arXiv subjects

M. Long

Publications and source records attributed to M. Long.

14 recordsLinked to original sources

arXiv2025

Strange metallicity encompasses high magnetic field-induced superconductivity in UTe2

A strange metallic state - characterized by an electrical resistivity that rises linearly in temperature $T$ at a rate governed by Planckian energetic dissipation - is present in a number of unconventional superconductors. Despite widespread investigative efforts, the microscopic properties of strange metallicity continue to defy elementary description. Here we investigate heavy fermion UTe$_2$, which hosts a suite of exotic spin-triplet superconducting phases, the most extreme of which resides in a narrow angular window of intense magnetic fields $>$ 40 T. Through angle-dependent magnetotransport measurements in pulsed magnetic fields, we find that this superconductive phase emerges from a strange metallic state with Planckian $T$-linear resistivity confined to a small region of phase space where the field-induced superconductivity is strongest, suggesting a shared underlying mechanism. These findings reveal a novel setting for strange metallicity - in contrast to the general case of singlet superconductivity emerging on the border of antiferromagnetism, here strange metallicity accompanies field-induced triplet superconductivity within a spin-polarized magnetic state. This result demonstrates the remarkable ubiquity of strange metallicity across diverse materials settings, and highlights UTe$_2$ as a unique platform for exploring the interplay between unconventional superconductivity and quantum criticality.

cond-mat.str-el↗

arXiv2024

A quantum critical line bounds the high field metamagnetic transition surface in UTe$_2$

Quantum critical phenomena are widely studied across various materials families, from high temperature superconductors to magnetic insulators. They occur when a thermodynamic phase transition is suppressed to zero temperature as a function of some tuning parameter such as pressure or magnetic field. This generally yields a point of instability - a so-called quantum critical point - at which the phase transition is driven exclusively by quantum fluctuations. Here we show that the heavy fermion metamagnet UTe$_2$ possesses a quantum phase transition at extreme magnetic field strengths of over 70 T. Rather than terminating at one singular point, we find that the phase boundary is sensitive to magnetic field components in each of the three Cartesian axes of magnetic field space. This results in the three-dimensional transition surface being bounded by a continuous ring of quantum critical points, the locus of which forms an extended line of quantum criticality - a novel form of quantum critical phase boundary. Within this quantum critical line sits a magnetic field-induced superconducting state in a toroidal shape, which persists to fields over 70~T. We model our data by a phenomenological free energy expansion, and show how a three-dimensional quantum critical phase boundary - rather than a more conventional singular point of instability - anchors the remarkable high magnetic field phase landscape of UTe$_2$.

cond-mat.supr-con↗

arXiv2023

The X-ray variation of M81* resolved by Chandra and NuSTAR

Despite advances in our understanding of low luminosity active galactic nuclei (LLAGNs), the fundamental details about the mechanisms of radiation and flare/outburst in hot accretion flow are still largely missing. We have systematically analyzed the archival Chandra and NuSTAR X-ray data of the nearby LLAGN M81*, whose $L_{\rm bol}\sim 10^{-5} L_{\rm Edd}$. Through a detailed study of X-ray light curve and spectral properties, we find that the X-ray continuum emission of the power-law shape more likely originates from inverse Compton scattering within the hot accretion flow. In contrast to Sgr A*, flares are rare in M81*. Low-amplitude variation can only be observed in soft X-ray band (amplitude usually $\lesssim 2$). Several simple models are tested, including sinusoidal-like and quasi-periodical. Based on a comparison of the dramatic differences of flare properties among Sgr A*, M31* and M81*, we find that, when the differences in both the accretion rate and the black hole mass are considered, the flares in LLAGNs can be understood universally in a magneto-hydrodynamical model.

astro-ph.HE↗

arXiv2015

Micro-physics simulations of columnar recombination along nuclear recoil tracks in high-pressure Xe gas for directional dark matter searches

Directional sensitivity is one of the most important aspects of WIMP dark matter searches. Yet, making the direction of nuclear recoil visible with large target masses is a challenge. To achieve this, we are exploring a new method of detecting directions of short nuclear recoil tracks in high-pressure Xe gas, down to a few micron long, by utilizing columnar recombination. Columnar recombination changes the scintillation and ionization yields depending on the angle between a track and the electric field direction. In order to realize this, efficient cooling of electrons is essential. Trimethylamine(TMA) is one of the candidate additives to gaseous Xe in order to enhance the effect, not only by efficiently cooling the electrons, but also by increasing the amount of columnar recombination by Penning transfer. We performed a detailed simulation of ionization electrons transport created by nuclear recoils in a Xe + TMA gas mixture, and evaluated the size of the columnar recombination signal. The results show that the directionality signal can be obtained for a track longer than a few micrometers in some ideal cases. Although more studies with realistic assumptions are still needed in order to assess feasibility of this technique, this potentially opens a new possibility for dark matter searches.

physics.ins-det↗

arXiv2010

Global 3D Simulations of Disc Accretion onto the classical T Tauri Star BP Tauri

The magnetic field of the classical T Tauri star BP Tau can be approximated as a superposition of dipole and octupole moments with respective strengths of the polar magnetic fields of 1.2 kG and 1.6 kG (Donati et al. 2008). We adopt the measured properties of BP Tau and model the disc accretion onto the star. We observed in simulations that the disc is disrupted by the dipole component and matter flows towards the star in two funnel streams which form two accretion spots below the dipole magnetic poles. The octupolar component becomes dynamically important very close to the star and it redirects the matter flow to higher latitudes. The spots are meridionally elongated and are located at higher latitudes, compared with the pure dipole case, where crescent-shaped, latitudinally elongated spots form at lower latitudes. The position and shape of the spots are in good agreement with observations. The disk-magnetosphere interaction leads to the inflation of the field lines and to the formation of magnetic towers above and below the disk. The magnetic field of BP Tau is close to the potential only near the star, inside the magnetospheric surface, where magnetic stress dominates over the matter stress. A series of simulation runs were performed for different accretion rates. They show that an accretion rate is lower than obtained in many observations, unless the disc is truncated close to the star. The torque acting on the star is about an order of magnitude lower than that which is required for the rotational equilibrium. We suggest that a star could lose most of its angular momentum at earlier stages of its evolution.

astro-ph.SR↗

arXiv2008

MHD simulations of disk-star interaction

We discuss a number of topics relevant to disk-magnetosphere interaction and how numerical simulations illuminate them. The topics include: (1) disk-magnetosphere interaction and the problem of disk-locking; (2) the wind problem; (3) structure of the magnetospheric flow, hot spots at the star's surface, and the inner disk region; (4) modeling of spectra from 3D funnel streams; (5) accretion to a star with a complex magnetic field; (6) accretion through 3D instabilities; (7) magnetospheric gap and survival of protoplanets. Results of both 2D and 3D simulations are discussed.

astro-ph↗

arXiv2008

Three-dimensional Simulations of Accretion to Stars with Complex Magnetic Fields

Disk accretion to rotating stars with complex magnetic fields is investigated using full three-dimensional magnetohydrodynamic (MHD) simulations. The studied magnetic configurations include superpositions of misaligned dipole and quadrupole fields and off-centre dipoles. The simulations show that when the quadrupole component is comparable to the dipole component, the magnetic field has a complex structure with three major magnetic poles on the surface of the star and three sets of loops of field lines connecting them. A significant amount of matter flows to the quadrupole "belt", forming a ring-like hot spot on the star. If the maximum strength of the magnetic field on the star is fixed, then we observe that the mass accretion rate, the torque on the star, and the area covered by hot spots are several times smaller in the quadrupole-dominant cases than in the pure dipole cases. The influence of the quadrupole component on the shape of the hot spots becomes noticeable when the ratio of the quadrupole and dipole field strengths $B_q/B_d\gtrsim0.5$, and becomes dominant when $B_q/B_d\gtrsim1$. In the case of an off-centre dipole field, most of the matter flows through a one-armed accretion stream, forming a large hot spot on the surface, with a second much smaller secondary spot. The light curves may have simple, sinusoidal shapes, thus mimicking stars with pure dipole fields. Or, they may be complex and unusual. In some cases the light curves may be indicators of a complex field, in particular if the inclination angle is known independently. We also note that in the case of complex fields, magnetospheric gaps are often not empty, and this may be important for the survival of close-in exosolar planets.

astro-ph↗

arXiv2006

Accretion to Stars with Non-dipole Magnetic Fields

Disc accretion to a rotating star with a non-dipole magnetic field is investigated for the first time in full three-dimensional (3D) magnetohydrodynamic (MHD) simulations. We investigated the cases of (1) pure dipole, (2) pure quadrupole, and (3) dipole plus quadrupole fields. Simulations have shown that in each case the structure of the funnel streams and associated hot spots on the surface of the star have specific features connected with the magnetic field configuration. In the case (1), matter accretes in two funnel streams which form two arch-like spots near the magnetic poles. In the case (3), most of the matter flows through the quadrupole "belt" forming a ring-shaped hot region on the magnetic equator. In the case (2), magnetic flux in the northern magnetic hemisphere is larger than that in the southern, and the quadrupole belt and the ring are displaced to the south. The stronger the quadrupole, the closer the ring is to the magnetic equator. At sufficiently large misalignment angle $Θ$, matter also flows to the south pole, forming a hot spot near the pole. The light curves have a variety of different features which makes it difficult to derive the magnetic field configuration from the light curves. There are specific features which are different in cases of dipole and quadrupole dominated magnetic field: (1) Angular momentum flow between the star and disc is more efficient in the case of the dipole field; (2) Hot spots are hotter and brighter in case of the dipole field because the matter accelerates over a longer distance compared with the flow in a quadrupole case.

astro-ph↗

arXiv2006

2D and 3D MHD Simulations of Disk Accretion by Rotating Magnetized Stars: Search for Variability

We performed 2D and full 3D magnetohydrodynamic simulations of disk accretion to a rotating star with an aligned or misaligned dipole magnetic field. We investigated the rotational equilibrium state and derived from simulations the ratio between two main frequencies: the spin frequency of the star and the orbital frequency at the inner radius of the disk. In 3D simulations we observed different features related to the non-axisymmetry of the magnetospheric flow. These features may be responsible for high-frequency quasi-periodic oscillations (QPOs). Variability at much lower frequencies may be connected with restructuring of the magnetic flux threading the inner regions of the disk. Such variability is specifically strong at the propeller stage of evolution.

astro-ph↗

arXiv2005

Locking of the Rotation of Disk-Accreting Magnetized Stars

We investigate the rotational equilibrium state of a disk accreting magnetized stars using axisymmetric magnetohydrodynamic (MHD) simulations. In this ``locked'' state, the spin-up torque balances the spin-down torque so that the net average torque on the star is zero. We investigated two types of initial conditions, one with a relatively weak stellar magnetic field and a high coronal density, and the other with a stronger stellar field and a lower coronal density. We observed that for both initial conditions the rotation of the star is locked to the rotation of the disk. In the second case, the radial field lines carry significant angular momentum out of the star. However, this did not appreciably change the condition for locking of the rotation of the star. We find that in the equilibrium state the corotation radius $r_{co}$ is related to the magnetospheric radius $r_A$ as $r_{co}/r_A\approx 1.2-1.3$ for case (1) and $r_{co}/r_A\approx 1.4-1.5$ for case (2). We estimated periods of rotation in the equilibrium state for classical T Tauri stars, dwarf novae and X-ray millisecond pulsars.

astro-ph↗

arXiv2004

Anomalous scaling of conductivity in integrable fermion systems

We analyze the high-temperature conductivity in one-dimensional integrable models of interacting fermions: the t-V model (anisotropic Heisenberg spin chain) and the Hubbard model, at half-filling in the regime corresponding to insulating ground state. A microcanonical Lanczos method study for finite size systems reveals anomalously large finite-size effects at low frequencies while a frequency-moment analysis indicates a finite d.c. conductivity. This phenomenon also appears in a prototype integrable quantum system of impenetrable particles, representing a strong-coupling limit of both models. In the thermodynamic limit, the two results could converge to a finite d.c. conductivity rather than an ideal conductor or insulator scenario.

cond-mat.str-el↗

arXiv2000

Reactive Hall response

The zero temperature Hall constant R_H, described by reactive (nondissipative) conductivities, is analyzed within linear response theory. It is found that in a certain limit, R_H is directly related to the density dependence of the Drude weight implying a simple picture for the change of sign of charge carriers in the vicinity of a Mott-Hubbard transition. This novel formulation is applied to the calculation of R_H in quasi-one dimensional and ladder prototype interacting electron systems.

cond-mat↗

arXiv1999

Hall constant of strongly correlated electrons on a ladder

The Hall constant R_H in a tight-binding model of correlated electrons on a ladder at T=0 is expressed in terms of derivatives of the ground state energy with respect to external magnetic and electric fields. This novel method is used for the analysis of the t-J model on finite size ladders. It is found that for a single hole R_H is hole-like and close to the semiclassical value, while for two holes it can vary with ladder geometry. In odd-leg ladders, R_H behaves quite regularly changing sign as a function of doping, the variation being quantitatively close to experimental results in cuprates.

cond-mat.str-el↗

arXiv1993

Hole-hole correlations in the $U=\infty $ limit of the Hubbard model and the stability of the Nagaoka state

We use exact diagonalisation in order to study the infinite - $U$ limit of the two dimensional Hubbard model. As well as looking at single-particle correlations, such as $n_{{\bf k}σ}=\langle c^\dagger _{{\bf k}σ}c_{{\bf k}σ} \rangle $, we also study {\it N-particle correlation functions} which compare the relative positions of {\it all} the particles in different models. In particular we study 16 and 18-site clusters and compare the charge correlations in the Hubbard model with those of spinless fermions and hard-core bosons. We find that although low densities of holes favour a `locally-ferromagnetic' fermionic description, the correlations at larger densities resemble those of pure hard-core bosons surprisingly well .

cond-mat↗