arXiv subjects

Timothy J. Haugan

Publications and source records attributed to Timothy J. Haugan.

12 recordsLinked to original sources

arXiv2025

Fibonacci sequence of twist angles in superconducting multi-layer graphene and hydrogenated graphitic fibers

A range of twist angles between adjacent surfaces/volumes are intrinsic to natural graphite or artificially design in multi-layer graphene. In addition, stacking faults can be created by the application of mechanic, electric or magnetic fields. Charge and spin transport then occur in relation to the existing twist-angle pattern. In two dimensions, a saddle point in the electronic band structure leads to divergence in the density of states, known as van Hove singularities (vHs). The energy difference between vHs for the conduction and valence bands was found to increase with the twist angle between neighboring graphite domains with respect to the c axis (perpendicular to the graphite planes). In this work, we estimate for the superconducting (SC)-like nano-size multi-layer granular domain in hydrogenated graphitic fibers [1]. We show that this value for and the values found by others for few-layer graphene might actually form the Fibonacci mathematical sequence. Moreover, SC hydrogenated graphite can harbour higher-order topology as reflected in at least quadratic energy gap flattening. Charge transport and magnetization measurements on hydrogenated graphitic fibers have been done using a Quantum Design Physical Properties Measurement System.

cond-mat.mes-hall↗

arXiv2025

Model for the commensurate charge-density waves in under-hole-doped cuprate superconductors

A simple model of the commensurate charge-density wave (CCDW) portion of the underdoped pseudogap regions of monolayer Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6-x}$ (Bi2201), bilayer Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212), and trilayer Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ (Bi2223) cuprate superconductors is presented and studied. Above the superconducting transition temperature $T_c$ but below the pseudogap transition temperature $T_p > T_c$, the CCDW forms on the oxygen sites in the CuO$_2$ layers with excess charges of $\pmδe$, where $e$ is the electronic charge, forming on alternating oxygen sites. This model is equivalent to $N$-layer versions of the two-dimensional Ising model for spins on a square lattice with repulsive interactions $J' , J>0$ between near-neighbor inter- and intralayer sites, respectively. For strong coupling, we show analytically for sections of $L\times M\times N$ sites that the partition function in the $J'\rightarrow\pm \infty$ limits reduces to that for an effective single layer with $L\times M$ sites and $J$ replaced by $NJ$. The CCDW is therefore strongly enhanced and stabilized by multilayer structures, likely accounting for the enhanced THz emission observed from the intrinsic Josephson junctions in underdoped Bi2212 mesas and for the many experiments on Bi2212 and related compounds purporting to provide evidence for a superconducting order parameter with $d_{x^2-y^2}$-wave symmetry.

cond-mat.supr-con↗

arXiv2023

Magneto-structural phase transitions and two-dimensional spin waves in graphite

We have previously found experimental evidence for several quantum phenomena in oxygen-ion implanted of hydrogenated graphite: ferromagnetism, antiferromagnetism, paramagentism, triplet superconductivity, Andreev states, Little-Parks oscillations, Lamb shift, Casimir effect, colossal magnetoresistance, and topologically-protected flat-energy bands [1-6]. Triplet superconductivity results in the formation of Josephson junctions, thus with potential of being used for spintronics applications in the critical area of quantum computing. In this paper, we are showing new experimental evidence for the formation of two-dimensional (2D) spin waves in oxygen-ion enriched and in hydrogenated highly oriented pyrolytic graphite. The temperature evolution of the remanent magnetization Mrem(T) data confirms the formation of spin waves that follow the 2D Heisenberg model with a weak uniaxial anisotropy. In addition, the step-like features also found in the temperature dependence of the electrical resistivity between insulating and metallic states suggest several outstanding possibilities, such as a structural transition, triplet superconductivity, and chiral properties.

cond-mat.supr-con↗

arXiv2023

Flat-band energy analysis of the temperature-dependent superconducting gap for hydrogenated graphite fibers found from nonlocal electrical conductance experimental data

Experimental evidence of novel phenomena in hydrogenated graphite fibers is found. An indirect excitonic mechanism is likely leading to a SC state below the temperature Tc = 50 K, where the gap is divergent. Analysis of the gap within the framework provided by the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity shows that this is a multigap system. The energy gap data can be better explained within the framework of topologically protected flat bands applied to systems in which superconductivity occurs on the surface or at the internal interfaces of the samples. The temperature dependence of the SC gap is linear above 50 K. We use nonlocal differential conductance Gdiff(V) = dI(V)/dV experimental data to show clear evidence of topological phenomena such as interference of chiral asymmetric Andreev edge states and crossed Andreev conversion. Gdiff(V) has a negative part that results from the nonlocal coherence between electron and holes in the Andreev edge states. We conclude that hydrogenated graphite bears the marks of an unconventional high-temperature superconductor (HTSC).

cond-mat.supr-con↗

arXiv2022

Magnetism and Superconductivity in Hydrogenated Graphite Foils

Unique to certain unconventional superconductors is the coexistence of magnetism and superconductivity. We have previously found ferromagnetism and superconductivity in hydrogenated graphitic materials. Herein we present similar as well as completely new findings this time applicable to hydrogenated graphite foils. As the strength of the magnetic field is increased, the temperature-dependent magnetization shows several important transitions. From a Neel paramagnetic-antiferromagnetic transition, to a ferromagnetic superconductor state, to an orbital paramagnetic glass high-temperature superconductor with critical temperature for the dominant phase at Tc = 50 to 60 K. The ferromagnetic state is observed up to room temperature. Thus, the magnetism of hydrogenated low-density carbon graphite foils plays an important role in establishing electronic correlations of which some are superconducting in nature.

cond-mat.supr-con↗

arXiv2022

Quantum spin Hall effect in two-dimensional metals without spin-orbit coupling

The quantum spin Hall effect has been observed in topological insulators using spin-orbit coupling as the probe, but it has not yet been observed in a metal. An experiment is proposed to measure the quantum spin Hall effect of an electron or hole in a two-dimensional (2D) metal by using the previously unexplored but relativistically generated 2D quantum spin Hall Hamiltonian, but without using spin-orbit coupling. A long cylindrical solenoid lies normally through the inner radius of a 2D metallic Corbino disk. The current $I_S$ surrounding the solenoid produces an azimuthal magnetic vector potential but no magnetic field in the disk. In addition, a radial electric field is generated across the disk by imposing either (a) a potential difference $Δv$ or (b) a radial charge current ${\bm I}$ across its inner and outer radii. Combined changes in $I_S$ and in either $Δv$ or ${\bm I}$ generate spontaneously quantized azimuthal charge and spin currents. The experiment is designed to measure these quantized azimuthal charge and spin currents in the disk consistently. The quantum Hamiltonians for experiments (a) and (b) are both solved exactly. A method to control the Joule heating is presented, which could potentially allow the quantum spin Hall measurements to be made at room temperature. Extensions of this design to an array of thermally-managed solenoids, each surrounded by thermally-managed stacks of 2D metallic Corbino disks, could function as a quantum computer that could potentially operate at room temperature.

cond-mat.mes-hall↗

arXiv2021

Superconducting-like and magnetic transitions in oxygen-implanted diamond-like and amorphous carbon films, and in highly oriented pyrolytic graphite

In our previously published work, we have reported colossal magnetoresistance, Andreev oscillations, ferromagnetism, and granular superconductivity in oxygen-implanted carbon fibers, graphite foils, and highly oriented pyrolytic graphite. In this follow-up research, more results on these oxygen-implanted graphite samples are presented. We show results from transport measurements on oxygen-implanted diamond-like carbon thin coatings, amorphous carbon films, and highly oriented pyrolytic graphite. Significantly, a three-order magnitude drop in the electrical resistance of the oxygen-implanted diamond-like carbon films is observed at the 50 K temperature that we have previously reported for the transition to the superconducting state. Below 50 K, the films resistance oscillates between the high and low resistance states, less when the sample is under a transverse magnetic field. This metastability between the insulating and superconducting-like states possibly reflects the evolution of the amplitude for the superconducting order parameter also known as the longitudinal Higgs mode. Transitions to low resistance state and metastability are also observed for amorphous carbon films. Finally, the highly oriented pyrolytic graphite samples resistance have a thermally activated term that can be understood on the basis of the LAMH model applied to narrow SC channels in which thermal fluctuations can cause phase slips. We also find that in oxygen-implanted carbon materials, the electron charge and spin correlations do not compete and their interplay rather facilitates the emergence of high-temperature superconductivity, and thus, additional unexpected effects like Heisenberg spin waves and magneto-structural transitions are observed.

cond-mat.supr-con↗

arXiv2020

Boron Content and the Superconducting Critical Temperature of Carbon-Based Materials

In this paper, we present results on magnetization properties of boron nitride-carbon (BN-C) and boron carbide-carbon (B4C-C) granular mixtures. The temperature-dependent magnetization for field-cooled during cooling and field-cooled during warming shows a kind of thermal hysteresis that is always seen around a metamagnetic phase transition from an antiferromagnetic martensite to a ferromagnetic austenite phase. The low-temperature magnetization has an upward turn that can be attributed to superparamagnetism, diamagnetic shielding, and trapped flux characteristic to high-temperature superconducting materials. After subtracting the diamagnetic background, the field-dependent magnetization loops M(B) are ferromagnetic-like, more significant for the BN-C than for the B4C-C mixture. In addition, the magnetization loops show the kink feature characteristic to granular superconductivity. The irreversibility temperature for a B4C-C mixture having 37.5 wt% B is Tc = 76 K. Combining our data with previous results on B-doped diamond and Q-carbon, we find that Tc increases linearly with the B concentration.

cond-mat.supr-con↗

arXiv2020

Electric-field induced strange metal states and possible high-temperature superconductivity in hydrogenated graphitic fibers

In this work, we have studied the effects from increasing the strength of the applied electric field on the charge transport of hydrogenated graphitic fibers. Resistivity measurements were carried out for direct currents in the nA - mA range and for temperatures from 1.9 K to 300 K. The high-temperature non-ohmic voltage-current dependence is well described by the nonlinear random resistor network model applied to systems that are disordered at all scales. The temperature-dependent resistivity shows linear, step-like transitions from insulating to metallic states as well as plateau features. As more current is being sourced, the fiber becomes more conductive and thus the current density goes up. The most interesting features is observed in high electric fields. As the fiber is cooled, the resistivity first decreases linearly with the temperature and then enters a plateau region at a temperature T ? 260 ? 280 K that is field-independent. These observations on a system made out of carbon, hydrogen, nitrogen, and oxygen atoms suggest possible electric-field induced superconductivity with a high critical temperature that was predicted from studying the role of chirality on the origin of life [1].

cond-mat.mtrl-sci↗

arXiv2020

Superconductivity in Hydrogenated Graphites

We report transport and magnetization measurements on graphitic materials that have been hydrogenated after being treated with octane. The temperature-dependent electrical resistivity shows anomalies manifested as re-entrant insulator-metal transitions. Below 50 K, the magnetoresistance data shows both antiferromagnetic and ferromagnetic behavior as the magnetic field is decrease or increased, respectively. The system is possibly an unconventional magnetic superconductor. The irreversible behavior observed in the field-cooled vs. the zero-field cooled data for a sufficiently high magnetic field suggests that the system might enter a superconducting state below 50 K. Energy gap data is obtained from nonlocal electric differential conductance measurements. An exciton-based mechanism is likely driving the system to the superconducting state below 50 K, where the gap is divergent. We find that the hydrogenated carbon fiber is a multiple gap system with critical temperatures estimates above room temperature. The temperature dependence of the superconducting gap follows the flat-band energy relationship, with the flat band gap parameter linearly increasing with the temperature above 50 K. Thus, we find that either a magnetic or an electric field can drive this hydrogenated graphitic system to superconducting state below 50 K. In addition, AF spin fluctuations creates pseudo-gap states above 50 K.

cond-mat.supr-con↗

arXiv2019

The Zeeman, Spin-Orbit, and Quantum Spin-Hall Interactions in Anisotropic and Low-Dimensional Conductors

When an electron or hole is in a conduction band of a crystal, it can be very different from 2, depending upon the crystalline anisotropy and the direction of the applied magnetic induction ${\bf B}$. In fact, it can even be 0! To demonstrate this quantitatively, the Dirac equation is extended for a relativistic electron or hole in an orthorhombically-anisotropic conduction band with effective masses $m_j$ for $j=1,2,3$ with geometric mean $m_g=(m_1m_2m_3)^{1/3}$. The appropriate Foldy-Wouthuysen transformations are extended to evaluate the non-relativistic Hamiltonian to $O({\rm m}c^2)^{-4}$, where ${\rm m}c^2$ is the particle's Einstein rest energy. For ${\bf B}||\hat{\bf e}_μ$, the Zeeman $g_μ$ factor is $2{\rm m}\sqrt{m_μ}/m_g^{3/2} + O({\rm m}c^2)^{-2}$. While propagating in a two-dimensional (2D) conduction band with $m_3\gg m_1,m_2$, $g_{||}<<2$, consistent with recent measurements of the temperature $T$ dependence of the parallel upper critical induction $B_{c2,||}(T)$ in superconducting monolayer NbSe$_2$ and in twisted bilayer graphene. While a particle is in its conduction band of an atomically thin one-dimensional metallic chain along $\hat{\bf e}_μ$, $g<<2$ for all ${\bf B}={\bf\nabla}\times{\bf A}$ directions and vanishingly small for ${\bf B}||\hat{\bf e}_μ$. The quantum spin Hall Hamiltonian for 2D metals with $m_1=m_2=m_{||}$ is $K[{\bf E}\times({\bf p}-q{\bf A})]_{\perp}σ_{\perp}+O({\rm m}c^2)^{-4}$, where ${\bf E}$ and ${\bf p}-q{\bf A}$ are the planar electric field and gauge-invariant momentum, $q=\mp|e|$ is the particle's charge, $σ_{\perp}$ is the Pauli matrix normal to the layer, $K=\pmμ_B/(2m_{||}c^2)$, and $μ_B$ is the Bohr magneton.

cond-mat.mes-hall↗

arXiv2019

Quantum effects in graphitic materials: Colossal magnetoresistance, Andreev reflections, Little-Parks effect, ferromagnetism, and granular superconductivity

Unlike the more common local conductance spectroscopy, nonlocal conductance can differentiate between nontopological zero-energy modes localized around inhomogeneities, and true Majorana edge modes in the topological phase. In particular, negative nonlocal conductance is dominated by the crossed Andreev reflection. In graphene, the Andreev reflection and the inter-band Klein tunneling couple electron-like and hole-like states through the action of either a superconducting (SC) pair potential or an electrostatic potential. We are here probing quantum phenomena in modified graphitic samples. Four-point contact transport measurements at cryogenic to room temperatures were conducted using a Quantum Design Physical Property Measurement System. The observed negative nonlocal differential conductance Gdiff probes the Andreev reflection at the walls of the SC grains coupled by Josephson effect through the semiconducting matrix. In addition, Gdiff shows the butterfly shape that is characteristic to resistive random-access memory devices. In a magnetic field, the Andreev reflection counters the effect of the otherwise lowered conduction. At low temperatures, the magnetoresistance shows irreversible yet strong colossal oscillations that are known to be quantum in nature. In addition, we have found evidence for seemingly granular SC as well as ferromagnetism. Moreover, the Little-Parks effect is revealed in both the classical small-amplitude and the phase-slip driven large-amplitude oscillations in the magnetoresistance. Thus, graphitic materials show potential for quantum electronics applications, including rectification and topological states.

cond-mat.mes-hall↗