrained flake, the M-H curve shows a rectangular-shaped hysteresis loop with perpendicular magnetic anisotropy and a large coercivity (up to 1.775 T at 10 K). Furthermore, the coercivity can be enhanced to a maximum of 2.6 T at 10 K under a 2.9% in-plane tensile strain. Our DFT calculations show that the magnetic anisotropy energy (MAE) can be dramatically increased after applying an in-plain tensile strain, which contributes to the enhancement of coercivity in the VI3 flake. Meanwhile, the strain tunability on the coercivity of CrI3, with a similar crystal structure, is limited. The main reason is the strong spin-orbital coupling in V3+ in VI6 octahedra in comparison with that in Cr3+. The strain tunability of coercivity in VI3 flakes highlights its potential for integration into vdW heterostructures, paving the way toward nanoscale spintronic devices and applications in the future.

We investigated the evolution of ferromagnetism in layered Fe3GeTe2 flakes under different pressures and temperatures using in situ magnetic circular dichroism (MCD) spectroscopy. We found that the rectangular shape of the hysteresis loop under an out-of-plane magnetic field sweep can be sustained below 7 GPa. Above that pressure, an intermediate state appears in the low-temperature region signaled by an 8-shaped skewed hysteresis loop. Meanwhile, the coercive field and Curie temperature decrease with increasing pressures, implying the decrease of the exchange interaction and the magneto-crystalline anisotropy under pressures. The intermediate phase has a labyrinthine domain structure, which is attributed to the increase of the ratio of exchange interaction to magneto-crystalline anisotropy based on Jagla's theory. Moreover, our calculations reveal a weak structural transition around 6 GPa that corresponds to a significant change in the FeI-FeI bond length, which has strong influences on magnetic interaction. Detailed analysis on exchange interaction and magneto-crystalline anisotropy with pressure shows a consistent trend with experiments.

Circularly polarized light carries light spin angular momentum, which may lead helicity-resolved Raman scattering to be sensitive to the electronic spin configuration in magnetic materials. Here, we demonstrate that all Raman modes in the 2D ferromagnet VI3 show different scattering intensities to left and right circularly polarized light at low temperatures, which gives direct evidence of the time-reversal symmetry breaking. By measuring the circular polarization of the dominant Raman mode with respect to the temperature and magnetic field, the ferromagnetic (FM) phase transition and hysteresis behavior can be clearly resolved. Besides the lattice excitations, quasielastic scattering is detected in the paramagnetic phase, and it gradually evolves into the acoustic magnon mode at 18.5 cm-1 in the FM state, which gives the spin wave gap that results from large magnetic anisotropy. Our findings demonstrate that helicity-resolved Raman spectroscopy is an effective tool to directly probe the ferromagnetism in 2D magnets.

We report experimental determination of compressibility and thermoelasticity for CrN based on in situ high pressure/high temperature synchrotron x-ray diffraction measurements. The pressure-induced...

How an interfacial superconductivity emerges during the nucleation and epitaxy is of great importance not only for unveiling the physical insights but also for finding a feasible way to tune the superconductivity via interfacial engineering. In this work, we report the nanoscale creation of a robust and relatively homogeneous interfacial superconductivity (TC ≈ 13 K) on the epitaxial FeTe surface, by van der Waals epitaxy of single-quintuple-layer topological insulator Bi2Te3. Our study suggests that the superconductivity in the Bi2Te3/FeTe heterostructure is generated at the interface and that the superconductivity at the interface does not enhance or weaken with the increase of the Bi2Te3 thickness beyond 1 quintuple layer (QL). The observation of the topological surface states crossing Fermi energy in the Bi2Te3/FeTe heterostructure with the average Bi2Te3 thickness of about 20 QL provides further evidence that this heterostructure may potentially host Majorana zero modes.

The recent discovery of 2D magnets has revealed various intriguing phenomena due to the coupling between spin and other degree of freedoms (such as helical photoluminescence, nonreciprocal SHG). Previous research on the spin-phonon coupling effect mainly focuses on the renormalization of phonon frequency. Here we demonstrate that the Raman polarization selection rules of optical phonons can be greatly modified by the magnetic ordering in 2D magnet CrI$_3$. For monolayer samples, the dominant A$\rm_{1g}$ peak shows abnormally high intensity in the cross polarization channel at low temperature, which is forbidden by the selection rule based on the lattice symmetry. While for bilayer, this peak is absent in the cross polarization channel for the layered antiferromagnetic (AFM) state and reappears when it is tuned to the ferromagnetic (FM) state by an external magnetic field. Our findings shed light on exploring the emergent magneto-optical effects in 2D magnets., Comment: 15 pages, 4 figures

Mermin-Wagner-Coleman theorem predicts no long-range magnetic order at finite temperature in the two-dimensional (2D) isotropic systems, but it does predict a quasi-long-range order with a divergent correlation length at the Kosterlitz-Thouless (KT) transition for planar magnets. As a representative of two-dimensional planar antiferromagnets, single-layer ${\mathrm{CoPS}}_{3}$ carries the promise of monolayer antiferromagnetic platforms for the ultimately thin spintronics. Here, with the aid of magnetostriction which is sensitive to the local magnetic order, we observe the signature phonon mode splitting of below ${T}_{\mathrm{KT}}$ in monolayer ${\mathrm{CoPS}}_{3}$. The two-magnon signal in the monolayer one manifests the associated magnetic transition. It indicates the quasi-long-range order in an exact 2D planar spin model below KT phase transition. The ratio ($J$\ensuremath{'}/$J$) between the interlayer and intralayer interactions, which characterizes the 2D behaviors, is evaluated to be around 0.03. Our results provide an efficient method to detect the quasi-long-range antiferromagnetic ordering in the two-dimensional magnets down to the monolayer limit.

Realization of the quantum anomalous Hall effect and axion electrodynamics in topological materials are among the paradigmatic phenomena in condensed matter physics. Recently, signatures of both phases are observed to exist in thin films of MnBi$_2$Te$_4$, a stoichiometric antiferromagnetic topological insulator. Direct evidence of the bulk topological magnetoelectric response in an axion insulator requires an energy gap at its topological surface state (TSS). However, independent spectroscopic experiments revealed that such a surface gap is absent, or much smaller than previously thought, in MnBi$_2$Te$_4$. Here, we utilize angle resolved photoemission spectroscopy and density functional theory calculations to demonstrate that a sizable TSS gap unexpectedly exists in Sb-doped MnBi$_2$Te$_4$. This gap is found to be topologically nontrivial, insensitive to the bulk antiferromagnetic-paramagnetic transition, while enlarges along with increasing Sb concentration. Our work shows that Mn(Bi$_{1-x}$Sb$_x$)$_2$Te$_4$ is a potential platform to observe the key features of the high-temperature axion insulator state, such as the topological magnetoelectric responses and half-integer quantum Hall effects.

The spin polarization in nonmagnetic materials is conventionally attributed to the outcome of spin-orbit coupling when the global inversion symmetry is broken. The recently discovered hidden spin polarization indicates that a specific atomic site asymmetry could also induce measurable spin polarization, leading to a paradigm shift in research on centrosymmetric crystals for potential spintronic applications. Here, combining spin- and angle-resolved photoemission spectroscopy and theoretical calculations, we report distinct spin-momentum-layer locking phenomena in a centrosymmetric, layered material, BiOI. The measured spin is highly polarized along the Brillouin zone boundary, while the same effect almost vanishes around the zone center due to its nonsymmorphic crystal structure. Our work demonstrates the existence of momentum-dependent hidden spin polarization and uncovers the microscopic mechanism of spin, momentum, and layer locking to each other, thus shedding light on the design metrics for future spintronic materials.

In this work, we demonstrate that the nonsuperconducting single-layer FeTe can become superconducting when its structure is properly tuned by epitaxially growing it on Bi2Te3 thin films. The properties of the single-layer FeTe deviate strongly from its bulk counterpart, as evidenced by the emergence of a large superconductivity gap (3.3 meV) and an apparent 8 × 2 superlattice (SL). Our first-principles calculations indicate that the 8 × 2 SL and the emergence of the novel superconducting phase are essentially the result of the structural change in FeTe due to the presence of the underlying Bi2Te3 layer. The structural change in FeTe likely suppresses the antiferromagnetic order in the FeTe and leads to superconductivity. Our work clearly demonstrates that moire pattern engineering in a heterostructure is a reachable dimension for investigating novel materials and material properties.

Layered honeycomb lattice antiferromagnetic $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ru}{\mathrm{Cl}}_{3}$ has been studied intensively recently as a Kitaev spin liquid candidate. Here we present strong evidence that this material host realistic Weyl fermions derived from Cl $2p$ electrons. Our theoretical analysis suggests the existence of quadratic Weyl fermions without spin-orbit coupling and linear Weyl fermion with spin-orbit coupling. Angle-resolved photoemission spectroscopy with systematic photon energy-dependent measurements demonstrates that this Weyl cone possesses linear dispersion at bulk Brillouin zone center along all the three reciprocal directions and hyperbolic dispersion off center. Our work reveals the rich topological physics lying in the band structure of Kitaev QSL candidates and will stimulate further investigation not only limited to $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ru}{\mathrm{Cl}}_{3}$ but also honeycomb iridate family of materials.

The recent discovery of superconductivity in doped infinite-layer nickelates has stimulated intensive interest, especially for similarities and differences compared to that in cuprate superconductors. In contrast to cuprates, although earlier magnetization measurement reveals a Curie-Weiss-like behavior in undoped infinite-layer nickelates, there is no magnetic ordering observed by elastic neutron scattering down to liquid helium temperature. Until now, the nature of the magnetic ground state in undoped infinite-layer nickelates was still elusive. Here, we perform a nuclear magnetic resonance (NMR) experiment through 139La nuclei to study the intrinsic spin susceptibility of infinite-layer LaNiO2. First, the signature for magnetic ordering or freezing is absent in the 139La NMR spectrum down to 0.24 K, which unambiguously confirms a paramagnetic ground state in LaNiO2. Second, a pseudogap-like behavior instead of Curie-Weiss-like behavior is observed in both the temperature-dependent Knight shift and nuclear spin-lattice relaxation rate (1/T1), which is widely observed in both underdoped cuprates and iron-based superconductors. Furthermore, the scaling behavior between the Knight shift and 1/T1T has also been discussed. Finally, the present results imply a considerable exchange interaction in infinite-layer nickelates, which sets a strong constraint for the proposed theoretical models., 14 pages, 4 figures, accepted by Physical Review Letters

Beyond the absence of long-range magnetic orders, the most prominent feature of the elusive quantum spin liquid (QSL) state is the existence of fractionalized spin excitations, i.e., spinons. When the system orders, the spin-wave excitation appears as the bound state of the spinon-antispinon pair. Although scarcely reported, a direct comparison between similar compounds illustrates the evolution from spinon to magnon. Here, we perform the Raman scattering on single crystals of two quantum kagome antiferromagnets, of which one is the kagome QSL candidate Cu3Zn(OH)6FBr, and another is an antiferromagnetically ordered compound EuCu3(OH)6Cl3. In Cu3Zn(OH)6FBr, we identify a unique one spinon-antispinon pair component in the E2g magnetic Raman continuum, providing strong evidence for deconfined spinon excitations. In contrast, a sharp magnon peak emerges from the one-pair spinon continuum in the Eg magnetic Raman response once EuCu3(OH)6Cl3 undergoes the antiferromagnetic order transition. From the comparative Raman studies, we can regard the magnon mode as the spinon-antispinon bound state, and the spinon confinement drives the magnetic ordering., Spinon excitations of a Kagome quantum spin liquid are expected to give rise to a magnetic continuum in Raman spectroscopy. Here, the authors report a magnetic Raman continuum in the Kagome spin liquid candidate Cu3Zn(OH)6FBr, in contrast to a sharp magnon Raman peak in the Kagome antiferromagnet EuCu3(OH)6Cl3.

Raman spectroscopy has been applied to investigate the lattice and magnetic excitations of the honeycomb Kitaev material $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{L}{\mathrm{i}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$. The temperature-dependent Raman spectra give evidence for an unusual magnetic excitation, typified by a broad continuum. The integrated intensity of this Raman continuum does not follow the bosonic statistics expected for the conventional insulating magnets, which indicates the emergence of itinerant Majorana fermions from spin fractionalization in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{L}{\mathrm{i}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$. In addition, the Raman results under pressures resolve one critical pressure at ${P}_{c}=4.1\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ with structural and magnetic phase transitions, signaled by the in-plane Ir-Ir bond dimerization and magnetic collapse, and the system turns into a correlated band insulator above ${P}_{c}$.

Topological insulator with antiferromagnetic order can serve as an ideal platform to realize axion electrodynamics. In this paper, we report a systematic study of the axion topological insulator candidate ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$. A linear energy dispersion across the Fermi level reveals a hole-type Fermi pocket. The orientation of the magnetic moment for ground state is determined within the $ab$-plane by anisotropic magnetic behavior. Besides long-range antiferromagnetic order, magnetization and magnetotransport measurements indicate existence of the ferromagnetic orders and ferromagnetic correlation, suggesting the formation of the magnetic polarons. These ferromagnetic clusters can persist above the antiferromagnetic transition leading to unconventional transport properties. Our results suggest multiple magnetic orders and states in ${\mathrm{EuIn}}_{2}{\mathrm{As}}_{2}$, which is vital to understanding its topological nature.

The authors combined the Heyd-Scuseria-Ernzerhof hybrid density functional first-principles calculation and the cluster exact diagonalization to study the strongly correlated electronic structures of the nickelate oxides Nd1\ensuremath{-}xSrxNiO2 and derive the effective one-band Hamiltonian model for the superconductivity.

We report the magnetotransport properties of $\mathrm{MnB}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ thin flakes through gate modulation at low temperatures. Under in-plane magnetic field, a large negative magnetoresistance (MR) maintains up to 10 T, which is related to the suppression of spin scattering when the magnetic order is gradually forced into the ferromagnetic (FM) state. Under perpendicular magnetic field, a steep resistance decrease is observed around \ensuremath{\sim}3 T, corresponding to the transition from an antiferromagnetic (AFM) to a canted AFM (CAFM) state. Due to the net Berry curvature, a notable anomalous Hall effect is observed and can be effectively tuned by gate voltages. The enhanced Hall coefficient would emerge under high magnetic fields when the Fermi level is close to the charge neutral point. Moreover, a transition from negative to positive MR is obtained when increasing the magnetic field. A large linear positive MR occurs around $\ensuremath{\sim}8\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, corresponding to the CAFM-FM transition. The nonsaturated positive MR here may have a similar mechanism to the one in Weyl semimetals, revealing the strong combination between topology and magnetism in $\mathrm{MnB}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{4}$.

The layered MnBi2nTe3n+1 family represents the first intrinsic antiferromagnetic topological insulator (AFM TI, protected by a combination symmetry ) ever discovered, providing an ideal platform to explore novel physics such as quantum anomalous Hall effect at elevated temperature and axion electrodynamics. Recent angle-resolved photoemission spectroscopy (ARPES) experiments on this family have revealed that all terminations exhibit (nearly) gapless topological surface states (TSSs) within the AFM state, violating the definition of the AFM TI, as the surfaces being studied should be -breaking and opening a gap. Here we explain this curious paradox using a surface-bulk band hybridization picture. Combining ARPES and first-principles calculations, we prove that only an apparent gap is opened by hybridization between TSSs and bulk bands. The observed (nearly) gapless features are consistently reproduced by tight-binding simulations where TSSs are coupled to a Rashba-split bulk band. The Dirac-cone-like spectral features are actually of bulk origin, thus not sensitive to the-breaking at the AFM surfaces. This picture explains the (nearly) gapless behaviour found in both Bi2Te3- and MnBi2Te4-terminated surfaces and is applicable to all terminations of MnBi2nTe3n+1 family. Our findings highlight the role of band hybridization, superior to magnetism in this case, in shaping the general surface band structure in magnetic topological materials for the first time., 18 pages, 4 figures

A topological insulator with magnetic elements shows surprising conductive behavior on its surface, presenting either a hurdle to realizing exotic quantum phenomena or a boon to devices with new charge transport mechanisms.

Herbertsmithite and Zn-doped barlowite are two compounds for experimental realization of twodimensional gapped kagome spin liquid. Theoretically, it has been proposed that charge doping a quantum spin liquid gives rise to exotic metallic states, such as high-temperature superconductivity. However, one recent experiment about herbertsmithite with successful Li-doping shows surprisingly the insulating state even under the heavy doped scenario, which can hardly be explained by many-body physics. Using first-principles calculation, we performed a comprehensive study about the Li intercalated doping effect of these two compounds. For the Li-doped herbertsmithite, we identified the optimized Li position at the Cl-(OH)$_3$-Cl pentahedron site instead of previously speculated Cl-(OH)$_3$ tetrahedral site. With the increase of Li doping concentration, the saturation magnetization decreases linearly due to the charge transfer from Li to Cu ions. Moreover, we found that Li forms chemical bonds with the nearby (OH)$^-$ and Cl$^-$ ions, which lowers the surrounding chemical potential and traps the electron, as evidenced by the localized charge distribution, explaining the insulating behavior measured experimentally. Though with different structure from herbertsmithite, Zn-doped Barlowite shows the same features upon Li doping. We conclude that Li doping this family of kagome spin liquid cannot realize exotic metallic states, other methods should be further explored, such as element substitution with different valence electrons., Comment: 6 figures

Quantum spin Hall system can exhibit exotic spin transport phenomena, mediated by its topological edge states. Here the concept of bending strain engineering to tune the spin transport properties of a quantum spin Hall system is demonstrated. We show that bending strain can be used to control the spin orientation of counter-propagating edge states of a quantum spin system to generate a non-zero spin current. This physics mechanism can be applied to effectively tune the spin current and pure spin current decoupled from charge current in a quantum spin Hall system by control of its bending curvature. Furthermore, the curved quantum spin Hall system can be achieved by the concept of topological nanomechanical architecture in a controllable way, as demonstrated by the material example of Bi/Cl/Si(111) nanofilm. This concept of bending strain engineering of spins via topological nanomechanical architecture affords a promising route towards the realization of topological nano-mechanospintronics., Strain engineering alters the topological properties of quantum spin Hall insulators, leading to potential applications in spintronics. Here the authors demonstrate that bending strain can be used to tune the spin transport properties and generate a non-zero spin current in curved in Bi/Cl/Si nanofilms.

The heterostructures of the ferromagnet (Cr2Te3) and topological insulator (Bi2Te3) have been grown by molecular beam epitaxy. The topological Hall effect as evidence of the existence of magnetic skyrmions has been observed in the samples in which Cr2Te3 was grown on top of Bi2Te3. Detailed structural characterizations have unambiguously revealed the presence of intercalated Bi bilayer nanosheets right at the interface of those samples. The atomistic spin-dynamics simulations have further confirmed the existence of magnetic skyrmions in such systems. The heterostructures of ferromagnet and topological insulator that host magnetic skyrmions may provide an important building block for next generation of spintronics devices.

Based on first-principles calculations and k .p effective models, we report physical properties of ferroelectric-like hexagonal polar metals with P63mc symmetry, which are distinct from those of conventional metals with spatial inversion symmetry. Spin textures exist on the Fermi surface of polar metals (e.g. ternary LiGaGe and elemental polar metal of Bi) and spin-momentum locking exist on accidental Dirac cones on the sixfold rotational axis, due to the spin-orbit coupling and lack of inversion symmetry in polar space group. This effect has potential applications in spin-orbitronics. Dirac points are also predicted in LiGaGe., 14 pages, 8 figures

A geometrically frustrated lattice may host frustrated electron spin or charge states that spawn exotic quantum phases. We show that a newly synthesized metal-organic framework of Cu-Hexaaminobenzene [Cu$_3$(HAB)$_2$] exhibits a multi spectra of unusual quantum phases long sought after in condensed matter physics. On one hand, the Cu$^{2+}$ ions form an ideal $S$-1/2 antiferromagnetic kagome lattice. On the other hand, the conjugated-electrons from the organic ligands give rise to completely dispersionless energy bands around the Fermi level, reproducing a frustrated $\pi_x$-$\pi_y$ hopping model on a honeycomb lattice. We propose to characterize the coexistence of frustrated local spins and conjugated electrons through scanning tunneling microscopy simulations. Most remarkably, their close energy proximity enables one to tune the system between the two frustrated states by doping up to one hole per HAB unit. Thus, Cu$_3$(HAB)$_2$ provides a unique exciting platform to investigate the interplay of frustrated spins and electrons in one single lattice, e.g. by gating experiments, which will undoubtedly raise interesting theoretical questions leading to possible new condensed-matter phases.

Emergent phenomena such as unconventional superconductivity, Mott-like insulators, and the peculiar quantum Hall effect in graphene-based heterostructures are proposed to stem from the superlattice-induced renormalization of (moire) Dirac fermions at the graphene Brillouin zone corners. Understanding the corresponding band structure commonly demands photoemission spectroscopy with both sub-meV resolution and large-momentum coverage, beyond the capability of the current state-of-the-art. Here the realization of moire Dirac cones around the Brillouin zone center in monolayer In2 Se3 /bilayer graphene heterostructure is reported. The renormalization is evidenced by reduced Fermi velocity (≈23%) of the moire Dirac cones and the reshaped Dirac point at the Γ point where they intersect. While there have been many theoretical predictions and much indirect experimental evidence, the findings here are the first direct observation of Fermi velocity reduction of the moire Dirac cones. These features suggest strong In2 Se3 /graphene interlayer coupling, which is comparable with that in twisted bilayer graphene. The strategy expands the choice of materials in the heterostructure design and stimulates subsequent broad investigations of emergent physics at the sub-meV energy scale.

We present a detailed study on the structural phase transition in α-TiBr3, which is deeply connected with the lattice and orbital degree of freedoms. A chemical vapor transport method is adopted to synthesize the α-TiBr3 single crystal samples, and the structural phase transition at about 180 K is characterized by x-ray diffraction (XRD), magnetic susceptibility, and specific heat capacity. To further the understanding in the physical nature of this phase transition, a systematic Raman spectroscopic study is performed on α-TiBr3 crystals. With temperature decreasing, a large frequency blue shift and peak width narrowing are observed in the vibrational mode associated with Ti in-plane relative movement, which indicates the formation of Ti–Ti bonding and orbital-fluctuation freezing at low temperatures. These results are fully consistent with magnetic–nonmagnetic phase transition resolved by the measurement of magnetic susceptibility and lattice changes by XRD.

Majorana fermions have been predicted to exist at the edge states of a two-dimensional topological superconductor. We fabricated single quintuple layer (QL) Bi2Te3/FeTe heterostructure with the step-flow epitaxy method and studied the topological properties of this system by using angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. We observed the coexistence of robust superconductivity and edge states on the single QL Bi2Te3 islands which can be potential evidence for topological superconductor.

${\mathrm{NaFe}}_{0.5}{\mathrm{Cu}}_{0.5}\mathrm{As}$ represents a rare exception in the metallic iron pnictide family, in which a small insulating gap is opened. Based on first-principles study, we provide a comprehensive theoretical characterization of this insulating compound. The ${\mathrm{Fe}}^{3+}$ spin degree of freedom is quantified as a quasi-one-dimensional (1D) $S=\frac{5}{2}$ Heisenberg model. The itinerant As hole state is downfolded to a ${p}_{xy}$-orbital hopping model on a square lattice. An orbital-dependent Hund's coupling between the spin and the hole is revealed. Several important material properties are analyzed, including (a) the factors affecting the small $p\ensuremath{-}d$ charge-transfer gap; (b) the role of extra interchain Fe atoms; and (c) quasi-1D spin excitation in the Fe chains. The experimental manifestations of these properties are discussed.

We describe electron spin resonance in a quantum spin liquid with significant spin-orbit coupling. We find that the resonance directly probes spinon continuum which makes it an efficient and informative probe of exotic excitations of the spin liquid. Specifically, we consider spinon resonance of three different spinon mean-field Hamiltonians, obtained with the help of projective symmetry group analysis, which model a putative quantum spin liquid state of the triangular rare-earth antiferromagnet YbMgGaO4. The band of absorption is found to be very broad and exhibit strong van Hove singularities of single spinon spectrum as well as pronounced polarization dependence., 14 pages, 9 figures (including supplement)

We report a new kagome quantum spin liquid candidate Cu$_3$Zn(OH)$_6$FBr, which does not experience any phase transition down to 50 mK, more than three orders lower than the antiferromagnetic Curie-Weiss temperature ($\sim$ 200 K). A clear gap opening at low temperature is observed in the uniform spin susceptibility obtained from $^{19}$F nuclear magnetic resonance measurements. We observe the characteristic magnetic field dependence of the gap as expected for fractionalized spin-1/2 spinon excitations. Our experimental results provide firm evidence for spin fractionalization in a topologically ordered spin system, resembling charge fractionalization in the fractional quantum Hall state., 5 pages and 3 figures. Supplementary Materials file is in the source file and also can be found in CPL published online version

Barlowite $Cu_4(OH)_6FBr$ is a newly found mineral containing $Cu^{2+}$ kagome planes. Despite similarities in many aspects to Herbertsmithite $Cu_3Zn(OH)_6Cl_2$, the well-known quantum spin liquid (QSL) candidate, intrinsic Barlowite turns out not to be a QSL, possibly due to the presence of $Cu^{2+}$ ions in between kagome planes that induce interkagome magnetic interaction [PRL, 113, 227203 (2014)]. Using first-principles calculation, we systematically study the feasibility of selective substitution of the interkagome Cu ions with isovalent nonmagnetic ions. Unlike previous speculation of using larger dopants, such as $Cd^{2+}$ and $Ca^{2+}$, we identify the most ideal stoichiometric doping elements to be Mg and Zn in forming $Cu_3Mg(OH)_6FBr$ and $Cu_3Zn(OH)_6FBr$ with the highest site selectivity and smallest lattice distortion. The equilibirium anti-site disorder in Mg/Zn- doped Barlowite is estimated to be one order of magnitude lower than that in Herbertsmithite. The single-electron band structure and orbital component analysis show that the proposed selective doping effectively mitigates the difference between Barlowite and Herbertsmithite., Comment: 5 pages, 3 figures

Cu(1,3-benzenedicarboxylate) [Cu(1,3-bdc)] contains structurally perfect kagome planes formed by ${\mathrm{Cu}}^{2+}$ ions without the presence of diamagnetic defects. This organometallic compound should serve as a precious platform to explore quantum frustrated magnetism, yet the experimental results so far are mysterious, leading to questions such as, ``Is Cu(1,3-bdc) just a trivial weak ferromagnet?'' Using the density functional theory, we have systematically studied the electronic and magnetic properties of Cu(1,3-bdc), putting forth a theoretical basis to clarify this novel material. We present numerical evidence of a dominating antiferromagnetic (AFM) exchange between nearest-neighbor (NN) ${\mathrm{Cu}}^{2+}$ as experimentally extracted from the high-temperature susceptibility data. We further show that beyond the NN AFM exchange, the additional interactions in Cu(1,3-bdc) have similar strength as those in the well-studied kagome antiferromagnet, herbertsmithite, by designing a comparative study. In the end, we discuss our understanding of the phase transition and FM signals observed under low temperature.

Gutzwiller projection is a way to construct many-body wave functions that could carry topological order or symmetry-protected topological (SPT) order. However, an important issue is to determine whether or not a given Gutzwiller-projected wave function (GWF) carries a nontrivial SPT order, and which SPT order is carried by the wave function. In this paper, we numerically study the SPT order in a spin $S=1$ GWF on the kagome lattice. Using the standard Monte Carlo method, we directly confirm that the GWF has (1) gapped bulk with short-range correlations, (2) a trivial topological order via a nondegenerate ground state, and zero topological entanglement entropy, (3) a nontrivial $U(1)\ifmmode\times\else\texttimes\fi{}U(1)$ SPT order via the Hall conductances of the protecting $U(1)\ifmmode\times\else\texttimes\fi{}U(1)$ symmetry, and (4) a symmetry-protected gapless boundary. This represents numerical evidence of continuous symmetry-protected topological order in two-dimensional bosonic lattice systems.

The discovery of high-temperature superconductivity in FeSe/STO has trigged great research interest to reveal a range of exotic physical phenomena in this novel material. Here we present a temperature dependent magnetotransport measurement for ultrathin FeSe/STO films with different thickness and protection layers. Remarkably, a surprising linear magnetoresistance (LMR) is observed around the superconducting transition temperatures but absent otherwise. The experimental LMR can be reproduced by magnetotransport calculations based on a model of magnetic field dependent disorder induced by spin fluctuation. Thus, the observed LMR in coexistence with superconductivity provides the first magnetotransport signature for spin fluctuation around the superconducting transition region in ultrathin FeSe/STO films.

Generally strong charge and magnetic inhomogeneities are observed in NQR/NMR experiments on underdoped cuprates. It is not the case for the underdoped HgBa$_2$CuO$_{4+\delta}$, the most symmetric and highest T$_c$ single layer cuprate, whose magnetic inhomogeneity is strongly suppressed. Also neutron scattering experiments reveal a unique pair of weakly dispersive magnetic modes in this material. We propose that these special properties stem from the symmetric positioning of the O-dopants between adjacent CuO$_2$ layers that lead to a strong superexchange interaction between a pair of hole spins. In this Letter we present a theoretical model, which gives a consistent explanation to the anomalous magnetic properties of this material.

We implement the universal wave function overlap (UWFO) method to extract modular $S$ and $T$ matrices for topological orders in Gutzwiller-projected parton wave functions (GPWFs). The modular $S$ and $T$ matrices generate a projective representation of $SL(2,\mathbb{Z})$ on the degenerate-ground-state Hilbert space on a torus and may fully characterize the 2+1D topological orders, i.e. the quasi-particle statistics and chiral central charge (up to $E_8$ bosonic quantum Hall states). We used the variational Monte Carlo method to computed the $S$ and $T$ matrices of the chiral spin liquid (CSL) constructed by the GPWF on the square lattice, and confirm that the CSL carries the same topological order as the $\nu=\frac{1}{2}$ bosonic Laughlin state. We find that the non-universal exponents in UWFO can be small and direct numerical computation is able to be applied on relatively large systems. We also discuss the UWFO method for GPWFs on other Bravais lattices in two and three dimensions by using the Monte Carlo method. UWFO may be a powerful method to calculate the topological order in GPWFs., Comment: 5 pages with 3 figures

The Fermi surface in underdoped cuprates is reconstructed by the charge density wave (CDW) order in the pseudogap phase. Theoretical proposals can be divided into two classes: one assumes the underlying Fermi surface without CDW as a conventional large surface; the other assumes small hole-like Fermi pockets. In both scenarios, we theoretically study the quantum oscillation and find three evenly spaced peaks in the oscillation spectra. The central dominant peak is induced by the CDW order. Its effective mass is strongly enhanced as the CDW vanishes in agreement with experiments. But the two scenarios have different understandings of the subdominant satellite peaks. In the large-surface scenario they are induced by the interlayer tunneling between the bilayer CuO$_{2}$ planes. Their effective masses are also enhanced with descreasing CDW. In the small-pocket scenario one of the subdominant peaks comes from the original small Fermi pockets of the pseudogap state. Its effective mass is nearly independent of the CDW strength and increases monotonically with the doping. We propose future quantum oscillation experiments to test these different predictions and thus to clarify the underlying Fermi surface structure of the pseudogap state., Comment: published version

Based on the NMR measurements on Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$ (La-Bi2201) in strong magnetic fields, we identify the non-superconducting pseudogap phase in the cuprates as a Luttinger-volume violating Fermi liquid (LvvFL). This state is a zero temperature quantum liquid that does not break translational symmetry, and yet, the Fermi surface encloses a volume smaller than the large one given by the Luttinger theorem. The particle number enclosed by the small Fermi surface in the LvvFL equals the doping level $p$, not the total electron number $n_e=1-p$. Both the phase string theory and the dopon theory are introduced to describe the LvvFL. For the dopon theory, we can obtain a semi-quantitative agreement with the NMR experiments., Comment: The final version in PRB

We show that a suitable combination of geometric frustration, ferromagnetism and spin-orbit interactions can give rise to nearly flat bands with a large bandgap and non-zero Chern number. Partial filling of the flat band can give rise to fractional quantum Hall states at high temperatures (maybe even room temperature). While the identification of material candidates with suitable parameters remains open, our work indicates intriguing directions for exploration and synthesis., 4 pages, 6 figures

We study the ground state phases for a mixture of two atomic spin-1 Bose-Einstein condensates (BECs) in the presence of a weak magnetic (B-) field. The ground state is found to contain a broken-axisymmetry (BA) phase due to competitions among intra- and inter-species spin exchange interactions and the linear Zeeman shifts. This is in contrast to the case of a single species spin- 1 condensate, where the axisymmetry breaking results from competitions among the linear and quadratic Zeeman shifts and the intra-species ferromagnetic interaction. All other remaining ground state phases for the mixture are found to preserve axisymmetry. We further elaborate on the ground state phase diagram and calculate their Bogoliubov excitation spectra. For the BA phase, there exist three Goldstone modes attempting to restore the broken U(1) and SO(2) symmetries., 10 pages, 7 figures

We identify a new kind of elementary excitations, spin-rotons, in the doped Mott insulator. They play a central role in deciding the superconducting transition temperature Tc, resulting in a simple Tc formula,Tc=Eg/6, with Eg as the characteristic energy scale of the spin rotons. We show that the degenerate S=1 and S=0 rotons can be probed by neutron scattering and Raman scattering measurements, respectively, in good agreement with the magnetic resonancelike mode and the Raman A1g mode observed in the high-Tc cuprates., 10 pages, 9 figures