arXiv subjects

Sangjun Lee

Publications and source records attributed to Sangjun Lee.

25 records · Page 2Linked to original sources

arXiv2019

Sixfold enhancement of superconductivity in a tunable electronic nematic system

The electronic nematic phase, wherein electronic degrees of freedom lower the crystal rotational symmetry, is a common motif across a number of high-temperature superconductors. However, understanding the role and influence of nematicity and nematic fluctuations in Cooper pairing is often complicated by the coexistence of other orders, particularly long-range magnetic order. Here we report the enhancement of superconductivity in a model electronic nematic system absent of magnetism, and show that the enhancement is directly born out of strong nematic fluctuations emanating from a tuned quantum phase transition. We use elastoresistance measurements of the Ba$_{1-x}$Sr$_{x}$Ni$_2$As$_2$ substitution series to show that strontium substitution promotes an electronically driven $B_{1g}$ nematic order in this system, and that the complete suppression of that order to absolute zero temperature evokes a dramatic enhancement of the pairing strength, as evidenced by a sixfold increase in the superconducting transition temperature. The direct relation between enhanced pairing and nematic fluctuations in this model system, as well as the interplay with a unidirectional charge density wave order comparable to that found in the cuprates, offers a means to elucidate the role of nematicity in boosting superconductivity.

cond-mat.supr-con↗

arXiv2018

Ultrafast time-resolved x-ray scattering reveals diffusive charge order dynamics in La$_{2-x}$Ba$_x$CuO$_4$

Charge order is universal among high-T$_c$ cuprates but its relevance to superconductivity is not established. It is widely believed that, while static order competes with superconductivity, dynamic order may be favorable and even contribute to Cooper pairing. We use time-resolved resonant soft x-ray scattering to study the collective dynamics of the charge order in the prototypical cuprate, La$_{2-x}$Ba$_x$CuO$_4$. We find that, at energy scales $0.4$ meV $ \lesssim ω\lesssim 2$ meV, the excitations are overdamped and propagate via Brownian-like diffusion. At energy scales below 0.4 meV the charge order exhibits dynamic critical scaling, displaying universal behavior arising from propagation of topological defects. Our study implies that charge order is dynamic, so may participate tangibly in superconductivity.

cond-mat.str-el↗

arXiv2018

Gravitational waves from first-order phase transitions: Towards model separation by bubble nucleation rate

We study gravitational-wave production from bubble collisions in a cosmic first-order phase transition, focusing on the possibility of model separation by the bubble nucleation rate dependence of the resulting gravitational-wave spectrum. By using the method of relating the spectrum with the two-point correlator of the energy-momentum tensor $\left< T(x)T(y) \right>$, we first write down analytic expressions for the spectrum with a Gaussian correction to the commonly used nucleation rate, $Γ\propto e^{βt}\rightarrow e^{βt-γ^2t^2}$, under the thin-wall and envelope approximations. Then we quantitatively investigate how the spectrum changes with the size of the Gaussian correction. It is found that the spectral shape shows ${\mathcal O}(10)\%$ deviation from $Γ\propto e^{βt}$ case for some physically motivated scenarios. We also briefly discuss detector sensitivities required to distinguish different spectral shapes.

hep-ph↗

arXiv2017

A Directional Antenna based Leader-Follower Relay System for End-to-End Robot Communications

In this paper, we present a directional antenna-based leader-follower robotic relay system capable of building end-to-end communication in complicated and dynamically changing environments. The proposed system consists of multiple networked robots - one is a mobile end node and the others are leaders or followers acting as radio relays. Every follower uses directional antennas to relay a communication radio and to estimate the location of the leader robot as a sensory device. For bearing estimation, we employ a weight centroid algorithm (WCA) and present a theoretical analysis of the use of WCA for this work. Using a robotic convoy method, we develop online, distributed control strategies that satisfy the scalability requirements of robotic network systems and enable cooperating robots to work independently. The performance of the proposed system is evaluated by conducting extensive real-world experiments that successfully build actual communication between two end nodes.

cs.RO↗

arXiv2017

Attack-Aware Multi-Sensor Integration Algorithm for Autonomous Vehicle Navigation Systems

In this paper, we propose a fault detection and isolation based attack-aware multi-sensor integration algorithm for the detection of cyberattacks in autonomous vehicle navigation systems. The proposed algorithm uses an extended Kalman filter to construct robust residuals in the presence of noise, and then uses a parametric statistical tool to identify cyberattacks. The parametric statistical tool is based on the residuals constructed by the measurement history rather than one measurement at a time in the properties of discrete-time signals and dynamic systems. This approach allows the proposed multi-sensor integration algorithm to provide quick detection and low false alarm rates for applications in dynamic systems. An example of INS/GNSS integration of autonomous navigation systems is presented to validate the proposed algorithm by using a software-in-the-loop simulation.

math.OC↗

arXiv2017

Observation of a Charge Density Wave Incommensuration Near the Superconducting Dome in CuxTiSe2

X-ray diffraction was employed to study the evolution of the charge density wave (CDW) in CuxTiSe2 as a function of copper intercalation in order to clarify the relationship between the CDW and superconductivity. The results show a CDW incommensuration arising at an intercalation value coincident with the onset of superconductivity at around x=0.055(5). Additionally, it was found that the charge density wave persists to higher intercalant concentrations than previously assumed, demonstrating that the CDW does not terminate inside the superconducting dome. A charge density wave peak was observed in samples up to x=0.091(6), the highest copper concentration examined in this study. The phase diagram established in this work suggests that charge density wave incommensuration may play a role in the formation of the superconducting state.

cond-mat.supr-con↗

arXiv2016

Comment on "Axion Dark Matter Coupling to Resonant Photons via Magnetic Field"

A recent Letter, Phys. Rev. Lett. 116, 161804 (2016), claims that for typical dark matter axion search experiments using cylindrical haloscopes, the power gain depends on the relative position of a cavity with respect to the center of a solenoidal magnetic field due to different electric and magnetic couplings. We review this Letter and find a misinterpretation of the coordinate system. We correct this and see no dependence of the coupling strength on the cavity location and the electric and magnetic energies stored in a cavity mode are equal.

hep-ex↗