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A remarkable property of twisted bilayer graphene (TBG) with small twist angle is the presence of a well-defined and conserved low-energy valley degrees of freedom1, which can potentially bring about new types of valley-associated spontaneous-symmetry breaking phases. Electron-electron (e-e) interactions in the TBG near the magic angle 1.1 degree can lift the valley degeneracy, allowing for the realization of orbital magnetism and topological phases2-11. However, direct measurement of the orbital-based magnetism in the TBG is still lacking up to now. Here we report evidence for orbital magnetic moment generated by the moire-scale current loops in a TBG with a twist angle {\theta} ~ 1.68 degree. The valley degeneracy of the 1.68 degree TBG is removed by e-e interactions when its low-energy van Hove singularity (VHS) is nearly half filled. A large and linear response of the valley splitting to magnetic fields is observed, attributing to coupling to the large orbital magnetic moment induced by chiral current loops circulating in the moire pattern. According to our experiment, the orbital magnetic moment is about 10.7 uB per moire supercell. Our result paves the way to explore magnetism that is purely orbital in slightly twisted graphene system., Comment: 4 figures in main text

In the vicinity of the magic angle in twisted bilayer graphene (TBG), many exotic correlated states, such as superconductivity, ferromagnetism, and topological phases, are observed because the two low-energy Van Hove singularities (VHSs) become exceedingly narrow, i.e., become flatbands. Heterostrain, strain in van der Waals structures where each layer is strained independently, can modify the single-particle band structure of the TBG and lead to various properties. Here, we show that heterostrain in a TBG near the magic angle generates a zero-energy flatband between the two VHSs. Doping the TBG to partially fill the zero-energy flatband, we observe a correlation-induced valley-polarized gap of \ensuremath{\sim}10 meV. By applying perpendicular magnetic fields, a large and linear response of the gap to magnetic fields is observed, attributing to the large orbital magnetic moments in TBG. The measured orbital magnetic moment per moir\'e unit cell is $\ensuremath{\sim}15\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}$ in the TBG, which is well consistent with our calculations.

Coulomb interaction is of central importance in localized energy levels (bound states) or electronic flat bands and can result in many exotic quantum phases. In a graphene monolayer, the relativistic massless Dirac fermion nature of the charge carriers enables us to realize unprecedented quasibound states, which are trapped temporarily via whispering-gallery modes (WGMs), in circular graphene quantum dots (GQDs). Here we show that Coulomb interaction still plays a dominating role in determining the electronic properties of the temporarily confined quasibound states with the lifetime (trapping time) of \ensuremath{\sim}10 fs. Our scanning tunneling microscopy and spectroscopy measurements demonstrate that the discrete quasibound state in a GQD will split into two peaks when it is partially filled. The energy separation of the two split peaks increases linearly with inverse effective radius of the GQDs, indicating that the splitting arises from the Coulomb interaction. Moreover, we show that the real-space distribution of the two split states separates in different regions of the GQD to reduce the Coulomb interaction, leading to the breaking of the WGM of the quasibound states.

A number of quantum Hall isospin ferromagnetic (QHIFM) states have been predicted in the ``relativistic'' zero Landau level (LL) of the graphene monolayer. However, identification of these high-field broken-symmetry states has mostly relied on macroscopic transport techniques, which lack spatial resolution. Here, we demonstrate a direct approach by imaging the QHIFM states at atomic scale with a scanning tunneling microscope. At half filling of the zero LL ($\ensuremath{\nu}=0$), the system is in a spin unpolarized state and we observe a linear magnetic-field-scaling of valley splitting. The wave functions of the QHIFM states at $\ensuremath{\nu}=0$ are directly imaged at the atomic scale and we observe an interaction-driven density wave featuring a Kekul\'e distortion, which is responsible for the large gap in high magnetic fields. Moreover, our experiment demonstrates that both the valley and spin splittings depend on the filling factor. For example, the spin splitting in the zero LL is dramatically enhanced (in excess of about 200% at maximum) when the Fermi level lies inside spin-polarized states (at $\ensuremath{\nu}=1$ or \ensuremath{-}1), accounting for strong many-body effects.

Theoretically, coupling of phonons with quasiparticles in the Landau levels (LLs) of graphene was predicted to generate a new sequence of discrete states in the density of states, i.e., phonon peaks. However, it was believed that these phonon peaks are extremely weak due to weak electron-phonon coupling in graphene. Here, we report the experimental observation of giant phonon peaks in a graphene monolayer induced by enhanced electron-phonon coupling. We demonstrate that emission or absorption of phonons of quasiparticles in the LLs of graphene generates the phonon peaks, which even exhibits fine structures corresponding to bound states of an electron and of a phonon. We also show that it is possible to tune the relative strength of the phonon peaks at nanoscale by controlling interactions between graphene and the supporting substrate. The local strain in graphene may play a vital role in enhancing the electron-phonon coupling, which results in the observed giant phonon peaks. Our experiment indicates that it is possible to explore interesting emergent phenomena, such as superconductivity, induced by electron-phonon coupling in graphene.

Electron-doped $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ has been the subject of much recent attention because of its similarities with hole-doped $\mathrm{L}{\mathrm{a}}_{2}\mathrm{Cu}{\mathrm{O}}_{4}$. However, direct evidence of its superconductivity has not yet been observed, which has led to various unresolved issues related to this material. In this study, a 14-nm-thick La-doped $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ film was grown on a (001) $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrate. We found that the fourfold component of the in-plane anisotropic magnetoresistance disappears and is replaced by a twofold symmetry as the temperature increases. Variable-temperature x-ray diffraction and spatially resolved optical second-harmonic generation experiments eliminate the possibility of crystal structural distortion. The temperature dependence of direct current magnetization measurements indicates that a change in the spin structure may account for the symmetry breaking, similar to the nematic ordering of iron-based and other superconductors. Remarkably, the transition temperature of the symmetry breaking is well consistent with the previously reported onset (50 K) of the low-energy gap in K-doped $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ observed by scanning tunneling spectroscopy. Our results provide a perspective on these materials that helps elucidate the nature of magnetic interactions and explores the expected superconductivity of iridates.

Twisted graphene bilayers (TGBs) have low-energy van Hove singularities (VHSs) that are strongly localized around AA-stacked regions of the moire pattern. Therefore, they exhibit novel many-body electronic states, such as Mott-like insulator and unconventional superconductivity. Unfortunately, these strongly correlated states were only observed in magic angle TGBs with the twist angle theta~1.1{\deg}, requiring a precisely tuned structure. Is it possible to realize exotic quantum phases in the TGBs not limited at the magic angle? Here we studied electronic properties of a TGB with theta~1.64{\deg} and demonstrated that a VHS splits into two spin-polarized states flanking the Fermi energy when the VHS is close to the Fermi level. Such a result indicates that localized magnetic moments emerge in the AA-stacked regions of the TGB. Since the low-energy VHSs are quite easy to be reached in slightly TGBs, our result therefore provides a facile direction to realize novel quantum phases in graphene system., Comment: 4 figures

We use temperature-dependent Hall measurements to identify contributions of spin Hall, magnetic proximity, and sublattice effects to the anomalous Hall signal in heavy metal/ferrimagnetic insulator heterostructures with perpendicular magnetic anisotropy. This approach enables detection of both the magnetic proximity effect onset temperature and the magnetization compensation temperature and provides essential information regarding the interfacial exchange coupling. Onset of a magnetic proximity effect yields a local extremum in the temperature-dependent anomalous Hall signal, which occurs at higher temperature as magnetic insulator thickness increases. This magnetic proximity effect onset occurs at much higher temperature in Pt than W. The magnetization compensation point is identified by a sharp anomalous Hall sign change and divergent coercive field. We directly probe the magnetic proximity effect using x-ray magnetic circular dichroism and polarized neutron reflectometry, which reveal an antiferromagnetic coupling between W and the magnetic insulator. Finally, we summarize the exchange-coupling configurations and the anomalous Hall-effect sign of the magnetized heavy metal in various heavy metal/magnetic insulator heterostructures.

Very recently, twisted graphene bilayers (TGBs) around the first magic angle $\ensuremath{\theta}\ensuremath{\approx}1.{1}^{\ensuremath{\circ}}$ have attracted much attention for the realization of exotic quantum states, such as correlated insulator behavior and unconventional superconductivity. Here we elaborately study a series of TGBs around the first magic angle engineered by heterostrain, where each layer is strained independently. Our experiment indicates that a moderate heterostrain enables the structural evolution from the small-angle TGB (\ensuremath{\theta} \ensuremath{\sim} 1.5\ifmmode^\circ\else\textdegree\fi{}) to the strained magic-angle TGB (\ensuremath{\theta} \ensuremath{\sim} 1.1\ifmmode^\circ\else\textdegree\fi{}), exhibiting the characteristic low-energy flat bands. The heterostrain can even drive the system into highly strained tiny-angle TGBs (\ensuremath{\theta}$\ensuremath{\ll}1.1$\ifmmode^\circ\else\textdegree\fi{}) with large deformed tetragonal superlattices, where a unique network of topological helical edge states emerges. Furthermore, the predicted domain wall modes, which are strongly localized and result in a hexagon-triangle-mixed frustrated lattice derived from the Kagome lattice, are observed in the strained tiny-angle TGBs.

The out-of-plane magnetoresistance of a $\mathrm{LaAl}{\mathrm{O}}_{3}$/$\mathrm{SrTi}{\mathrm{O}}_{3}$ interface is commonly positive due to significant orbital effect. In this paper, a large negative magnetoresistance is observed at a $\mathrm{LaAl}{\mathrm{O}}_{3}$ and Fe-doped $\mathrm{SrTi}{\mathrm{O}}_{3}$ interface upon the application of an out-of-plane magnetic field. Such large negative magnetoresistance is attributed to enhanced two-dimensional weak localization and Kondo effect. We think that these two effects are mainly the consequences of intentional Fe dopants in the substrate, which introduce holes, disorders, and spin scattering centers into the electronically conducting interface, directly modifying the spin-orbit coupling, orbital effect, scale of electrons motion, and local magnetic moments at the $\mathrm{LaAl}{\mathrm{O}}_{3}$/$\mathrm{SrTi}{\mathrm{O}}_{3}$ interface. The ${\mathrm{LaAlO}}_{3}\text{/Fe-doped}\phantom{\rule{0.28em}{0ex}}{\mathrm{SrTiO}}_{3}$ interfacial system provides a prospective platform for studying quantum interference between strongly correlated electrons. The substrate doping method may facilitate the application of $\mathrm{LaAl}{\mathrm{O}}_{3}$/$\mathrm{SrTi}{\mathrm{O}}_{3}$ interfaces in versatile electronic devices.

Negative differential conductance (NDC), characterized by the decreasing current with increasing voltage, has attracted continuous attention for its various novel applications. The NDC typically exists in a certain range of bias voltages for a selected system and controlling the regions of NDC in curves of current versus voltage ($I\text{\ensuremath{-}}V$) is experimentally challenging. Here, we demonstrate a magnetic-field-controlled NDC in scanning tunneling spectroscopy of graphene npn junction resonators. The magnetic field not only can switch on and off the NDC, but also can continuously tune the regions of the NDC in the $I\text{\ensuremath{-}}V$ curves. In the graphene npn junction resonators, magnetic fields generate sharp and pronounced Landau-level peaks with the help of the Klein tunneling of massless Dirac fermions. A tip of scanning tunneling microscope induces a relatively shift of the Landau levels in graphene beneath the tip. Tunneling between the misaligned Landau levels results in the magnetic-field-controlled NDC.

Creation of high-quality $p\ensuremath{-}n$ junctions in graphene monolayer is vital in studying many exotic phenomena of massless Dirac fermions. However, even with the fast progress of graphene technology for more than ten years, it remains conspicuously difficult to generate nanoscale and atomically sharp $p\ensuremath{-}n$ junctions in graphene. Here, we realized nanoscale $p\ensuremath{-}n$ junctions with atomically sharp boundaries in graphene monolayer by using monolayer vacancy island of Cu surface. The generated sharp $p\ensuremath{-}n$ junctions with the height as high as 660 meV isolate the graphene above the Cu monolayer vacancy island as nanoscale graphene quantum dots (GQDs) in a continuous graphene sheet. Massless Dirac fermions are confined by the $p\ensuremath{-}n$ junctions for a finite time to form quasibound states in the GQDs. By using scanning tunneling microscopy, we observe resonances of quasibound states in the GQDs with various sizes and directly visualize effects of geometries of the GQDs on the quantum interference patterns of the quasibound states, which allow us to test the quantum electron optics based on graphene in atomic scale.

By comparing the in-plane and out-of-plane orbitally dependent magnetoresistance under a magnetic field of 9 T, the conductive layer thickness of the ${\mathrm{LaAlO}}_{3}$/${\mathrm{SrTiO}}_{3}$ interface is determined for temperatures between 45 and 300 K. As the influence of spin-orbit coupling cannot be ignored when considering the conductive layer thickness dependence at temperatures below 45 K, a lower magnetic field is employed. The results show that the thickness increases gradually and then remains constant as the temperature decreases. Such a tendency is well reproduced theoretically by considering the equilibrium between drift and diffusion of electrons near the interface. We discover that the thickness mainly depends on the relative permittivity of ${\mathrm{SrTiO}}_{3}$ and electron mobility, and it is independent of the sheet carrier density. Our investigations show that the dimensionality of the confined zone for electrons can be controlled and can be used as a reference for other interfacial systems in condensed matter physics.

Finite-size twisted bilayer graphene (TBG, where here the TBG is of nanoscale size) is quite unstable and will change its structure to a Bernal (or $AB$-stacking) bilayer with a much lower energy. Therefore, the lack of finite-size TBG makes its electronic properties difficult to access in experiments. In this paper, a special confined TBG is obtained in the overlaid area of two continuous misoriented graphene sheets. The width of the confined region of the TBG changes gradually from about 22 to 0 nm. By using scanning tunneling microscopy, we study carefully the structure and the electronic properties of finite-size TBG. Our results indicate that the low-energy electronic properties, including twist-induced Van Hove singularities (VHSs) and spatial modulation of the local density of states, are strongly affected by the translational symmetry breaking of the finite-size TBG. However, the electronic properties above the energy of the VHSs are almost not influenced by quantum confinement even when the width of the TBG is reduced to only a single moir\'e spot.

Massless Dirac fermions in graphene provide unprecedented opportunities to realize the Klein paradox, which is one of the most exotic and striking properties of relativistic particles. In the seminal theoretical work [Katsnelson et al., Nat. Phys. 2 620 (2006)], it was predicted that the massless Dirac fermions can pass through one-dimensional (1D) potential barriers unimpededly at normal incidence. Such a result seems to preclude confinement of the massless Dirac fermions in graphene by using 1D potential barriers. Here, we demonstrate, both experimentally and theoretically, that massless Dirac fermions can be trapped in quasi-1D npn junction of a continuous graphene monolayer. Because of highly anisotropic transmission of the massless Dirac fermions at n-p junction boundaries (the so-called Klein tunneling in graphene), charge carries incident at large oblique angles will be reflected from one edge of the junction with high probability and continue to bounce from the opposite edge. Consequently, these electrons are trapped for a finite time to form quasi-bound states in the quasi-1D npn junction. The quasi-bound states seen as pronounced resonances are probed and the quantum interference patterns arising from these states are directly visualized in our scanning tunneling microscope measurements., 4 Figures in main text

Assembling different two-dimensional (2D) crystals, covering a very broad range of properties, into van der Waals (vdW) heterostructures enables unprecedented possibilities for combining the best of different ingredients in one objective material. So far, metallic, semiconducting, and insulating 2D crystals have been used successfully in making functional vdW heterostructures with properties by design. Here, we expand 2D superconducting crystals as a building block of vdW hererostructures. One-step growth of large-scale high-quality vdW heterostructures of graphene and 2D superconducting $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{M}{\mathrm{o}}_{2}\mathrm{C}$ by using chemical vapor deposition is reported. The superconductivity and its 2D nature of the heterostructures are characterized by our scanning tunneling microscopy measurements. This adds 2D superconductivity, the most attractive property of condensed matter physics, to vdW heterostructures.

To generate a moderate band gap in a graphene monolayer is a very important but rather difficult task. A rare working solution of this problem is to cut it into one-dimensional (1D) nanometer-wide ribbons. Here we show that, instead of cutting the graphene monolayer, a wide band gap can be created in a unique 1D strained structure, i.e., a wrinkled graphene-nanoribbon-like (GNR-like) structure, of a continuous graphene sheet via strong hybridization between the graphene and the metal substrate. The wrinkled GNR-like structures with widths of only a few nanometers are observed in a continuous graphene sheet grown on a Rh foil by using thermal strain engineering. Spatially resolved scanning tunneling spectroscopy revealed a band-gap opening of a few hundred meV in the GNR-like structure in an otherwise continuous metallic graphene sheet, directly demonstrating the realization of a metallic-semiconducting-metallic junction entirely in a graphene monolayer.

Theoretically, it has been demonstrated that armchair Graphene nanoribbons (GNRs) can be divided into three families, i.e., ${N}_{a}=3p,\phantom{\rule{0.16em}{0ex}}{N}_{a}=3p+1$, and ${N}_{a}=3p+2$ (here ${N}_{a}$ is the number of dimer lines across the ribbon width and $p$ is an integer), according to their electronic structures, and the energy gaps for the three families are quite different even with the same $p$. However, a systematic experimental verification of this fundamental prediction is still lacking, owing to very limited atomic-level control of the width of the armchair GNRs investigated. Here, we studied electronic structures of the armchair GNRs with atomically well-defined widths ranging from ${N}_{a}=6$ to ${N}_{a}=26$ by using a scanning tunneling microscope. Our result demonstrated explicitly that all the studied armchair GNRs exhibit semiconducting gaps and, more importantly, the observed gaps as a function of ${N}_{a}$ are well grouped into the three categories, as predicted by density-functional theory calculations. Such a result indicated that the electronic properties of the armchair GNRs can be tuned dramatically by simply adding or cutting one carbon dimer line along the ribbon width.

In strained graphene, lattice deformation can create pseudomagnetic fields affecting the behavior of massless Dirac fermions and result in zero-field Landau level-like quantization. In the presence of an external magnetic field, valley-polarized Landau levels are predicted to be observed because the strain-induced pseudomagnetic fields are of opposite directions in the $K$ and ${K}^{\ensuremath{'}}$ valleys of graphene. However, an experimental verification of such a unique valley-polarized Landau quantization has not been reported so far. Here, we present experimental spectroscopic measurements in strained graphene on Rh foil by scanning tunneling microscopy. We directly observed the splitting of the Landau level in the quantum Hall regime and we interpreted the experimental result as the valley-polarized Landau level induced by the coexistence of the pseudomagnetic fields and external magnetic fields. The observed result paves the way to exploit novel electronic properties in graphene through the combination of the spatially varying strain (or the pseudomagnetic fields) and the external magnetic fields.

We investigated current induced depairing in the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}/\mathrm{FeTe}$ topological insulator-chalcogenide interface superconductor. The measured depairing current density provides information on the magnetic penetration depth and superfluid density, which in turn shed light on the nature of the normal state that underlies the interfacial superconductivity.

Recently, the rhombohedral trilayer graphene (r-TLG) has attracted much attention because of its low-energy flat bands, which are predicted to result in many strongly correlated phenomena. Here, we demonstrate that it is possible to probe the stacking order and electronic spectrum of the r-TLG directly with a scanning tunneling microscopy around a monoatomic step edge of the top graphene layer. The tunneling spectra of the r-TLG exhibit four adjacent peaks, which are generated by the low-energy flat bands, flanking the charge neutrality point. Based on these spectra, the true energy gap and the energy gap at the K-point of the r-TLG are determined as about 9 meV and 23 meV, respectively. The observed features are well reproduced by a low-energy effective Hamiltonian., 3 Figures

Carrier density $({n}_{\mathrm{s}})$ is a crucial parameter that governs the properties of correlated oxides, so the field-effect approach is an ideal tool to investigate the novel physics of the system. Here, the carrier-mediated transport of slightly doped anatase ${\mathrm{TiO}}_{2}$ epitaxial films were studied by electric double layer transistor (EDLT) gating. The ${n}_{\mathrm{s}}$ has been increased hugely, and concomitantly, the channels of anatase ${\mathrm{TiO}}_{2}$ films undergo an insulator-metal transition with a decrease in resistivity by almost three orders of magnitude. More fascinating, the Kondo effect depends very strongly on ${n}_{\mathrm{s}}$, and the Hall mobility could be enhanced about one order of magnitude with increasing ${n}_{\mathrm{s}}$. This study shows that EDLT gating is a powerful method to stimulate and mediate novel phenomena of correlated oxides.

Despite the structures and properties of tilt grain boundaries of graphite surface and graphene have been extensively studied, their effect on the structures and electronic spectra of graphene layers has not been fully addressed. Here we study effects of one-dimensional tilt grain boundaries on structures and electronic spectra of graphene multilayers by scanning tunneling microscopy and spectroscopy. A tilt grain boundary of a top graphene sheet in graphene multilayers leads to a twist between consecutive layers and generates superstructures (Moir\'e patterns) on one side of the boundary. Our results demonstrate that the twisting changes the electronic spectra of Bernal graphene bilayer and graphene trilayers dramatically. We also study quantum-confined twisted graphene bilayer generated between two adjacent tilt grain boundaries and find that the band structure of such a system is still valid even when the number of superstructures is reduced to two in one direction. It implies that the electronic structure of this system is driven by the physics of a single Moir\'e spot., Comment: 4 figures in main text and 8 figures in SI

It is well established that some kinds of lattice deformations in graphene monolayer, which change electron hopping in the sublattice and affect in-plane motion of electrons, may induce out-of-plane pseudomagnetic fields as large as 100 T. Here, we demonstrate that stacking misorientation in graphene bilayers mimics the effect of huge in-plane pseudomagnetic fields greater than 1000 T on the interlayer hopping of electrons. As well as addressing the similarity between the effect of in-plane pseudomagnetic fields and the twisting on the electronic band structure of the Bernal graphene bilayer, we point out that the in-plane magnetic fields (or twisting) could modify the low-energy pseudospin texture of the graphene bilayer (the pseudospin winding number is reduced from 2 to 1), thereby changing the chiralities of quasiparticles from those of spin 1 to those of spin 1/2. Our results illustrate the possibility of controllably manipulating electronic properties of Bernal graphene bilayer by introducing the in-plane magnetic field or twisting.

We consider the electronic structure of a slightly twisted graphene bilayer and show the coexistence of van Hove singularities (VHSs) and superlattice Dirac points in a continuum approximation. The graphene-on-graphene moir\'e pattern gives rise to a periodic electronic potential, which leads to the emergence of the superlattice Dirac points due to the chiral nature of the charge carriers. Owning to the distinguishing real and reciprocal structures, the sublattice exchange even and odd structures of the twisted graphene bilayer (the two types of commensurate structures) result in two different structures of the superlattice Dirac points. We further calculate the effect of a strain on the low-energy electronic structure of the twisted graphene bilayer and demonstrate that the strain affects the position of the VHSs dramatically., Comment: 5 figures, to appear in Phys. Rev. B

Recent studies show that periodic potentials can generate superlattice Dirac points at energies $\ifmmode\pm\else\textpm\fi{}\ensuremath{\hbar}{\ensuremath{\nu}}_{\mathrm{F}}|\mathbf{G}|/2$ in graphene (where $\ensuremath{\hbar}$ is the reduced Planck constant, ${\ensuremath{\nu}}_{\mathrm{F}}$ is the Fermi velocity of graphene, and G is the reciprocal superlattice vector). Here, we perform scanning tunneling microscopy and spectroscopy studies of a corrugated graphene monolayer on Rh foil. We show that the quasiperiodic ripples of nanometer wavelength in the corrugated graphene give rise to weak one-dimensional electronic potentials and thereby lead to the emergence of the superlattice Dirac points. The position of the superlattice Dirac point is space dependent and shows a wide distribution of values. We demonstrate that the space-dependent superlattice Dirac points are closely related to the space-dependent Fermi velocity, which may arise from the effect of the local strain and the strong electron-electron interaction in the corrugated graphene.

Recent studies show that the electronic structures of graphene can be modified by strain, and it was predicted that strain in graphene can induce peaks in the local density of states (LDOS), mimicking Landau levels (LLs) generated in the presence of a large magnetic field. Here we report the scanning tunneling spectroscopy (STS) observation of nine strain-induced peaks in LDOS at 77 K along a graphene ridge created when the graphene layer was cleaved from a sample of highly oriented pyrolytic graphite (HOPG). The energies of these peaks follow the progression of LLs of massless ``Dirac fermions'' (DFs) in a magnetic field of 230 T. The results presented here suggest a possible route to realize zero-field quantum Hall-like effects at 77 K.

Structural stability, electronic, and optical properties of InN under high pressure are studied using the first-principles calculations. The lattice constants and electronic band structure are found consistent with the available experimental and theoretical values. The pressure of the wurtzite-to-rocksalt structural transition is 13.4 GPa, which is in an excellent agreement with the most recent experimental values. The optical characteristics reproduce the experimental data thus justifying the feasibility of our theoretical predictions of the optical properties of InN at high pressures.

Magnetic properties with chains of hcp Co hollow spheres have been studied. The diameter of the spheres ranges from 500 to 800 nm, with a typical shell thickness of about 60 nm. The shell is polycrystalline with an average crystallite size of 20 to 35 nm. The blocking temperature determined by the zero-field-cooling MZFC(T) measurement at H = 90 Oe is about 325 K. The corresponding effective anisotropy is determined as, Keff = 4.6*10^4 J/m^3. In addition, the blocking temperature and the effective anisotropy determined by the analysis on HC(T) are 395 K and 5.7*10^4 J/m^3, respectively. The experimentally determined anisotropy is smaller by one order of magnitude than the magnetocrystalline anisotropy of the bulk hcp Co, which is about 3 to 5*10^5 J/m^3. A further analysis on HC(T) shows that the magnetization reversal follows a nucleation rotational mode with an effective switching volume, V* = 2.3*10^3 nm^3. The corresponding effective diameter is calculated as 16.4 nm. It is slightly larger than the coherence length of Co, about 15 nm. The possible reason for the much reduced magnetic anisotropy is discussed briefly., Comment: 10 pages, 5 figures, submitted to PRB

The temperature variation effect of shape anisotropy on the coercivity, HC(T), for the aligned Stoner-Wohlfarth (SW) soft ferromagnets, such as fcc Ni, fcc Co and bcc Fe, are investigated within the framework of Neel-Brown (N-B) analysis. An extended N-B equation is thus proposed,by introducing a single dimensionless correction function, the reduced magnetization, m(\tao) = MS(T)/MS(0), in which \tao = T/TC is the reduced temperature, MS(T) is the saturation magnetization, and TC is the Curie temperature. The factor, m(\tao), accounts for the temperature-dependent effect of the shape anisotropy. The constants, H0 and E0, are for the switching field at zero temperature and the potential barrier at zero field, respectively. According to this newly derived equation, the blocking temperature above which the properties of superparamagnetism show up is described by the expression, TB = E0m^2(\tao)/[kBln(t/t0)], with the extra correction factor m^2(\tao). The possible effect on HC(T) and the blocking temperature, TB, attributed to the downshift of TC resulting from the finite size effect has been discussed also., Comment: 22 pages, 2 figures, 1 table, Accepted by Phys. Rev. B