arXiv subjects

J. Dai

Publications and source records attributed to J. Dai.

At least 37 records · Page 2Linked to original sources

arXiv2020

Transverse coronal loop oscillations excited by homologous circular-ribbon flares

We report our multiwavelength observations of two homologous circular-ribbon flares (CRFs) in active region 11991 on 2014 March 5, focusing on the transverse oscillations of an extreme-ultraviolet (EUV) loop excited by the flares. The transverse oscillations are of fast standing kink-mode. The first-stage oscillation triggered by the C2.8 flare is decayless with lower amplitudes (310$-$510 km). The periods (115$-$118 s) in different wavelengths are nearly the same, indicating coherent oscillations. The magnetic field of the loop is estimated to be 65$-$78 G. The second-stage oscillation triggered by the M1.0 flare is decaying with larger amplitudes (1250$-$1280 km). The periods decreases from 117 s in 211 Å to 70 s in 171 Å, implying a decrease of loop length or an implosion after a gradual expansion. The damping time, being 147$-$315 s, increases with the period, so that the values of $τ/P$ are close to each other in different wavelengths. The thickness of the inhomogeneous layer is estimated to be $\sim$0\farcs45 under the assumption of resonant absorption. This is the first observation of the excitation of two kink-mode loop oscillations by two sympathetic flares. The results are important for understanding of the excitation of kink oscillations of coronal loops and hence the energy balance in the solar corona. Our findings also validate the prevalence of significantly amplified amplitudes of oscillations by successive drivers.

astro-ph.SR↗

arXiv2020

Tunable two-dimensional electron system at the (110) surface of SnO$_2$

We report the observation of a two-dimensional electron system (2DES) at the $(110)$ surface of the transparent bulk insulator SnO$_2$, and the tunability of its carrier density by means of temperature or Eu deposition. The 2DES is insensitive to surface reconstructions and, surprisingly, it survives even after exposure to ambient conditions --an extraordinary fact recalling the well known catalytic properties SnO$_2$. Our data show that surface oxygen vacancies are at the origin of such 2DES, providing key information about the long-debated origin of $n$-type conductivity in SnO$_2$, at the basis of a wide range of applications. Furthermore, our study shows that the emergence of a 2DES in a given oxide depends on a delicate interplay between its crystal structure and the orbital character of its conduction band.

cond-mat.mtrl-sci↗

arXiv2020

Machine-learning-corrected quantum dynamics calculations

Quantum scattering calculations for all but low-dimensional systems at low energies must rely on approximations. All approximations introduce errors. The impact of these errors is often difficult to assess because they depend on the Hamiltonian parameters and the particular observable under study. Here, we illustrate a general, system and approximation-independent, approach to improve the accuracy of quantum dynamics approximations. The method is based on a Bayesian machine learning (BML) algorithm that is trained by a small number of rigorous results and a large number of approximate calculations, resulting in ML models that accurately capture the dependence of the dynamics results on the quantum dynamics parameters. Most importantly, the present work demonstrates that the BML models can generalize quantum results to different dynamical processes. Thus, a ML model trained by a combination of approximate and rigorous results for a certain inelastic transition can make accurate predictions for different transitions without rigorous calculations. This opens the possibility of improving the accuracy of approximate calculations for quantum transitions that are out of reach of rigorous scattering calculations.

physics.chem-ph↗

arXiv2018

High-density two-dimensional electron system induced by oxygen vacancies in ZnO

We realize a two-dimensional electron system (2DES) in ZnO by simply depositing pure aluminum on its surface in ultra-high vacuum, and characterize its electronic structure using angle-resolved photoemission spectroscopy. The aluminum oxidizes into alumina by creating oxygen vacancies that dope the bulk conduction band of ZnO and confine the electrons near its surface. The electron density of the 2DES is up to two orders of magnitude higher than those obtained in ZnO heterostructures. The 2DES shows two $s$-type subbands, that we compare to the $d$-like 2DESs in titanates, with clear signatures of many-body interactions that we analyze through a self-consistent extraction of the system self-energy and a modeling as a coupling of a 2D Fermi liquid with a Debye distribution of phonons.

cond-mat.mtrl-sci↗

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

Converting topological insulators into topological metals within the tetradymite family

We report the electronic band structures and concomitant Fermi surfaces for a family of exfoliable tetradymite compounds with the formula $T_2$$Ch_2$$Pn$, obtained as a modification to the well-known topological insulator binaries Bi$_2$(Se,Te)$_3$ by replacing one chalcogen ($Ch$) with a pnictogen ($Pn$) and Bi with the tetravalent transition metals $T$ $=$ Ti, Zr, or Hf. This imbalances the electron count and results in layered metals characterized by relatively high carrier mobilities and bulk two-dimensional Fermi surfaces whose topography is well-described by first principles calculations. Intriguingly, slab electronic structure calculations predict Dirac-like surface states. In contrast to Bi$_2$Se$_3$, where the surface Dirac bands are at the $Γ-$point, for (Zr,Hf)$_2$Te$_2$(P,As) there are Dirac cones of strong topological character around both the $\bar Γ$- and $\bar {M}$-points which are above and below the Fermi energy, respectively. For Ti$_2$Te$_2$P the surface state is predicted to exist only around the $\bar {M}$-point. In agreement with these predictions, the surface states that are located below the Fermi energy are observed by angle resolved photoemission spectroscopy measurements, revealing that they coexist with the bulk metallic state. Thus, this family of materials provides a foundation upon which to develop novel phenomena that exploit both the bulk and surface states (e.g., topological superconductivity).

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↗

arXiv2015

Spin frustration and magnetic ordering in triangular lattice antiferromagnet Ca$_3$CoNb$_2$O$_9$

We synthesized a quasi-two-dimensional distorted triangular lattice antiferromagnet Ca$_3$CoNb$_2$O$_9$, in which the effective spin of Co$^{2+}$ is 1/2 at low temperatures, whose magnetic properties were studied by dc susceptibility and magnetization techniques. The x-ray diffraction confirms the quality of our powder samples. The large Weiss constant $θ_{CW}\sim$ $-55$ K and the low Neel temperature($T_N\sim$ 1.45 K) give a frustration factor $f$ ($=\midθ_{CW}/T_N \mid$) $\approx$ 38, suggesting that Ca$_3$CoNb$_2$O$_9$ resides in strong frustration regime. Slightly below $T_N$, deviation between the susceptibility data under zero-field cooling (ZFC) and field cooling (FC) is observed. A new magnetic state with 1/3 of the saturate magnetization $M_s$ is suggested in the magnetization curve at 0.46 K. Our study indicates that Ca$_3$CoNb$_2$O$_9$ is an interesting material to investigate magnetism in triangular lattice antiferromagnets with weak anisotropy.

cond-mat.str-el↗

arXiv2015

Magnetism and a field-cycling induced effect in staircase Kagomé antiferromagnet PbCu$_3$TeO$_7$ revealed by NMR

We report $^{125}$Te and $^{63,65}$Cu nuclear magnetic resonance (NMR) studies on single crystals of staircase Kagomé antiferromagnet PbCu3TeO7 ($T_{N1}\approx$ 36 K). A Curie constant as large as $Θ$~140 K is obtained by a Curie-Weiss fitting of the high-temperature Knight shift. The frustration factor f (= $Θ/T_N $)$\approx$ 4 implies intermediate frustration in this system. From the high-temperature Knight shift data of $^{125}$Te, its hyperfine coupling constant is estimated to be $^{125}A_{hf}$ = -67 kOe/$μ_B$, which suggests a strong interlayer coupling bridging the neighboring Kagomé layers. At $T$= 2 K, we find two types of zero-field NMR signals. One has no dependence on the history of the field treatment, which is consistent with Cu NMR signals from inequivalent Cu sites with different internal hyperfine fields. The other one is only seen after a field-cycling process, which is consistent with domain wall contributions in this frustrated antiferromagnet.

cond-mat.str-el↗

arXiv2015

Nearly-critical spin and charge fluctuations in KFe2As2 observed by high-pressure NMR

We report a high-pressure 75As NMR study on the heavily hole-doped iron pnictide superconductor KFe2As2 (Tc~3.8 K). The low-energy spin fluctuations are found to decrease with applied pressure up to 2 GPa, but then increase again, changing in lockstep with the pressure-induced evolution of Tc. Their diverging nature suggests close proximity to a magnetic quantum critical point at a negative pressure of P~-0.6 GPa. Above 2.4 GPa, the 75As satellite spectra split below 40 K, indicating a breaking of As site symmetry and an incipient charge order. These pressure-controlled phenomena demonstrate the presence of nearly-critical fluctuations in both spin and charge, providing essential input for the origin of superconductivity.

cond-mat.supr-con↗

arXiv2014

Near field thermal memory device

We report the concept of a near-field memory device based on the radiative bistability effect in the system of two closely separated parallel plates of SiO$_2$ and VO$_2$ which exchange heat by thermal radiation in vacuum. We demonstrate that the VO$_2$ plate, having metal-insulator transition at 340 K, has two thermodynamical steady-states. One can switch between the states using an external laser impulse. We show that due to near-field photon tunneling between the plates, the switching time is found to be only 5 ms which is several orders lower than in case of far field.

cond-mat.mtrl-sci↗

arXiv2014

Phase Separation, Competition, and Volume Fraction Control in NaFe$_{1-x}$Co$_x$As

We report a detailed nuclear magnetic resonance (NMR) study by combined $^{23}$Na and $^{75}$As measurements over a broad range of doping to map the phase diagram of NaFe$_{1-x}$Co$_x$As. In the underdoped regime ($x \le$ 0.017), we find a magnetic phase with robust antiferromagnetic (AFM) order, which we denote the {\it s}-AFM phase, cohabiting with a phase of weak and possibly proximity-induced AFM order ({\it w}-AFM) whose volume fraction $V \simeq 8$\% is approximately constant. Near optimal doping, at $x = 0.0175$, we observe a phase separation between static antiferromagnetism related to the {\it s}-AFM phase and a paramagnetic (PM) phase related to {\it w}-AFM. The volume fraction of AFM phase increases upon cooling, but both the N{é}el temperature and the volume fraction can be suppressed systematically by applying a $c$-axis magnetic field. On cooling below $T_c$, superconductivity occupies the PM region and its volume fraction grows at the expense of the AFM phase, demonstrating a phase separation of the two types of order based on volume exclusion. At higher dopings, static antiferromagnetism and even critical AFM fluctuations are completely suppressed by superconductivity. Thus the phase diagram we establish contains two distinct types of phase separation and reflects a strong competition between AFM and superconducting phases both in real space and in momentum space. We suggest that both this strict mutual exclusion and the robustness of superconductivity against magnetism are consequences of the extreme two-dimensionality of NaFeAs.

cond-mat.supr-con↗

arXiv2014

Spin Fluctuations and Frustrated Magnetism in the Multiferroic FeVO$_4$

We report $^{51}$V nuclear magnetic resonance (NMR) studies on single crystals of the multiferroic material FeVO$_4$. The high-temperature Knight shift shows Curie-Weiss behavior, $^{51}K = a/(T + θ)$, with a large Weiss constant $θ\approx$ 116 K. However, the $^{51}$V spectrum shows no ordering near these temperatures, splitting instead into two peaks below 65 K, which suggests only short-ranged magnetic order on the NMR time scale. Two magnetic transitions are identified from peaks in the spin-lattice relaxation rate, $1/^{51}T_1$, at temperatures $T_{N1} \approx$ 19 K and $T_{N2} \approx$ 13 K, which are lower than the estimates obtained from polycrystalline samples. In the low-temperature incommensurate spiral state, the maximum ordered moment is estimated as 1.95$μ_B$/Fe, or 1/3 of the local moment. Strong low-energy spin fluctuations are also indicated by the unconventional power-law temperature dependence $1/^{51}T_1 \propto T^2$. The large Weiss constant, short-range magnetic correlations far above $T_{N1}$, small ordered moment, significant low-energy spin fluctuations, and incommensurate ordered phases all provide explicit evidence for strong magnetic frustration in FeVO$_4$.

cond-mat.str-el↗

arXiv2014

Orbital selective correlations between nesting/scattering/Lifshitz transition and the superconductivity in AFe1-xCoxAs (A=Li, Na)

The correlations between the superconductivity in iron pnictides and their electronic structures are elusive and controversial so far. Here through angle-resolved photoemission spectroscopy measurements, we show that such correlations are rather distinct in AFe$_{1-x}$Co$_x$As (A=Li, Na), but only after one realizes that they are orbital selective. We found that the superconductivity is enhanced by the Fermi surface nesting, but only when it is between $d_{xz}/d_{yz}$ Fermi surfaces, while for the $d_{xy}$ orbital, even nearly perfect Fermi surface nesting could not induce superconductivity. Moreover, the superconductivity is completely suppressed just when the $d_{xz}/d_{yz}$ hole pockets sink below Fermi energy and evolve into an electron pocket. Our results thus establish the orbital selective relation between the Fermiology and the superconductivity in iron-based superconductors, and substantiate the critical role of the $d_{xz}/d_{yz}$ orbitals. Furthermore, around the zone center, we found that the $d_{xz}/d_{yz}$-based bands are much less sensitive to impurity scattering than the $d_{xy}$-based band, which explains the robust superconductivity against heavy doping in iron-based superconductors.

cond-mat.supr-con↗

arXiv2013

Simultaneous optimization of spin fluctuations and superconductivity under pressure in an iron-based superconductor

We present a high-pressure NMR study of the overdoped iron pnictide superconductor NaFe$_{0.94}$Co$_{0.06}$As. The low-energy antiferromagnetic spin fluctuations in the normal state, manifest as the Curie-Weiss upturn in the spin-lattice relaxation rate $1/^{75}T_1T$, first increase strongly with pressure but fall again at $P > P_{\rm opt} =$ 2.2 GPa. Neither long-ranged magnetic order nor a structural phase transition is encountered up to 2.5 GPa. The superconducting transition temperature $T_c$ shows a pressure-dependence identical to the spin fluctuations. Our observations demonstrate that magnetic correlations and superconductivity are optimized simultaneously as a function of the electronic structure, thereby supporting very strongly a magnetic origin of superconductivity.

cond-mat.supr-con↗

arXiv2013

Tunable interplay between 3d and 4f electrons in Co-doped iron pnictides

We study the interplay of 3d and 4f electrons in the iron pnictides CeFe$_{1-x}$Co$_x$AsO and GdFe$_{1-y}$Co$_y$AsO, which correspond to two very different cases of $4f$-magnetic moment. Both CeFeAsO and GdFeAsO undergo a spin-density-wave (SDW) transition associated with Fe 3d electrons at high temperatures, which is rapidly suppressed by Fe/Co substitution. Superconductivity appears in a narrow doping range: $0.05 < x < 0.2$ for CeFe$_{1-x}$Co$_x$AsO and $0.05 < y < 0.25$ for GdFe$_{1-y}$Co$_y$AsO, showing a maximum transition temperature $T_\textup{sc}$ of about 13.5 K for Ce and 19 K for Gd. In both compounds, the $4f$-electrons form an antiferromagnetic (AFM) order at low temperatures over the entire doping range and Co 3d electrons are ferromagnetically ordered on the Co-rich side; the Curie temperature reaches $T_\textup{C}^\textup{Co} \approx$ 75 K at $x = 1$ and $y = 1$. In the Ce-compounds, the Néel temperature $T_\textup{N}^\textup{Ce}$ increases upon suppressing the SDW transition of Fe and then remains nearly unchanged with further increasing Co concentration up to $x \simeq 0.8$ ($T_\textup{N}^\textup{Ce}\approx$ 4 K). Furthermore, evidence of Co-induced polarization on Ce-moments is observed on the Co-rich side. In the Gd-compounds, the two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic behavior in the magnetic susceptibility on the Co-rich side. For $0.7 \leq y < 1.0$, the system undergoes a possible magnetic reorientation below the Néel temperature of Gd ($T_\textup{N}^\textup{Gd}$). Our results suggest that the effects of both electron hybridizations and magnetic exchange coupling between the 3d-4f electrons give rise to a rich phase diagram in the rare-earth iron pnictides.

cond-mat.str-el↗

arXiv2012

Robust magnetic order of Ce 4f-electrons coexisting with superconductivity in CeFeAsO$_{1-x}$F$_x$

The electrical resistance of CeFeAsO$_{1-x}$F$_x$ (x = 0.06 and 0.08) has been measured in a magnetic field up to 40T. At zero field, the sample with x = 0.06 shows a structural phase transition around T$_S$~100K, followed by a spin-density-wave (SDW) transition around T$_{SDW}$~30K. For x = 0.08, the structural phase transition is suppressed down to T$_S$~60K without a clear anomaly associated with the Fe-SDW transition, and superconductivity shows up at T$_C$~25K. At lower temperatures, both samples show a clear resistive peak around T$_N$~4K, which is associated with the antiferromagnetic (AFM) transition of Ce-4f electrons. Strikingly, zero resistance is recovered upon further lowering temperature below T$_N$ for x = 0.08. Moreover, we found that the AFM transition of Ce 4f-electrons at 4K hardly changes with applying a magnetic field up to 40T, even in the case of x = 0.08, where superconductivity has been partially suppressed at such a large field.

cond-mat.supr-con↗

arXiv2012

Quenched Fe Moment in the Collapsed Tetragonal Phase of Ca$_{1-x}$Pr$_{x}$Fe$_2$As$_2$

We report $^{75}$As NMR studies on single crystals of rare-earth doped iron pnictides superconductor Ca$_{1-x}$Pr$_{x}$Fe$_{2}$As$_{2}$ ($x$=0.075 and 0.15). The $^{75}$As spectra show a chemical pressure effect with doping and a first order structure transition to the collapsed tetragonal phase upon cooling. A sharp drop of the Knight shift is seen below the structural transition, whereas $1/T_1$ is strongly enhanced at low-temperatures. These evidences indicate quenching of Fe local magnetism and short-range ordering of Pr$^{3+}$ moment in the collapsed tetragonal phase. The quenched Fe moment through structure collapse suggests a strong interplay of structure and magnetism, which is important for understanding the nature of the collapsed tetragonal phase.

cond-mat.supr-con↗