arXiv subjects

David B. Shrekenhamer

Publications and source records attributed to David B. Shrekenhamer.

3 recordsLinked to original sources

arXiv2025

Wide-Angle, Multiplexed Backscatter Communications Using a Dynamic Metasurface-Backed Luneburg Lens

Backscatter communications is attractive for its low power requirements due to the lack of actively radiating components; however, commonly used devices are typically limited in range and functionality. Here, we design and demonstrate a backscatter device consisting of a flattened Luneburg lens combined with a spatially-tunable dynamic metasurface. Using quasi-conformal transformation optics (QCTO), we design a flattened, additively manufactured Luneburg lens that focuses incoming waves over a wide field-of-view onto its flattened focal plane. When a reflective surface is placed at the focal plane, the flattened Luneburg lens retroreflects, enabling long-range backscatter communications over an extremely large field-of-view ($\pm30\degree$) and bandwidth. The dynamic metasurface is designed to modulated the reflected phase across the S-band (2-4 GHz) with fine spatial control. Thus, when combined with the flattened Luneburg lens, the device is able to modulate the retroreflected signal to achieve backscatter communications. We experimentally demonstrate full phase control of the backscattered signal across a range of incidence angles, spatial multiplexing, and secure communications against eavesdroppers by actively suppressing or randomizing signals in unwanted directions. The metasurface-backed Luneburg lens device offers a low-power solution for long-range wireless networks with advanced capabilities.

physics.optics↗

arXiv2016

Fermi-surface topologies and low-temperature phases of the filled Skutterudite compounds CeOs$_4$Sb$_{12}$ and NdOs$_4$Sb$_{12}$

MHz conductivity, torque magnetometer and magnetization measurements are reported on single crystals of CeOs$_4$Sb$_{12}$ and NdOs$_4$Sb$_{12}$ using temperatures down to 0.5~K and magnetic fields of up to 60~tesla. The field-orientation dependence of the de Haas-van Alphen and Shubnikov-de Haas oscillations is deduced by rotating the samples about the $[010]$ and $[0\bar{1}1]$ directions. The results indicate that NdOs$_4$Sb$_{12}$ has a similar Fermi surface topology to that of the unusual superconductor PrOs$_4$Sb$_{12}$, but with significantly smaller effective masses, supporting the importance of local phonon modes in contributing to the low-temperature heat capacity of NdOs$_4$Sb$_{12}$. By contrast, CeOs$_4$Sb$_{12}$ undergoes a field-induced transition from an unusual semimetal into a high-field, high-temperature state characterized by a single, almost spherical Fermi-surface section. The behavior of the phase boundary and comparisons with models of the bandstructure lead us to propose that the field-induced phase transition in CeOs$_4$Sb$_{12}$ is similar in origin to the well-known $α-γ$ transition in Ce and its alloys.

cond-mat.str-el↗

arXiv2013

Spin Induced Optical Conductivity in the Spin Liquid Candidate Herbertsmithite

A quantum spin liquid (QSL) is a state of matter in which magnetic spins interact strongly, but quantum fluctuations inhibit long-range magnetic order even at zero temperature. A QSL has been predicted to have a host of exotic properties, including fractionalized excitations and long-range quantum entanglement. Despite the numerous theoretical studies, experimental realization of a QSL has proved to be challenging due to the lack of candidate materials. The triangular organic salts EtMe3Sb[Pd(dmit)2]2 and κ-(BEDT-TTF)2Cu2(CN)3, and kagome ZnCu3(OH)6Cl2 (Herbertsmithite) have recently emerged as promising candidates of exhibiting a QSL state, but the nature of their ground states is still elusive. Here we studied a large-area high-quality single crystal of Herbertsmithite by means of time-domain terahertz (THz) spectroscopy. We observed in the low-frequency (0.6-2.2 THz) optical conductivity evidence for the nature of the spin system. In particular, the in-plane absorption spectrum exhibits a unique frequency dependence that can be described by a power-law with an exponent of approximately 1.4, in sharp contrast with the ω^4 dependence expected for an ordered Mott insulator. The absorption is also found to increase as the temperature decreases, a behavior unexpected for conventional insulators. Such features are consistent with recent theory based on the interactions between the charge and spin degrees of freedom in a QSL system.

cond-mat.str-el↗