arXiv subjects

T. R. Li

Publications and source records attributed to T. R. Li.

5 recordsLinked to original sources

arXiv2020

Suspended and restored activities of A Nearby Super Massive Black Hole

The discovery of spectral type transition of active galactic nuclei (AGNs), the so-called changing-look(CL) phenomenon, challenges the widely accepted AGN paradigm, not only in the orientation based Unified Model, but also in the standard disk model. In past decades, only a couple of nearby repeat "changing-look" active galactic nuclei (CL-AGNs) have been identified. Here we report spectroscopic observations of UGC 3223 over the course of 18 years, from 2001 onwards. Combining the spectrum taken in 1987 by Stirpe, we have witnessed its type transitions from $1.5\rightarrow2.0\rightarrow1.8$ over 32 years, and captured a long-lived (at least 10 years) thorough "turn-off" state with a spectrum typical of a Seyfert 2 galaxy. The long-term thorough "turn-off" state probably suggests a once-dormant and an awakening central engine in UGC3223. We argue the (dis)appearance of the broad Balmer emission lines can be explained by the disk-wind BLR model given the evolution of the calculated Eddington ratio of accretion of the supermassive black hole.

astro-ph.CO↗

arXiv2018

Mermin-Wagner physics, (H,T) phase diagram, and candidate quantum spin-liquid phase in the spin-1/2 triangular-lattice antiferromagnet Ba8CoNb6O24

Ba$_8$CoNb$_6$O$_{24}$ presents a system whose Co$^{2+}$ ions have an effective spin 1/2 and construct a regular triangular-lattice antiferromagnet (TLAFM) with a very large interlayer spacing, ensuring purely two-dimensional character. We exploit this ideal realization to perform a detailed experimental analysis of the $S = 1/2$ TLAFM, which is one of the keystone models in frustrated quantum magnetism. We find strong low-energy spin fluctuations and no magnetic ordering, but a diverging correlation length down to 0.1 K, indicating a Mermin-Wagner trend towards zero-temperature order. Below 0.1 K, however, our low-field measurements show an nexpected magnetically disordered state, which is a candidate quantum spin liquid. We establish the $(H,T)$ phase diagram, mapping in detail the quantum fluctuation corrections to the available theoretical analysis. These include a strong upshift in field of the maximum ordering temperature, qualitative changes to both low- and high-field phase boundaries, and an ordered regime apparently dominated by the collinear "up-up-down" state. Ba$_8$CoNb$_6$O$_{24}$ therefore offers fresh input for the development of theoretical approaches to the field-induced quantum phase transitions of the $S = 1/2$ Heisenberg TLAFM.

cond-mat.str-el↗

arXiv2017

Experimental Approach to the Thermodynamics of the Pure Two-Dimensional Spin-1/2 Triangular Lattice Antiferromagnet in Ba8CoNb6O24

Frustrated quantum magnets pose well-defined questions concerning quantum fluctuation effects and the nature of the many-body wavefunction, which challenge theory, numerics, experiment and materials synthesis. The S = 1/2 triangular-lattice antiferromagnet (TLAF) presents a case where classical order is strongly suppressed by quantum fluctuations, leading to extensive renormalization of physical properties at all energy scales. However, purely two-dimensional (2D) models are difficult to realise in the 3D world and their physics is controlled by the Mermin-Wagner theorem, which describes the dominant effects of additional thermal fluctuations. Here we report the magnetic properties Ba8CoNb6O24, whose Co2+ions have an effective spin 1/2 and construct a regular TLAF with very large interlayer spacing. We find no magnetic ordering down to 0.028 K, strong low-energy spin fluctuations in qualitative agreement with theoretical analysis and a diverging correlation length, all indicating a Mermin-Wagner trend towards zero-temperature ordering in this ideal 2D system.

cond-mat.str-el↗

arXiv2017

Robust short-range-ordered nematicity in FeSe evidenced by high-pressure NMR

We report high-pressure $^{77}$Se NMR studies on FeSe single crystals that reveal a prominent inhomogeneous NMR linewidth broadening upon cooling, with the magnetic field applied along the tetragonal [110] direction. The data indicate the existence of short-range-ordered, inhomogeneous electronic nematicity, which has surprisingly long time scales over milliseconds. The short-range order survives temperatures up to $8$ times the structural transition temperature, and remains robust against pressure, in contrast to the strong pressure-dependence of the orbital ordering, structural transition, and the ground state magnetism. Such an extended region of static nematicity in the ($P$,$T$) space of FeSe indicates an enormously large fluctuating regime, and provide fresh insights and constraints to the understanding of electronic nematicity in iron-based superconductors.

cond-mat.supr-con↗

arXiv2016

Pressure Induced Stripe-order Antiferromagnetism and First-order Phase Transition in FeSe

To elucidate the magnetic structure and the origin of the nematicity in FeSe, we perform a high-pressure $^{77}$Se NMR study on FeSe single crystals. We find a suppression of the structural transition temperature with pressure up to about 2 GPa from the anisotropy of the Knight shift. Above 2 GPa, a stripe-order antiferromagnetism that breaks the spatial four-fold rotational symmetry is determined by the NMR spectra under different field orientations and with temperatures down to 50 mK. The magnetic phase transition is revealed to be first-order type, implying the existence of a concomitant structural transition via a spin-lattice coupling. Stripe-type spin fluctuations are observed at high temperatures, and remain strong with pressure. These results provide clear evidences for strong coupling between nematicity and magnetism in FeSe, and therefore support a universal scenario of magnetic driven nematicity in iron-based superconductors.

cond-mat.supr-con↗