Session D1: Focus Session: High Magnetic Fields, THz Spectroscopy, MRI

Novel magnetic textures in SrCu$_{2}$(BO$_{3})_{2}$ from magnetostriction up to 97.4 tesla

Quantum magnets are model systems wherein strongly frustrated spin interactions generate a variety of exotic magnetic phases of current interest, including quantum spin ices, spin liquids, spin supersolids and complex magnetic superstructures. SrCu$_{2}$(BO$_{3})_{2}$, the only classic realization of the spin-1/2 Heisenberg antiferromagnet in the Shastry-Sutherland (orthogonal spin dimer) lattice is known to exhibit numerous magnetization plateaus due formation of stripe-like magnetic textures in high fields. However, the fine structure of these plateaus remains controversial on both experimental and theoretical fronts due to the existing limits for achievable magnetic fields in the laboratory, the sensitivity of current magnetization techniques, and the uncontrolled nature of available theoretical approaches for highly frustrated magnetic lattices. This talk will describe how we probe magnetic textures in SrCu$_{2}$(BO$_{3})_{2}$ via a recently-developed \textit{magnetostriction} technique based on optical fiber Bragg gratings [1]. We achieve microstrain (nm-resolution) sensitivity in ultrahigh pulsed fields to 97.4 T using the NHMFL 100 tesla multi-pulse magnet system [2]. The magnetostriction data reveal fine structure corresponding to all magnetization plateaus, and a significant lattice response to the long-predicted 1/2-saturation plateau at 82 T, as well as a new feature at 73.6 T that we attribute to a never before observed structure corresponding to 2/5 of magnetization saturation [3]. These data are complemented by simultaneous magnetocaloric-effect measurements, and are supported by numerical results obtained using a controlled density matrix renormalization group method.\\[4pt] [1] Daou R. et al., \textit{Rev. Sci. Instrum}. \textbf{81}, 033909 (2010).\\[0pt] [2] Sims J.R., et al. IEEE Trans. Appl. Supercond. 18, 587-591 (2008).\\[0pt] [3] M. Jaime et al., submitted. In collaboration with R. Daou, S.A. Crooker, F. Weickert, A. Uchida, A. Feiguin, C.D. Batista, H. Dabkowska, and B. Gaulin.   

Superconducting critical current measurements in pulsed magnets

Measurements of critical current in single crystals of high temperature superconductor using pulsed magnetic fields are tricky due to short time scale, fast field sweep rate and sheer absolute current values in restricted sample space. We will present a measurement system design that addresses the challenges via a combination of Field Programmable Gate Array (FPGA) fast-response signal generation and detection architecture and Focused Ion Beam crystal shaping.   

Development of a high-field electron paramagnetic resonance spectrometer

Electron paramagnetic resonance (EPR) spectroscopy is a powerful and versatile technique to study structure and dynamics of biomolecules. Structural investigations of biological molecules begin with site-directed spin labeling (SDSL). Using SDSL, a nitroxide spin label containing a stable unpaired electron is covalently attached at a specific site within a bio-macromolecule. The time resolution and the sensitivity of EPR spectrometer become higher when the system is operated at higher frequencies and magnetic fields. In addition, a fine spectral resolution obtained with a high-field EPR (HFEPR) enables us to study details of the conformation in biological molecule by determining the orientation of a spin-label or the relative orientation of two spin-labels embedded in the molecule. In this presentation, we will report the development of a 115/230 GHz continuous wave (cw) and pulsed EPR spectrometer at USC. The spectrometer is based on a 700/100 mW solid-state source at 115/230 GHz respectively, a 12-Tesla magnet and a superheterodyne detection system. The system also has the 2nd synthesizer for double electron-electron resonance (DEER) spectroscopy. HFEPR measurements with spin-labeled CS DNA will be discussed.   

Sensitivity of charge transport measurements to local inhomogeneities

We derive analytic expressions for the sensitivity of resistive and Hall measurements to local variations in a specimen's material properties in the combined linear limit of both small magnetic fields and small perturbations, presenting exact, algebraic expressions both for four-point probe measurements on an infinite plane and for symmetric, circular van der Pauw discs. We then generalize the results to obtain corrections to the sensitivities both for finite magnetic fields and for finite perturbations. Calculated functions match published results and computer simulations, and provide an intuitive, visual explanation for experimental misassignment of carrier type in n-type ZnO and agree with published experimental results for holes in a uniform material. These results simplify calculation and plotting of the sensitivities on an $N\times N$ grid from a problem of order $N^5$ to one of order $N^3$ in the arbitrary case and of order $N^2$ in the handful of cases that can be solved exactly, putting a powerful tool for inhomogeneity analysis in the hands of the researcher: calculation of the sensitivities requires little more than the solution of Laplace's equation on the specimen geometry.   

Polarization Modulation THz TDS of Topological Insulators

Optical hall conductivity measurements are powerful alternatives to DC transport measurements in samples in which the latter are challenging. They provide a deeper understanding of interactions in correlated systems and also serve as a measure of disorder in such systems. Aoki et al[1] has studied Quantum Hall Effect in graphene theoretically and has predicted that Optical Hall Conductivity should be measureable with an accurate detection of the Hall angle in the THz regime. Shimano et al [2] has reported evidence for Quantum Hall Plateau in the longitudinal conductivity $\sigma _{xy}$ in the THz region in a 2DEG system. In this work, we have developed a new broadband technique which rapidly measures complex Faraday and Kerr angles. Our technique is capable of measuring the entire complex conductivity tensor with a single scan, with an accuracy of 5mRad in the frequency range 02 to 2.5THz. We have employed this to study topological insulators and have observed a magnetic field dependent absorption around 0.5THz. 1. Morimoto, T., Jour. of Phy: Conference Series, 2009. 150(2). 2. Ikebe, Y., et al., PRL, 2010. 104(25): p. 256802.   

Analysis of complex mixtures using a new, high resolution Trapped Ion Mobility Spectrometer -- Mass Spectrometer

Over the last decade, a variety of new types of Ion Mobility Spectrometry (IMS) analyzers have been developed (e.g., periodic focusing DC ion guide, segmented quadrupole drift cell, multistage IMS, field asymmetric IMS and transient wave ion guide). High resolution IMS (R$>$50) has been mainly restricted to long IMS cells (1-2 m), where ions are separated based on size-to-charge ratio as they are pushed by an electric field through a stationary bath gas. In the present work we describe a Trapped Ion Mobility Spectrometer (TIMS) and its applications. In as much as TIMS uses an electric field to hold ions stationary in a moving bath gas, it represents a paradigm shift in mobility analysis. This results in an analyzer capable of high resolution mobility separations (R$>$80) in a compact ($<$ 10 cm), low voltage ($<$ 300 V) design. Hybridization with a mass analyser (TIMS-MS) provides versatility for the analysis, separation and structural characterization of a variety of chemical compounds with increasing complexity. In particular, examples of TIMS -- MS separation for complex biological and heteroatom hydrocarbons will be shown.   

Measurement of statistical nuclear spin polarization in a nanoscale GaAs sample

We measure the statistical polarization of quadrupolar nuclear spins in a sub-micrometer ($0.6\ \mu \mathrm{m}^3$) particle of GaAs using magnetic resonance force microscopy. The crystalline sample is cut out of a GaAs wafer and attached to a micro-mechanical cantilever force sensor using a focused ion beam technique. Nuclear magnetic resonance is demonstrated on ensembles containing less than $5 \times 10^8$ nuclear spins and occupying a volume of around (300 nm)$^3$ in GaAs with reduced volumes possible in future experiments. We discuss how the further reduction of this detection volume will bring the spin ensemble into a regime where random spin fluctuations, rather than Boltzmann polarization, dominate its dynamics. The detection of statistical polarization in GaAs therefore represents an important first step toward 3D magnetic resonance imaging of III-V materials on the nanometer-scale.   

Carbon nanotube quantum dots as highly sensitive THz spectrometers

We show that carbon nanotube quantum dots (CNT-Dots) coupled to antennas are extremely sensitive, broad-band, terahertz quantum detectors. Their response is due to photon-assisted single-electron tunneling (PASET)[1], but cannot be fully understood with orthodox PASET models[2]. We consider intra-dot excitations and non-equilibrium cooling to explain the anomalous response. REFERENCES: [1] Y. Kawano, S. Toyokawa, T. Uchida and K. Ishibashi, THz photon assisted tunneling in carbon-nanotube quantum dots, Journal of Applied Physics 103, 034307 (2008). [2] P. K. Tien and J. P. Gordon, Multiphoton Process Observed in the Interaction of Microwave Fields with the Tunneling between Superconductor Films, Phys. Rev. 129, 647 (1963).   

Ultrafast electro-optic modulation for Terahertz upconversion spectroscopy

Traditional time-domain terahertz (THz) spectroscopy techniques require the combination of sub-picosecond laser systems and time-delay stages for spectral analysis, making the design of a portable THz spectrometer challenging. In this work, we demonstrate an alternative method of spectrally resolving coherent THz radiation using narrow band optical light and passive optical spectrum analyzers. This method utilizes THz induced electro-optic modulation of a narrow band laser to upconvert the spectral content of the THz radiation to the visible portion of the electro-magnetic spectrum. This device does not require any movable parts and is well suited for spectrally analyzing both broadband and narrowband THz radiation. The spectral resolution of this technique is limited by the bandwidth of the optical radiation and the non-linear medium. Further advancements will include the development of a portable THz spectrometer, suitable for either research or clinical applications.   

Detection of internal molecular structural motions using anisotropic spectroscopy

The far infrared spectroscopy of molecular crystals reveals both intra and inter molecular vibrational modes [1,2]. With the significant increase in complexity of structures, one finds increasing overlap in the internal modes. As an overall strategy to measure the correlated structural motions in protein, we use anisotropic and birefringent behavior of molecular crystals to develop a new technique called MOSTS (Modulated Orientation Sensitive THz Spectroscopy). We achieve high sensitivity and mode separation by using single molecular crystal such as sucrose and rapid modulation of the relative alignment of the terahertz polarization and the crystal axes by rotating the sample. By locking into the signal at the rotation frequency we determine the polarization sensitive signal and map out the optically active vibrational resonances. To illustrate the technique we compare our measured spectra with the calculated and find a close agreement. \\[4pt] [1] D.G. Allis, J.A. Zeitler, P.F.Taday and T.M.Korter, Chem. Phys. Lett., 463, 84 (2008).\\[0pt] [2] P.U. Jepsen and J.C. Stewart, Chem. Phys. Lett., 442, 275 (2007).   

Ultra-Low Frequency MRI: Novel Imaging Sequences

Our ultra-low field MRI system operates at a field of 0.132 mT with the signal detected by a Superconducting QUantum Interference Device (SQUID) coupled to an untuned, superconducting, second-derivative gradiometer. Operation at such low fields requires that we prepolarize the protons of our specimen in a field of about 150 mT prior to imaging to increase the sample magnetization. With the ultimate goal of \textit{in vivo} imaging of prostate tissue by taking advantage of the enhanced longitudinal-relaxation-time at low fields, we seek to decrease the imaging time and optimize the signal-to-noise ratio of our imaging pulse sequences to make \textit{in vivo} imaging viable in a clinical setting. To achieve this, we begin with standard imaging sequences used in other frequency domains and adapt them to our specific purposes and requirements, in particular the need to prepolarize. We describe modified inversion recovery and multiple echo imaging sequences, specifically Carr-Purcell-Meiboom-Gill (CPMG) pulse train and fully balanced Steady-State Free Precession (SSFP) sequences. We present the results of applying these sequences to imaging agarose gel phantoms.   

Millimeter-wave and terahertz spectroscopy for the detection of ionized air and chemical vapor

Our previous work has demonstrated that the time domain terahertz spectroscopy (TDTS) of chemical vapor or ionized air produces characteristic spectrum from which one can identify chemicals or an ionization source, such as nuclear isotopes. While the average power of TDTS is only less than micro-Watts, the peak power of terahertz pulse can exceed kilo-Watts. When terahertz pulse is concentrated within a small area this large peak power produces a very large electric field, exceeding several kV/m. We investigate the field strength dependence of TDTS to see how it affects the detection capability of TDTS for chemical vapor and ionized air when the peak power is varied over the range from a few mW to kW. At the same time we performed similar experiments using a CW millimeter-wave spectroscopy over the frequency range from 75 GHz to 110 GHz and the power strength range from a few micro-Watts to several mW. We will present the details of our experimental results and discuss the merits of both systems for accuracy and long range detection of vapors. We will also examine some theory to understand the data.