arXiv subjects

Xianhui Chen

Publications and source records attributed to Xianhui Chen.

At least 37 records · Page 2Linked to original sources

arXiv2025

Electronic-correlation-assisted charge stripe order in a Kagome superconductor

A central mystery in high-temperature cuprate superconductors is the coexistence of multiple exotic orders, which is presumably associated with strong electronic correlation. The ongoing interest in this enigmatic phenomenon is further energized when similar electronic orders and states emerge and coexist in less correlated Kagome superconductors. Here, by utilizing angle-resolved photoemission spectroscopy (ARPES), nuclear magnetic resonance (NMR) spectroscopy, scanning tunneling microscopy (STM) and first-principles calculations, we reveal the sudden emergence of a distinct short-range charge stripe order in Sn-doped CsV$_3$Sb$_5$ Kagome superconductors when the long-range $2 \times 2$ charge density wave order in pristine CsV$_3$Sb$_5$ is suppressed. This short-range stripe order features a modulation vector of approximately 1/3 along one of the three lattice directions, induces remarkable quasiparticle scattering between the original quasi-1D Kagome-d-bands and their replica of folding, and clearly suppresses the electron density of states at the Fermi level. Our first-principles calculations reveal that $3 \times 1$ supermodulation represents a hidden secondary instability in pristine CsV$_3$Sb$_5$. This instability is further enhanced by in-plane chemical pressure induced by Sn substitution, and coupled to the electronic correlation, leading to a unique charge stripe order in the system. As such, our results reveal a new route toward emergent electronic orders via cooperative interactions between the lattice and electronic degrees of freedom.

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arXiv2025

Recent progress in nickelate superconductors

The discovery of superconductivity in nickelate compounds has opened new avenues in the study of high-temperature superconductors. Here we provide a comprehensive overview of recent progress in the field, including all different nickelate systems, reduced-Ruddlesden-Popper-type infinite layer LaNiO$_2$, Ruddlesden-Popper-type bilayer La$_3$Ni$_2$O$_7$ and trilayer La$_4$Ni$_3$O$_{10}$. We begin by introducing the superconducting properties of the hole-doped LaNiO$_2$ system, which marked the starting point for nickelate superconductivity. We then turn to the bilayer La$_3$Ni$_2$O$_7$ system, discussing both its high-pressure and thin-film superconducting phases. This is followed by an examination of the trilayer La$_4$Ni$_3$O$_{10}$ system and other related multilayer nickelates. Throughout the review, we highlight emerging trends, key challenges, and open questions. We conclude by addressing current limitations in materials synthesis and characterization, and future directions that may help uncover the mechanisms driving superconductivity in these complex oxide systems.

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arXiv2025

Unveiling the landscape of Mottness and its proximity to superconductivity in 4Hb-TaS$_2$

Mott physics is at the root of a plethora of many-body quantum phenomena in quantum materials. Recently, the stacked or twisted structures of van der Waals (vdW) materials have emerged as a unique platform for realizing exotic correlated states in the vicinity of the Mott transition. However, the definitive feature of Mottness and how it rules the low-energy electronic state remain elusive and experimentally inaccessible in many interesting regimes. Here, we quantitatively describe a filling-controlled Mott state and its interplay with superconductivity by scanning tunnelling spectroscopy in a vdW bulk heterostructure, 4Hb-TaS$_2$, that interleaves strongly correlated 1T-TaS$_2$ layers with superconducting 1H-Ta$_2$ layers. The fine tunability of electron doping induced by interlayer charge transfer allows us to continuously track the spectral function with unsurpassed energy resolution from a depleted narrow band (0.2 electrons per site) toward a Mott transition at half filling. The gradually emerging Mott-Hubbard bands, followed by the sharpening and vanishing of the central quasiparticle peak as predicted in the Brinkman-Rice scenario, unambiguously demonstrate the Mott physics at play. Importantly, the renormalization of the low-energy electrons acts destructively on the superconducting pairing potential, leaving behind nonsuperconducting, paramagnetic puddles at the nanoscale. Our results reveal a seminal system near the border of the Mott criterion that enables us to illustrate the predictive power of the Hubbard model, and set such heterostructures as promising ground for realizing new correlated states in the heavily doped Mott regime.

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arXiv2025

Anomalous narrow-band correlation in a natural superconducting heterostructure

A new frontier in condensed matter physics is to stack atomically thin layered-materials with different properties and create intriguing phenomena which do not exist in any of the constituent layers. Transition metal dichalcogenide 4Hb-TaS$_2$, with an alternating stacking of a spin liquid candidate 1T-TaS$_2$ and a superconductor 1H-TaS$_2$, is a natural heterostructure for such a purpose. Recently, rare phenomena are indeed observed, including chiral superconductivity, two-component nematic superconductivity, topological surface superconductivity and enigmatic magnetic memory. A widely proposed starting point to understand such a mysterious heterostructure requires strong electronic correlation, presumably provided by 1T-TaS$_2$ layers with a narrow flat band near the Fermi level ($E_F$). Here, by using angle-resolved photoemission spectroscopy, we reveal the theoretically expected flat band near $E_F$ in the energy-momentum space for the first time. However, this flat band only exists on the 1T-TaS$_2$ terminated surface layer with broken translational symmetry, but not on the 1T-TaS$_2$ layers buried in the bulk. These results directly challenge the foundation of the current theoretical paradigm. On the 1T-TaS$_2$ terminated surface layer, we further reveal a pseudogap and an anomalous doping effect. These phenomena and the dichotomy between surface and bulk layers also shed new light on the unusual coexistence of distinct electronic orders in this mysterious heterostructure.

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arXiv2025

Origin of local magnetic exchange interaction in infiite-layer nickelates

Significant magnetic exchange interactions have been observed in infinite-layer nickelates RNiO2 (R = La, Pr, Nd), which exhibit unconventional superconductivity upon hole doping. Despite their structural and Fermi surface similarities to cuprates, infinite-layer nickelates possess a larger charge transfer gap, which influences their magnetic exchange interactions via oxygen. In this work, we performed 17O nuclear magnetic resonance (NMR) measurements on LaNiO2 and Sr-doped LaNiO2, revealing glassy spin dynamics originating from Ni-O planes. This indicates that infinite-layer nickelates are in proximity to magnetic ordering and that magnetic correlations play a crucial role in their physics. More importantly, our analysis of the Knight shift and hyperfine coupling of 17O nuclei revealed that the Ni-Ni superexchange interaction, mediated by the σ bond between the Ni-dx2-y2 and O-p orbitals, is one order of magnitude weaker than that in cuprates. This alone cannot account for the total magnetic exchange interaction observed in nickelates. First-principles many-body calculations indicate that an interstitial s orbital near the Fermi level, coupled with the Ni-d3z2-r2 orbital, significantly enhances the superexchange interaction. This contrasts with cuprates, where magnetic interactions are predominantly governed by Cu-dx2-y2 superexchange via oxygen. Our findings provide new insights into the distinct magnetic interactions in infinite-layer nickelates and their potential role in unconventional superconductivity.

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arXiv2025

Symmetry-Broken Kondo Screening and Zero-Energy Mode in the Kagome Superconductor CsV3Sb5

The quantum states of matter reorganize themselves in response to defects, giving rise to emergent local excitations that imprint unique characteristics of the host states. While magnetic impurities are known to generate Kondo screening in a Fermi liquid and Yu-Shiba-Rusinov (YSR) states in a conventional superconductor, it remains unclear whether they can evoke distinct phenomena in the kagome superconductor AV3Sb5 (where A is K, Rb or Cs), which may host an orbital-antiferromagnetic charge density wave (CDW) state and an unconventional superconducting state driven by the convergence of topology, geometric frustration and electron correlations. In this work, we visualize the local density of states induced near various types of impurities in both the CDW and superconducting phases of CsV3-xMxSb5 (M = Ta, Cr) using scanning tunneling microscopy. We observe Kondo resonance states near magnetic Cr dopants. Notably, unlike in any known metal or CDW compound, the spatial pattern of Kondo screening breaks all in-plane mirror symmetries of the kagome lattice, suggesting an electronic chirality due to putative orbital loop currents. While Cooper pairs show relative insensitivity to nonmagnetic impurities, native V vacancies with weak magnetic moments induce a pronounced zero-bias conductance peak (ZBCP). This ZBCP coexists with trivial YSR states within the superconducting gap and does not split in energy with increasing tunneling transmission, tending instead to saturate. This behavior is reminiscent of signature of Majorana zero modes, which could be trapped by a sign-change boundary in the superconducting order parameter near a V vacancy, consistent with a surface topological superconducting state. Our findings provide a new approach to exploring novel quantum states on kagome lattices.

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arXiv2025

Pressure-enhanced spin-density-wave transition in double-layer nickelate $La_{3}Ni_{2}O_{7-δ}$

Recently, a signature of high-temperature superconductivity above the liquid nitrogen temperature (77 K) was reported for $La_{3}Ni_{2}O_{7-δ}$ under pressure. This finding immediately stimulated intense interest in the possible mechanism of high-$T_{c}$ superconductivity in double-layer nickelates. Notably, the pressure-dependent phase diagram inferred from transport measurements indicates that the superconductivity under high pressure emerges from the suppression of density-wave-like order at ambient pressure, which is similar to high-temperature superconductors. Here, nuclear magnetic resonance (NMR) spectroscopy of $^{139}La$ nuclei was performed to study the density-wave-like transition in a single crystal of $La_{3}Ni_{2}O_{7-δ}$. At high temperatures, two sets of sharp $^{139}La$ NMR peaks are clearly distinguishable from a broad background signals, which are ascribed to La(1) sites from two bilayer Ruddlesden-Popper phases with different oxygen vacancy $δ$. As the temperature decreases, the temperature-dependent $^{139}La$ NMR spectra and nuclear spin-lattice relaxation rate $(1/T_{1}$) for both La(1) sites provide evidence of spin-density-wave (SDW) ordering below the transition temperature ($T_{SDW}$), which is ~ 150 K. The anisotropic splitting in the NMR spectra suggests the formation of a possible double spin stripe with magnetic moments aligned along the c-axis. Furthermore, we studied the pressure-dependent SDW transition up to ~ 2.7 GPa. Surprisingly, the $T_{SDW}$ inferred from NMR measurements of both La(1) sites increases with increasing pressure, which is opposite to the results from previous transport measurements under pressure and suggests an intriguing phase diagram between superconductivity and SDW. All these results will be helpful for building a connection between superconductivity and magnetic interactions in double-layer nickelates.

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arXiv2025

Superconductivity of the hybrid Ruddlesden-Popper La5Ni3O11 single crystals under high pressure

The discovery of high-temperature superconductivity in La3Ni2O7 and La4Ni3O10 under high pressure indicates that the Ruddlesden-Popper (RP) phase nickelates Rn+1NinO3n+1 (R = rare earth) is a new material family for high-temperature superconductivity. Exploring the superconductivity of other RP or hybrid RP phase nickelates under high pressure has become an urgent and interesting issue. Here, we report a novel hybrid RP nickelate superconductor of La5Ni3O11. The hybrid RP nickelate La5Ni3O11 is formed by alternative stacking of La3Ni2O7 with n=2 and La2NiO4 with n=1 along the c axis. The transport and magnetic torque measurements indicate a density-wave transition at approximately 170 K near ambient pressure, which is highly similar to both La3Ni2O7 and La4Ni3O10. With increasing pressure, high-pressure transport measurements reveal that the density-wave transition temperature (TDW) continuously increases to approximately 210 K with increasing pressure up to 12 GPa before the appearance of pressure-induced superconductivity, and the density-wave transition abruptly fades out in a first-order manner at approximately 12 GPa. The optimal superconductivity with Tconset = 64 K and Tczero = 54 K is achieved at approximately 21 GPa. On the other hand, high-pressure X-ray diffraction experiments reveal a structural phase transition from an orthorhombic structure to a tetragonal structure at approximately 4.5 GPa. In contrast to La3Ni2O7 and La4Ni3O10, the pressure-induced structural transition has no significant effect on either the density-wave transition or the superconductivity, suggesting a minor role of lattice degree of freedom in La5Ni3O11. The present discovery extends the superconducting member in the RP nickelate family and sheds new light on the superconducting mechanism.

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arXiv2025

Prerequisite of superconductivity: SDW rather than tetragonal structure in double-layer La3Ni2O7-x

The pressure-induced high-temperature superconductivity(Tc) in nickelates La3Ni2O7-x has sparked significant interest to explore its superconductivity at ambient pressure.Lan+1NinO3n+1(n=2,3)adopts an orthorhombic structure with tilted NiO6 octahedra and undergoes a spin-density-wave(SDW) transition at ambient pressure, while the octahedral tilting and the SDW are suppressed by pressure, and high pressure induces a structural transition from orthorhombic to tetragonal, and the high-Tc superconductivity is achieved in the tetragonal structure. This tetragonal structure is widely believed to be crucial for the pressure-induced superconductivity. Whether the pressure-stabilized tetragonal structure is a prerequisite for achieving nickelate superconductivity at ambient pressure is under hot debate. Here, by post-annealing of the orthorhombic La3Ni2O7-x as grown microcrystals with noticeable oxygen defects in high oxygen pressure environment, tetragonal La3Ni2O6.96 single crystals are successfully obtained at ambient pressure. In contrast to the orthorhombic La3Ni2O7-x, the tetragonal La3Ni2O7-x exhibits metallic behavior without a SDW transition at ambient pressure. Moreover, no superconductivity is observed at high pressure up to ~ 70 GPa. On the other hand, by utilizing Helium as the pressure medium, we have revisited the superconducting structure in pressurized orthorhombic La3Ni2O6.93. Our results indicate that the orthorhombic structure is quite robust against pressure, and no structural transition from orthorhombic to tetragonal happens, and the superconductivity under high pressure is achieved in orthorhombic structure rather than tetragonal structure claimed previously. All these results suggest that tetragonal structure is not prerequisite for achieving superconductivity in La3Ni2O7-x.

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arXiv2025

Absence of superconductivity and density-wave transition in ambient-pressure tetragonal La$_4$Ni$_3$O$_{10}$

The recent discovery of superconductivity in La$_3$Ni$_2$O$_7$ and La$_4$Ni$_3$O$_{10}$ under high pressure stimulates intensive research interests. These nickelates crystallize in an orthogonal/monoclinic structure with tilted NiO$_6$ octahedra at ambient pressure and enter a density-wave-like phase at low temperatures. The application of pressure suppresses the octahedral tilting and triggers a transition to tetragonal structure (I4/mmm), which is believed to be a key prerequisite for the emergence of superconducting state. Here, by developing a high oxidative environment growth technology, we report the first tetragonal nickelates La$_4$Ni$_3$O$_{10}$ microcrystals without octahedral tilting at ambient pressure. In tetragonal La$_4$Ni$_3$O$_{10}$, transport measurements find that both density-wave and superconducting transitions are absent up to 160 GPa, indicating a robust tetragonal metallic ground state. Density functional theory calculations reveal that the band structure of ambient-pressure tetragonal La$_4$Ni$_3$O$_{10}$ involves more $d_{z2}$ orbital contribution to the Fermi surface, compared to the monoclinic phase or the high-pressure superconducting tetragonal phase. The concurrent absence of density-wave state and high-pressure superconductivity in our ambient-pressure tetragonal crystals of La$_4$Ni$_3$O$_{10}$ suggests an underlying correlation between these two orders. It suggests that the tetragonal structure is not necessary, while the density-wave state is crucial for the superconductivity in nickelates. Our findings impose important constraints on the mechanism of pressure-induced superconductivity in nickelates and sheds new light on exploring ambient pressure high-temperature Ni-based superconductors.

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arXiv2024

Fermiology with nodal structures in nonsymmorphic superconductor LaNiGa$_2$: A de Haas-van Alphen study

Topological metals possess various types of symmetry-protected degenerate band crossings. When a topological metal becomes superconducting, the low-energy electronic excitations stemming from the band crossings located close to the Fermi level may contribute to highly unusual pairing symmetry and superconducting states. In this work, we study the electronic band structure of the time-reversal symmetry breaking superconductor LaNiGa$_2$ by means of quantum oscillation measurements. A comprehensive investigation combining angle-resolved high-field de Haas-van Alphen (dHvA) spectroscopy and first-principles calculations reveals the fermiology of LaNiGa$_2$ and verifies its nonsymmorphic $Cmcm$ lattice symmetry, which promises nodal band crossings pinned at the Fermi level with fourfold degeneracies. Moreover, such nodal structures, proposed to play a crucial role giving rise to the interorbital triplet pairing, are indeed captured by our dHvA analysis. Our results identify LaNiGa$_2$ as a prototypical topological crystalline superconductor and highlight the putative contribution of low-energy nodal quasiparticles to unconventional superconducting pairing.

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arXiv2024

Abnormally enhanced Hall Lorenz number in the magnetic Weyl semimetal NdAlSi

In Landau's celebrated Fermi liquid theory, electrons in a metal obey the Wiedemann--Franz law at the lowest temperatures. This law states that electron heat and charge transport are linked by a constant $L_0$, i.e., the Sommerfeld value of the Lorenz number ($L$). Such relation can be violated at elevated temperatures where the abundant inelastic scattering leads to a reduction of the Lorenz number ($L < L_0$). Here, we report a rare case of remarkably enhanced Lorenz number ($L > L_0$) discovered in the magnetic topological semimetal NdAlSi. Measurements of the transverse electrical and thermal transport coefficients reveal that the Hall Lorenz number $L_{xy}$ in NdAlSi starts to deviate from the canonical value far above its magnetic ordering temperature. Moreover, $L_{xy}$ displays strong nonmonotonic temperature and field dependence, reaching its maximum value close to 2$L_0$ in an intermediate parameter range. Further analysis excludes charge-neutral excitations as the origin of enhanced $L_{xy}$. Alternatively, we attribute it to the Kondo-type elastic scattering off localized 4$f$ electrons, which creates a peculiar energy distribution of the quasiparticle relaxation time. Our results provide insights into the perplexing transport phenomena caused by the interplay between charge and spin degrees of freedom.

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arXiv2024

Two-dimensional superconductivity and anomalous vortex dissipation in newly-discovered transition metal dichalcogenide-based superlattices

Properties of layered superconductors can vary drastically when thinned down from bulk to monolayer, owing to the reduced dimensionality and weakened interlayer coupling. In transition metal dichalcogenides (TMDs), the inherent symmetry breaking effect in atomically thin crystals prompts novel states of matter, such as Ising superconductivity with an extraordinary in-plane upper critical field. Here, we demonstrate that two-dimensional (2D) superconductivity resembling those in atomic layers but with more fascinating behaviours can be realized in the bulk crystals of two new TMD-based superconductors Ba0.75ClTaS2 and Ba0.75ClTaSe2. They comprise an alternating stack of H-type TMD layers and Ba-Cl layers. In both materials, intrinsic 2D superconductivity develops below a Berezinskii-Kosterlitz-Thouless transition. The upper critical field along ab plane exceeds the Pauli limit (Hp); in particular, Ba0.75ClTaSe2 exhibits an extremely high in plane Hc2 (14Hp) and a colossal superconducting anisotropy of 150. Moreover, the temperature-field phase diagram of Ba0.75ClTaSe2 under an in-plane magnetic field contains a large phase regime of vortex dissipation, which can be ascribed to the Josephson vortex motion, signifying an unprecedentedly strong fluctuation effect in TMD-based superconductors. Our results provide a new path towards the establishment of 2D superconductivity and novel exotic quantum phases in bulk crystals of TMD-based superconductors.

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arXiv2024

Two-dimensional superconductivity in new niobium dichalcogenides-based bulk superlattices

Transition metal dichalcogenides exhibit many unexpected properties including two-dimensional (2D) superconductivity as the interlayer coupling being weakened upon either layer-number reduction or chemical intercalation. Here we report the realization of 2D superconductivity in the newly-synthesized niobium dichalcogenides-based bulk superlattices Ba$_{0.75}$ClNbS$_{2}$ and Ba$_{0.75}$ClNbSe$_{2}$, which consists of the alternating stacking of monolayer $H$-NbS$_{2}$ (or $H$-NbSe$_{2}$) and monolayer inorganic insulator spacer Ba$_{0.75}$Cl. Magnetic susceptibility and resistivity measurements show that both superlattices belong to type-II superconductor with $T_{c}$ of 1 K and 1.25 K, respectively. Intrinsic 2D superconductivity is confirmed for both compounds below a Berezinskii-Kosterlitz-Thouless transition and a large anisotropy of the upper critical field. Furthermore, the upper critical field along $ab$ plane ($H_{c2}^{\parallel ab}$) exceeds the Pauli limit ($μ_{0}H_{p}$) in Ba$_{0.75}$ClNbSe$_{2}$, highlighting the influence of spin-orbit interactions. Our results establish a generic method for realizing the 2D superconducting properties in bulk superlattice materials.

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arXiv2024

Nodeless multigap superconductivity in organic-ion-intercalated (tetrabutyl~ammonium)$_{0.3}$FeSe

We probe the superconducting order parameter of the organic-ion-intercalated FeSe-based superconductor (tetrabutyl ammonium)$_{0.3}$FeSe [(TBA)$_{0.3}$FeSe] using muon-spin relaxation/rotation ($μ$SR). Zero-field $μ$SR measurements show only a weak temperature dependence with no evidence for magnetic ordering or broken time-reversal symmetry in the superconducting state. The temperature dependence of the superfluid density is deduced from transverse-field $μ$SR measurements with fields applied both parallel and perpendicular to the $c$~axis axis, and can be well described by a nodeless two-gap $s+s$ wave model. These properties are reminiscent of those of (Li$_{1-x}$Fe$_x$)OHFe$_{1-y}$Se, which also has a comparably enhanced $T_c$, suggesting that such a gap structure is a common feature of quasi-two-dimensional intercalated FeSe-based superconductors.

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arXiv2024

Large Oscillatory Thermal Hall Effect in Kagome Metals

The thermal Hall effect recently provided intriguing probes to the ground state of exotic quantum matters. These observations of transverse thermal Hall signals lead to the debate on the fermionic versus bosonic origins of these phenomena. The recent report of quantum oscillations (QOs) in Kitaev spin liquid points to a possible resolution. The Landau level quantization would most likely capture only the fermionic thermal transport effect. However, the QOs in the thermal Hall effect are generally hard to detect. In this work, we report the observation of a large oscillatory thermal Hall effect of correlated Kagome metals. We detect a 180-degree phase change of the oscillation and demonstrate the phase flip as an essential feature for QOs in the thermal transport properties. More importantly, the QOs in the thermal Hall channel are more profound than those in the electrical Hall channel, which strongly violates the Wiedemann Franz (WF) law for QOs. This result presents the oscillatory thermal Hall effect as a powerful probe to the correlated quantum materials.

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arXiv2024

Electrical switching of Ising-superconducting nonreciprocity for quantum neuronal transistor

Nonreciprocal quantum transport effect is mainly governed by the symmetry breaking of the material systems and is gaining extensive attention in condensed matter physics. Realizing electrical switching of the polarity of the nonreciprocal transport without external magnetic field is essential to the development of nonreciprocal quantum devices. However, electrical switching of superconducting nonreciprocity remains yet to be achieved. Here, we report the observation of field-free electrical switching of nonreciprocal Ising superconductivity in Fe3GeTe2/NbSe2 van der Waals (vdW) heterostructure. By taking advantage of this electrically switchable superconducting nonreciprocity, we demonstrate a proof-of-concept nonreciprocal quantum neuronal transistor, which allows for implementing the XOR logic gate and faithfully emulating biological functionality of a cortical neuron in the brain. Our work provides a promising pathway to realize field-free and electrically switchable nonreciprocity of quantum transport and demonstrate its potential in exploring neuromorphic quantum devices with both functionality and performance beyond the traditional devices.

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arXiv2024

Electrically tunable, rapid spin-orbit torque induced modulation of colossal magnetoresistance in Mn$_3$Si$_2$Te$_6$ nanoflakes

As a quasi-layered ferrimagnetic material, Mn$_3$Si$_2$Te$_6$ nanoflakes exhibit magnetoresistance behaviour that is fundamentally different from their bulk crystal counterparts. They offer three key properties crucial for spintronics. Firstly, at least 10^6 times faster response comparing to that exhibited by bulk crystals has been observed in current-controlled resistance and magnetoresistance. Secondly, ultra-low current density is required for resistance modulation (~ 5 A/cm$^2$). Thirdly, electrically gate-tunable magnetoresistance has been realized. Theoretical calculations reveal that the unique magnetoresistance behaviour in the Mn$_3$Si$_2$Te$_6$ nanoflakes arises from a magnetic field induced band gap shift across the Fermi level. The rapid current induced resistance variation is attributed to spin-orbit torque, an intrinsically ultra-fast process (~nanoseconds). This study suggests promising avenues for spintronic applications. In addition, it highlights Mn$_3$Si$_2$Te$_6$ nanoflakes as a suitable platform for investigating the intriguing physics underlying chiral orbital moments, magnetic field induced band variation and spin torque.

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