D. A. Yarotski

Scanning tunneling microscopy of DNA-wrapped carbon nanotubes.

We employ scanning tunneling microscopy (STM) to reveal the structure of DNA-carbon nanotube complexes with unprecedented spatial resolution and compare our experimental results to molecular dynamics simulations. STM images show strands of DNA wrapping around (6,5) nanotubes at approximately 63 degrees angle with a coiling period of 3.3 nm, in agreement with the theoretical predictions. In addition, we observe width modulations along the DNA molecule itself with characteristic lengths of 1.9 and 2.5 nm, which remain unexplained. In our modeling we use a helical coordinate system, which naturally accounts for tube chirality along with an orbital charge density distribution and allows us to simulate this hybrid system with the optimal pi-interaction between DNA bases and the nanotube. Our results provide novel insight into the self-assembling mechanisms of nanotube-DNA hybrids and can be used to guide the development of novel DNA-based nanotube separation and self-assembly methods, as well as drug delivery and cancer therapy techniques.

Optical spectroscopy of the Weyl semimetal TaAs

B. Xu, ∗ Y. M. Dai, ∗ L. X. Zhao, K. Wang, R. Yang, W. Zhang, J. Y. Liu, H. Xiao, 3 G. F. Chen, 4 A. J. Taylor, D. A. Yarotski, R. P. Prasankumar, † and X. G. Qiu 4, ‡ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA Center for High Pressure Science and Technology Advanced Research, Beijing 100094, China Collaborative Innovation Center of Quantum Matter, Beijing 100190, China Associate Directorate for Chemistry, Life and Earth Sciences, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA (Dated: October 5, 2015)

OBSERVATION OF CHIRPED SOLITON DYNAMICS AT LAMBDA = 1.55 MU M IN A SINGLE-MODE OPTICAL FIBER WITH FREQUENCY-RESOLVED OPTICAL GATING

We present an experimental observation of the dynamics of an initially chirped optical soliton at 1.55microm that is propagating through a single-mode optical fiber, using frequency-resolved optical gating (FROG). FROG permits observation of both the amplitude and the phase profiles of ultrashort pulses, providing complete information on the pulse evolution. The features that are detected, which include what is believed to be the first experimental observation of phase slips, are in quantitative agreement with numerical simulations that employ the nonlinear Schrödinger equation.

Time-resolved terahertz dynamics in thin films of the topological insulator Bi2Se3

We use optical pump–THz probe spectroscopy at low temperatures to study the hot carrier response in thin Bi2Se3 films of several thicknesses, allowing us to separate the bulk from the surface transient response. We find that for thinner films the photoexcitation changes the transport scattering rate and reduces the THz conductivity, which relaxes within 10 picoseconds (ps). For thicker films, the conductivity increases upon photoexcitation and scales with increasing both the film thickness and the optical fluence, with a decay time of approximately 5 ps as well as a much higher scattering rate. These different dynamics are attributed to the surface and bulk electrons, respectively, and demonstrate that long-lived mobile surface photo-carriers can be accessed independently below certain film thicknesses for possible optoelectronic applications.

Ultrafast carrier dynamics in the large magnetoresistance material WTe2

In this study, ultrafast optical pump-probe spectroscopy is used to track carrier dynamics in the large-magnetoresistance material WTe2. Our experiments reveal a fast relaxation process occurring on a subpicosecond time scale that is caused by electron-phonon thermalization, allowing us to extract the electron-phonon coupling constant. An additional slower relaxation process, occurring on a time scale of ~5–15 ps, is attributed to phonon-assisted electron-hole recombination. As the temperature decreases from 300 K, the time scale governing this process increases due to the reduction of the phonon population. However, below ~50 K, an unusual decrease of the recombination time sets in, most likely due to a change in the electronic structure that has been linked to the large magnetoresistance observed in this material.

Unveiling Stability Criteria of DNA-Carbon Nanotubes Constructs by Scanning Tunneling Microscopy and Computational Modeling.

We present a combined approach that relies on computational simulations and scanning tunneling microscopy (STM) measurements to reveal morphological properties and stability criteria of carbon nanotube-DNA (CNT-DNA) constructs. Application of STM allows direct observation of very stable CNT-DNA hybrid structures with the well-defined DNA wrapping angle of 63.4° and a coiling period of 3.3 nm. Using force field simulations, we determine how the DNA-CNT binding energy depends on the sequence and binding geometry of a single strand DNA. This dependence allows us to quantitatively characterize the stability of a hybrid structure with an optimal π-stacking between DNA nucleotides and the tube surface and better interpret STM data. Our simulations clearly demonstrate the existence of a very stable DNA binding geometry for (6,5) CNT as evidenced by the presence of a well-defined minimum in the binding energy as a function of an angle between DNA strand and the nanotube chiral vector. This novel approach demonstrates the feasibility of CNT-DNA geometry studies with subnanometer resolution and paves the way towards complete characterization of the structural and electronic properties of drug-delivering systems based on DNA-CNT hybrids as a function of DNA sequence and a nanotube chirality.

Microwave frequency comb attributed to the formation of dipoles at the surface of a semiconductor by a mode-locked ultrafast laser

The generation of terahertz radiation by focusing a mode-locked ultrafast laser on the surface of a semiconductor was demonstrated by Zhang in 1990, and others have made numerous measurements and analyses of this effect. We have measured the surge current which causes this radiation, showing that this current, and presumably the radiation, are frequency combs with harmonics at integer multiples of the pulse repetition rate of the laser. The harmonics in the current are enhanced by placing the semiconductor in a tunneling junction, where the fundamental is increased by 8 dB with a DC tunneling current of 100 pA.

Microwave frequency-comb generation in a tunneling junction by intermode mixing of ultrafast laser pulses

We present a method for hyper-spectral characterization of the nonlinear effects in a tunneling junction. Harmonics up to 1 GHz were measured in a frequency comb in the tunneling current when 15-fs laser pulses at a repetition rate of 74.25 MHz were focused on the tunneling junction of a scanning tunneling microscope. The typical output power is −120 dBm at the fundamental frequency, which is the pulse repetition rate and decreases by several dB for the higher harmonics. The observed square-law dependence of the signal power on the tunneling current and incident laser power is in good agreement with theoretical predictions.

Probing the Interplay between Quantum Charge Fluctuations and Magnetic Ordering in LuFe2O4

The mechanisms producing strong coupling between electric and magnetic order in multiferroics are not always well understood, since their microscopic origins can be quite different. Hence, gaining a deeper understanding of magnetoelectric coupling in these materials is the key to their rational design. Here, we use ultrafast optical spectroscopy to show that the influence of magnetic ordering on quantum charge fluctuations via the double-exchange mechanism can govern the interplay between electric polarization and magnetism in the charge-ordered multiferroic LuFe2O4.

Phase analysis of nonlinear femtosecond pulse propagation and self-frequency shift in optical fibers

Abstract Phase sensitive analysis of femtosecond pulse propagation in optical fibers employing frequency resolved optical gating (FROG) is presented and compared to numerical simulations employing a modified cubic nonlinear Schrodinger equation (NLSE). Phase information obtained from deconvolution of the experimental traces allows the observation and characterization of specific pulse propagation features as a function of energy and distance. The experimental observation of the phase signature of a soliton during propagation and the phase properties of the soliton self-frequency shift are described and are found to be in remarkable agreement with the simulations.