Abstract

Magnetic honeycomb iridates are thought to show strongly spin-anisotropic exchange interactions which, when highly frustrated, lead to an exotic state of matter known as the Kitaev quantum spin liquid. However, in all known examples these materials magnetically order at finite temperatures, the scale of which may imply weak frustration. Here we show that the application of a relatively small magnetic field drives the three-dimensional magnet \( \beta-\mathrm{Li}_2\mathrm{IrO}_3 \) from its incommensurate ground state into a quantum correlated paramagnet. Interestingly, this paramagnetic state admixes a zig-zag spin mode analogous to the zig-zag order seen in other Mott-Kitaev compounds. The rapid onset of the field-induced correlated state implies the exchange interactions are delicately balanced, leading to strong frustration and a near degeneracy of different ground states.

Abstract

We study the two-dimensional superconductor-insulator transition (SIT) in thin films of tantalum nitride. At zero magnetic field, films can be disorder-tuned across the SIT by adjusting thickness and film stoichiometry; insulating films exhibit classical hopping transport. Superconducting films exhibit a magnetic-field-tuned SIT, whose insulating ground state at high field appears to be a quantum-corrected metal. Scaling behavior at the field-tuned SIT shows classical percolation critical exponents \( z ν \approx 1.3 \), with a corresponding critical field \( H_c \ll H_{c2} \), the upper critical field. The Hall effect exhibits a crossing point near \( H_c \), but with a nonuniversal critical value \( ρ_{xy}^{c} \) comparable to the normal-state Hall resistivity. We propose that high-carrier-density metals will always exhibit this pattern of behavior at the boundary between superconducting and (trivially) insulating ground states.

Abstract

Honeycomb iridates such as $\gamma$-Li2IrO3 are argued to realize Kitaev spin-anisotropic magnetic exchange, along with Heisenberg and possibly other couplings. While systems with pure Kitaev interactions are candidates to realize a quantum spin-liquid ground state, in $\gamma$-Li2IrO3 it has been shown that the presence of competing magnetic interactions leads to an incommensurate spiral spin order at ambient pressure below 38 K. We study the pressure sensitivity of this magnetically ordered state in single crystals of $\gamma$-Li2IrO3 using resonant x-ray scattering (RXS) under applied hydrostatic pressures of up to 3 GPa. RXS is a direct probe of electronic order, and we observe the abrupt disappearance of the q sp=(0.57, 0, 0) spiral order at a critical pressure Pc= 1.4 GPa with no accompanying change in the symmetry of the lattice.

Abstract

Two methods of quantifying the spatial resolution of a camera are described, performed, and compared, with the objective of designing an imaging-system experiment for students in an undergraduate optics laboratory. With the goal of characterizing the resolution of a typical digital single-lens reflex (DSLR) camera, we motivate, introduce, and show agreement between traditional test-target contrast measurements and the technique of using Fourier analysis to obtain the modulation transfer function (MTF). The advantages and drawbacks of each method are compared. Finally, we explore the rich optical physics at work in the camera system by calculating the MTF as a function of wavelength and f-number. For example, we find that the Canon 40D demonstrates better spatial resolution at short wavelengths, in accordance with scalar diffraction theory, but is not diffraction-limited, being significantly affected by spherical aberration. The experiment and data analysis routines described here can be built and written in an undergraduate optics lab setting.

Abstract

The complex antiferromagnetic orders observed in the honeycomb iridates are a double-edged sword in the search for a quantum spin-liquid: both attesting that the magnetic interactions provide many of the necessary ingredients, while simultaneously impeding access. Focus has naturally been drawn to the unusual magnetic orders that hint at the underlying spin correlations. However, the study of any particular broken symmetry state generally provides little clue about the possibility of other nearby ground states. Here we use magnetic fields approaching 100 tesla to reveal the extent of the spin correlations in \( \gamma \)-lithium iridate. We find that a small component of field along the magnetic easy-axis melts long-range order, revealing a bistable, strongly correlated spin state. Far from the usual destruction of antiferromagnetism via spin polarization, the high-field state possesses only a small fraction of the total iridium moment, without evidence for long-range order up to the highest attainable magnetic fields.

Abstract

Electrons confined to two dimensions display an unexpected diversity of behaviors as they are cooled to absolute zero. Noninteracting electrons are predicted to eventually “localize” into an insulating ground state, and it has long been supposed that electron correlations stabilize only one other phase: superconductivity. However, many two-dimensional (2D) superconducting materials have shown surprising evidence for metallic behavior, where the electrical resistivity saturates in the zero-temperature limit; the nature of this unexpected metallic state remains under intense scrutiny. We report electrical transport properties for two disordered 2D superconductors, indium oxide and tantalum nitride, and observe a magnetic field–tuned transition from a true superconductor to a metallic phase with saturated resistivity. This metallic phase is characterized by a vanishing Hall resistivity, suggesting that it retains particle-hole symmetry from the disrupted superconducting state.

Abstract

In this paper, we propose a novel powerful strategy to perform searches for new electroweak states. Uncolored electroweak states appear in generic extensions of the Standard Model (SM) and yet are challenging to discover at hadron colliders. This problem is particularly acute when the lightest state in the electroweak multiplet is neutral and all multiplet components are approximately degenerate. In this scenario, production of the charged fields of the multiplet is followed by decay into nearly invisible states; if this decay occurs promptly, the only way to infer the presence of the reaction is through its missing energy signature. Our proposal relies on emission of photon radiation from the new charged states as a means of discriminating the signal from SM backgrounds. We demonstrate its broad applicability by studying two examples: a pure Higgsino doublet and an electroweak quintuplet field.

Abstract

We report on the capillary-driven leveling of a topographical perturbation at the surface of a freestanding liquid nanofilm. The width of a stepped surface profile is found to evolve as the square root of time. The hydrodynamic model is in excellent agreement with the experimental data. In addition to exhibiting an analogy with diffusive processes, this novel system serves as a precise nanoprobe for the rheology of liquids at interfaces in a configuration that avoids substrate effects.

Abstract

We study pore nucleation in a model membrane system, a freestanding polymer film. Nucleated pores smaller than a critical size close, while pores larger than the critical size grow. Holes of varying size were purposefully prepared in liquid polymer films, and their evolution in time was monitored using optical and atomic force microscopy to extract a critical radius. The critical radius scales linearly with film thickness for a homopolymer film. The results agree with a simple model which takes into account the energy cost due to surface area at the edge of the pore. The energy cost at the edge of the pore is experimentally varied by using a lamellar-forming diblock copolymer membrane. The underlying molecular architecture causes increased frustration at the pore edge resulting in an enhanced cost of pore formation.

Recent Publications

Student authorFaculty author

11.

Alejandro Ruiz, Alex Frano, Nicholas P. Breznay, Itamar Kimchi, Toni Helm, Iain Oswald, Daniel Y. Chan, R. J. Birgeneau, Zahirul Islam, and James G. Analytis

Correlated States in \( \beta-\mathrm{Li}_2\mathrm{IrO}_3 \) Driven by Applied Magnetic Fields

Nature Communications 8 (2017) 961.
12.

Nicholas P. Breznay, Mihir Tendulkar, Li Zhang, Sang-Chul Lee, and Aharon Kapitulnik

Superconductor To Weak-Insulator Transitions in Disordered Tantalum Nitride Films

Physical Review B 96 (2017) 134522.
13.

Nicholas P. Breznay, Alejandro Ruiz, Alex Frano, Wenli Bi, Robert J. Birgeneau, Daniel Haskel, and James G. Analytis

Resonant X-Ray Scattering Reveals Possible Disappearance of Magnetic Order Under Hydrostatic Pressure in the Kitaev Candidate $\Gamma$-Li$_2$Iro$_3$

Physical Review B 96 (2017) 020402.
14.

Calvin Leung and Thomas D. Donnelly

Measuring the spatial resolution of an optical system in an undergraduate optics laboratory

American Journal of Physics 85 (2017) 429-438.
15.

K A Modic, B J Ramshaw, J B Betts, Nicholas P. Breznay, James G Analytis, Ross D McDonald, and Arkady Shekhter

Robust Spin Correlations at High Magnetic Fields in the Harmonic Honeycomb Iridates

Nature Communications 8 (2017) 2.
16.

Nicholas P. Breznay and Aharon Kapitulnik

Particle-Hole Symmetry Reveals Failed Superconductivity in the Metallic Phase of Two-Dimensional Superconducting Films

Science Advances 3 (2017) e1700612.
17.

Ahmed Ismael, Eder Izaguirre, and Brian Shuve

Illuminating New Electroweak States at Hadron Colliders

Physical Review D 94 (2016) 015001.
18.

Brian Shuve and Michael E. Peskin

Revision of the LHCb Limit on Majorana Neutrinos

Physical Review D 94 (2016) 113007.
19.

Mark Ilton, Miles M. P. Couchman, Cedric Gerbelot, Michael Benzaquen, Paul D. Fowler, Howard A. Stone, Elie Raphael, Kari Dalnoki-Veress, and Thomas Salez

Capillary Leveling of Freestanding Liquid Nanofilms

Physical Review Letters 117 (2016) .
20.

Mark Ilton, Christian DiMaria, and Kari Dalnoki-Veress

Direct Measurement of the Critical Pore Size in a Model Membrane

Physics Review Letters 117 (2016) .