[Show abstract][Hide abstract] ABSTRACT: We have studied the effect of structural disorder on the de Haas van Alphen and Shubnikov de Haas quantum oscillations measured in natural, Kish, and highly oriented pyrolytic graphite samples at temperatures down to 30 mK and at magnetic fields up to 14 T. The measurements were performed on different samples characterized by means of x-ray diffractometry, transmission electron microscopy, and atomic-force microscopy techniques. Our results reveal a correlation between the amplitude of quantum oscillations and the sample surface roughness.
Preview · Article · Jan 2016 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: We briefly review earlier and report original experimental results in the
context of metastable or possible superconducting materials. We show that
applied electric field induces conducting state in Copper Chloride (CuCl) whose
characteristics resemble behavior of sliding charge-density-wave(s) (CDW). We
discuss whether the sliding CDW or collective transport of similar ordered
charge phase(s) may account for the problem of "high-temperature
superconductivity" observed in this and other materials, including Cadmium
Sulfide (CdS), metal-ammonia solutions, polymers, amorphous carbon and tungsten
oxides. We also discuss a local superconductivity that occurs at the surface of
graphite and amorphous carbon under deposition of foreign atoms/molecules.
No preview · Article · Feb 2015 · Physica C Superconductivity
[Show abstract][Hide abstract] ABSTRACT: The carrier density in tens of nanometers thick graphite samples (multi-layer-graphene, MLG) has been modified by applying a gate voltage Vg perpendicular to the graphene planes. Surface potential microscopy shows inhomogeneities in the carrier density (n) in the sample near surface region and under different values of Vg at room temperature. Transport measurements on different MLG samples reveal that under a large enough applied electric field these regions undergo a superconducting-like transition at T < 17 K. A magnetic field applied parallel or normal to the graphene layers suppresses the transition without changing appreciably the transition temperature.
[Show abstract][Hide abstract] ABSTRACT: The structural, optical, and transport properties of graphene grown by chemical vapor deposition (CVD) of propane under hydrogen on the Si face of SiC substrates have been investigated. We show that little changes in temperature during the growth can trigger the passivation of the SiC surface by hydrogen. Depending on the growth condition, hole or electron doping can be achieved, down to a few 1011 cm−2. When the growth temperature is high (T≈1500–1550∘C), we obtain electron-doped graphene monolayers lying on a buffer layer. When the growth temperature is slightly lowered (T≈1450–1500∘C), hole-doped graphene layers are obtained, lying on a hydrogen-passivated SiC surface, as confirmed by the enhancement of the mobility (of the order of 4500 cm2/Vs) and the persistence of weak localization almost up to room temperature (250 K). The high homogeneity of this graphene allows the observation of the half-integer quantum Hall effect, typical of graphene, at the centimeter scale in the best cases. The influence of the SiC steps on the transport properties is discussed.
Full-text · Article · Feb 2014 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: We report a highly anisotropic in-plane magnetoresistance (MR) in graphite that possesses in-plane parallel line-like structural defects. In a current direction perpendicular to the line defects (LD), MR is negative and linear in low fields with a crossover to a positive MR at higher fields, while in a current direction parallel to LD, we observed a giant super-linear positive MR. These extraordinary MRs are respectively explained by a hopping magnetoresistance via non-zero angular momentum orbitals, and by the magnetoresistance of inhomogeneous media. The linear negative orbital MR is a unique signature of the broken time-reversal symmetry (TRS). We discuss the origin of the disorder-induced TRS-breaking in graphite.
Preview · Article · Oct 2013 · Journal of Physics Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: We studied the in-plane magnetoresistance R(B,T) anisotropy in epitaxial multilayer graphene films grown on the Si face of a 6H-SiC substrate that originates from steplike morphology of the SiC substrate. To enhance the anisotropy, a combination of argon atmosphere with graphite capping was used during the film growth. The obtained micro-Raman spectra demonstrated a complex multilayer graphene structure with the smaller film thickness on terraces as compared to the step edges. Several Hall bars with different current/steps mutual orientations have been measured. A clear anisotropy in the magnetoresistance has been observed, and attributed to variations in electron mobility governed by the steplike structure. Our data also revealed that (i) the graphene-layer stacking is mostly Bernal type, (ii) the carriers are massive, and (iii) the carriers are confined to the first 2–4 graphene layers following the buffer layer.
[Show abstract][Hide abstract] ABSTRACT: Highly-anisotropic in-plane magneto-resistance (MR) in graphite (HOPG)
samples has been recently observed (Y. Kopelevich et al., arXiv:1202.5642)
which is negative and linear in low fields in some current direction while it
is giant, super-linear and positive in the perpendicular direction. In the
framework of the hopping conductance theory via non-zero angular momentum
orbitals we link extraordinary MRs in graphite and in organic insulators (OMAR)
observed in about the same magnetic fields. The theory predicts quadratic
negative MR (NMR) when there is a time-reversal symmetry (TRS), and linear NMR
if TRS is broken. We argue that the observed linear NMR could be a unique
signature of the broken TRS both in graphite and organic compounds. While some
local paramagnetic centers are responsible for the broken TRS in organic
insulators, a large diamagnetism of our HOPG samples may involve a more
intriguing scenario of TRS breaking.
[Show abstract][Hide abstract] ABSTRACT: This comment addresses several issues in the paper by Sepioni et al.,
where it is stated that the ferromagnetism in pristine highly oriented
pyrolytic graphite (HOPG) reported by several groups in the previous
years is most likely due to impurity contamination. In this comment,
clear arguments are given why this statement is not justified.
Furthermore, it is pointed out, that there are already measurements
using element-sensitive microscopic techniques, e.g. X-ray Magnetic
Circular Dichroism (XMCD) that directly proved the intrinsic origin of
the ferromagnetism in graphite, also in pristine HOPG.
Full-text · Article · Jun 2012 · EPL (Europhysics Letters)
[Show abstract][Hide abstract] ABSTRACT: This comment addresses several issues in the paper by Sepioni et al., where
it is stated that the ferromagnetism in pristine highly oriented pyrolytic
graphite (HOPG) reported by several groups in the previous years is most likely
due to impurity contamination. In this comment, clear arguments are given why
this statement is not justified. Furthermore, it is pointed out, that there are
already measurements using element-sensitive microscopic techniques, e.g. X-ray
Magnetic Circular Dichroism (XMCD) that directly proved the intrinsic origin of
the ferromagnetism in graphite, also in pristine HOPG.
Full-text · Article · Jun 2012 · EPL (Europhysics Letters)
[Show abstract][Hide abstract] ABSTRACT: We report on magnetization, magnetoresistance, and Shubnikov-de Haas oscillations experiments in Na-implanted samples of highly oriented pyrolitic graphite (HOPG). Different ion fluences were applied so that samples with Na contents of 0.5, 1.0, 1.5, and 2.0 at. % were obtained in the implanted region. Ferromagnetic-like hysteresis was observed in magnetization experiments where the field was applied parallel to the graphene planes. The observed saturation moment increases systematically as a function of the implanted ion concentration up to Na 1 at. %, where it goes through a maximum before decreasing slightly towards Na 2 at. %. The planar magnetoresistance amplitude at fixed field and temperature closely correlates with the saturation magnetization data. This result suggests that the strong planar magnetoresistance in graphite is at least partially related to a spin dependent mechanism. The magnetoresistance experiments also reveal the occurrence of Shubnikov-de Haas oscillations. The characteristic frequencies and the effective masses could be estimated and do not depend on the Na concentration. The reported experiments show that the expressive enhancement observed in ferromagnetic-like response in Na-implanted HOPG is primarily due to point defects produced by the implantation process.
No preview · Article · May 2012 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: We have studied the temperature and magnetic field dependence of the
electrical resistance of mesoscopic, tens of nanometers thick multigraphene
samples as a function of a bias voltage applied perpendicular to the graphene
planes. We found that the resistance changes asymmetrically with the bias
voltage sign. For large and negative bias voltages the resistance shows a
non-percolative superconducting-like transition at $T \sim 15 ... 20 $K. A
large enough magnetic field suppresses the transition.
[Show abstract][Hide abstract] ABSTRACT: Magnetization measurements were performed on CeCoIn5 at temperatures down to 20 mK and magnetic fields up to 17 T applied along different crystallographic orientations. For field configurations nearly parallel to the ab plane (θ≲40∘ and T≤50 mK), we have found an intriguing vortex dynamics regime revealed by a hysteretic and metastable anomalous peak effect (APE), which gives evidence of surface barrier effects enhanced by antiferromagnetic fluctuations in the mixed state of CeCoIn5. Furthermore, we have observed crossover features in the torque and magnetization traces at fields below Hc2, which are consistent with vortices lattice phase transitions and with the anomalies speculated to be the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) superconducting state in CeCoIn5. All of the above features were found to be dramatically perturbed in Ce0.98Gd0.02CoIn5.
No preview · Article · Feb 2012 · Physical review. B, Condensed matter
[Show abstract][Hide abstract] ABSTRACT: Different experimental techniques reveal a nearly magnetic-field-independent depinning of the flux line lattice (FLL) between 20 and 30 K in highly anisotropic high temperature superconductors like Bi 2:2:1:2 and similar compounds. This anomalous behavior of the FLL has been partially unnoticed in the literature and is the subject of this experimental review. We compare data from literature obtained with different experimental techniques and discuss a possible common origin based on a “softening” of the interplane Josephson coupling.
No preview · Article · Nov 2011 · Modern Physics Letters B
[Show abstract][Hide abstract] ABSTRACT: We performed magnetization M(H,T) and magnetoresistance R(T,H) measurements
on powdered (grain size ~ 149 micrometers) as well as highly oriented
rhombohedral (A7) bismuth (Bi) samples consisting of single crystalline blocks
of size ~ 1x1 mm2 in the plane perpendicular to the trigonal c-axis. The
obtained results revealed the occurrence of (1) local superconductivity in
powdered samples with Tc(0) = 8.75 \pm 0.05 K, and (2) global superconductivity
at Tc(0) = 7.3 \pm 0.1 K in polycrystalline Bi triggered by low-resistance
Ohmic contacts with silver (Ag) normal metal. The results provide evidence that
the superconductivity in Bi is localized in a tiny volume fraction, probably at
intergrain or Ag/Bi interfaces. On the other hand, the occurrence of global
superconductivity observed for polycrystalline Bi can be accounted for by
enhancement of the superconducting order parameter phase stiffness induced by
the normal metal contacts, the scenario proposed in the context of "pseudogap
regime" in cuprates [E. Berg et al., PRB 78, 094509 (2008)].
Preview · Article · Sep 2011 · Low Temperature Physics
[Show abstract][Hide abstract] ABSTRACT: Both experimental and theoretical studies of the magnetic properties of
micrographite and nanographite indicate a crucial role of the partial oxidation
of graphitic zigzag edges in ferromagnetism. In contrast to total and partial
hydrogenation, the oxidation of half of the carbon atoms on the graphite edges
transforms the antiferromagnetic exchange interaction between graphite planes
and over graphite ribbons to the ferromagnetic interaction. The stability of
the ferromagnetism is discussed.
Full-text · Article · Jun 2011 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: We study the origins of high-temperature ferromagnetic behavior in graphite by means of unbiased ab-initio calculations and compare them with our data. The experimental results show that oxygen/sulfur-induced edges of graphitic fragments (via unzipping effect) play an essential role in this phenomenon, and that the finite magnetic moment appears if edges in a graphitic ribbon are occupied asymmetrically by either oxygen or sulphur. In particular, our ab-initio calculations performed within the LDA and GGA approximations showed that in the case of pure graphene ribbon, its zig-zag edge carbon atoms carry large magnetic moment (˜1 muB/C). In an oxidized or sulfurized graphene, however, the magnetic moment at the edge with absorbed atoms gets considerably reduced, leading to effective ferromagnetic (more precisely, ferri-magnetic) behavior of the sample.
[Show abstract][Hide abstract] ABSTRACT: Recently superconductivity in graphite-sulfur composites was experimentally observed. In this work we have analyzed the electronic structure changes associated with the presence of sulfur atoms in one and two dimensional graphite layers. We have considered ordered and disordered sulfur atoms distributions in many configurations. The density of states (DOS) of these structures were obtained using the negative factor counting (NFC) technique coupled to a tight-binding Hamiltonian (Hückel type). Our results indicate that the incorporation of sulfur atoms at edge graphite layers (changing their global geometric curvature and increasing the DOS at the Fermi level) might be in the origin of the graphite superconductivity.
No preview · Article · Jan 2011 · MRS Online Proceeding Library
[Show abstract][Hide abstract] ABSTRACT: Nobel Prize in Physics 2010 was given for "groundbreaking experiments regarding the two-dimensional material graphene." In fact, before graphene has been extracted from graphite and measured, some of its fundamental physical properties have already been experimentally uncovered in bulk graphite. In this Letter to the Nobel Committee we propose to include those findings in the Scientific Background
[Show abstract][Hide abstract] ABSTRACT: The electronic Raman scattering of bulk graphite at zero magnetic field
reveals a structureless signal characteristic of a metal. For B≳2T
, several peaks at energies scaling linearly with magnetic field were
observed and ascribed to transitions from the lowest energy Landau
level(s) [LL(s)] to excited states belonging to the same ladder. The LLs
are equally (unequally) spaced for high (low) quantum numbers, being
consistent with the LL sequence from massive chiral fermions
[m∗=0.033(2)me] with Berry’s phase
2π found in graphene bilayers. These results provide spectroscopic
evidence that some of the physics recently revealed by graphene
multilayers is also shared by bulk graphite.
No preview · Article · Jun 2010 · Physical review. B, Condensed matter