Session S3: Condensed Matter II

Quantum entanglement in photosynthetic light harvesting

Identification of non-trivial quantum mechanical effects in the functioning of biological systems has been a long-standing and elusive goal in the fields of physics, chemistry and biology. Recent progress in control and measurement technologies, especially in the optical spectroscopy domain, have made possible the identification of such effects. I examine light harvesting components of photosynthetic organisms -- complex, coupled, many-body quantum systems -- in which electronic coherence has recently been shown to survive for relatively long time scales despite the effects of their noisy environments. By constructing useful measures of entanglement for such systems, and using an accurate model of energy transfer dynamics in the presence of noise, I demonstrate the existence of quantum entanglement in a commonly studied light harvesting complex. The lifetimes and temperature dependency of entanglement are examined in detail. This study constitutes the first rigorous quantification of entanglement in a biological system.   

Analyzing Particle Size Effects in ZnS:Cu using X-ray Absorption Spectroscopy

We report XAS measurements on ZnS:Cu,Mn phosphor materials of various particle sizes, 1-25$\mu$m. These materials exhibit electroluminescence (EL) at AC voltages of order 100V for $\sim$25$\mu$m particles, but only from small points associated with CuS nanoprecipitates (which form along the 111 plane in ZnS); here the local electric fields are enhanced by a factor of about 100. To enable lower-voltage applications, it is desirable to make smaller particles so devices can be thinner. We investigated the local structure for smaller particles produced by mill-grinding to determine why grinding leads to reduced AC EL. The K-edge EXAFS data show a decreased Cu-S first peak amplitude for the ground particle samples, but little change for Mn-S or Zn-S peaks. The XANES data show a large change in the structure of the Cu K-edge for smaller particles but not for either the host Zn or Mn dopant edges. Clearly grinding affects the environment about Cu, in the CuS precipitates, much more than the ZnS host lattice. Possibly the ZnS particles fracture preferentially through the CuS precipitates or the ZnS particles are partially sheared along the 111 plane.   

Metallic-like photoluminescence and absorption in fused silica surface flaws

Using high-sensitivity confocal time-resolved photoluminescence (PL) techniques, we report an ultrafast PL (40 ps-5 ns) from impurity-free surface flaws on fused silica, including polished, indented, or fractured surfaces of fused silica, and from laser-heated evaporation pits. This PL is excited by the single-photon absorption of sub-band gap light, and is especially bright in fractures. Regions which exhibit this PL are strongly absorptive well below the band gap, as evidenced by a propensity to damage with 3.5 eV nanosecond-scale laser pulses.   

Cooling and Heating Processes in the Magnetocaloric Materials: Is Reversibility possible?

Irreversibility and reversibility of adiabatic processes in the magnetocaloric materials such as MnAs and YbInCu$_{4}$ have been a major concern for technological applications. We used a differential scanning calorimeter in order to record the heat flux as a function of the temperature and applied field. From the measured heat flux, we extracted the latent heat and entropy associated to cooling and heating processes. For materials with structural phase transition associated to magnetic ordering, we observed irreversibility of the thermodynamic cycle. On the other hand, for materials with valence transition, we observed a nearly reversible process. The thermomagnetic behavior can be understood as Zener's $p-d $exchange mechanism dominates for MnAs, i.e. the interaction range is weaker but long ranged, because the extended valence hole states mediate the ferromagnetic interaction.   

Comparative Study of PbS and CdSe quantum dots for use in Luminescent Solar Concentrators

A comparative study for absorption, redshift and photovoltaic cell (PV) response has been performed for Luminescent Solar Concentrators (LSCs) with embedded PbS and CdSe quantum dots (QDs). LSCs are planar non-tracking devices where the incident solar radiation is absorbed by a fluorescent species embedded in a polymer or glass plate which down-convert and re-emit the solar radiation at longer wavelengths. The emitted light is trapped in the concentrator plate by total internal reflection, transported and emitted at the four edges, where the photons are collected by PV cells. Based on nearly double the current generated by PV cell to prototype device, we have concluded that for the purpose of embedding in the LSC PbS quantum dots outperform CdSe. The results are linked to smaller self absorption observed in PbS QD solution and broader absorption spectrum of these QDs.   

The 1-d Long Range Diluted Heisenberg Spin Glass - A Monte Carlo Study

We present results from a finite size scaling (FSS) analysis of Monte Carlo simulations on a novel long range diluted 1-d Heisenberg model with power law decay of interactions with distance. The advantage of studying a model like this combined with Heisenberg-like spins is that it allows one to study very large sizes (32000), and therefore reinforces the use of the FSS method to a) Study the existence of a phase transition. b) Extract the critical exponents of the phase transition. Besides allowing large sizes there are other advantages of studying this model: a) Real material spin-glasses are believed to be Heisenberg-like, and this is therefore directly relevant. b) By tuning the power of the decaying interactions we are able to study a range of universality classes from mean-field to short-range. Additionally we also present data to explore the role of Ising-like variables called chiralities which have been argued to be the drivers of the spin-glass transition. We find that our data does not support this spin-chirality decoupling scheme.   

Optimization of a rubidium magnetometer based on nonlinear optical rotation

Atomic spin polarization of alkali atoms in the ground state can survive thousands of collisions with paraffin-coated cell walls. The resulting long spin-relaxation times achieved in evacuated, paraffin-coated cells enable precise measurement of atomic spin precession and energy shifts of ground-state Zeeman sublevels. In the present work, nonlinear magneto-optical rotation with frequency-modulated light (FM NMOR) is used to measure magnetic-field-induced spin precession for rubidium atoms contained in a paraffin-coated cell. We discuss optimization of the shot-noise-projected magnetometer sensitivity and practical implementation of the Rb magnetometer. The magnetometer will be applied to searches for anomalous spin-dependent interactions of the proton.   

Quantitative characterization of one-dimensional magnetic chains in organic semiconductors

Quasi one-dimensional iron chains are formed in thermally evaporated iron phthalocyanine thin films on silicon substrates. The chain length is modified by the substrate growth temperature and can be controlled within one order of magnitude. The surface morphology of organic thin films (80nm) is studied with atomic force microscopy. The grains are randomly oriented, have odd shapes, and are strongly elongated at high temperatures due to asymmetric shape of the small molecule. A height-height correlation function is applied to the data to extract the correlation length, roughness, and scaling parameter. A correlation between these structural characteristics and magnetic measurements performed in a vibrating sample magnetometer are presented.   

An electro-optic experimental study of an unusual liquid crystal phase transition

Liquid crystal phases are highly sensitive to their surroundings and they interact with light in unusual ways: the index of refraction is different depending on the polarization of the incident light. This combination of properties makes them ideal for low-power liquid crystal displays (LCD's), ubiquitous in today's portable electronic devices. They are also beautiful: optical textures of liquid crystals show bright colors, with the color corresponding to the amount of retardation in the light polarized along different axes. These phases are fluid, but can nevertheless be highly ordered. We have developed a novel experimental analysis using a photometric calculation of microscopy images to perform a series of experiments on several liquid crystal materials, called ``de Vries'' smectics. Using this system, we examined how the structure of these phases changed under the influence of different boundary conditions, temperature, and applied electric fields. These unusual materials show the bizarre behavior of appearing to become less ordered with decreasing temperature. This phase, which is not fully understood, has advantageous optical properties that could lead to the next generation of liquid crystal displays.   

Electronic structure and quantum critical behavior of NbFe$_{2}$

The C14 hexagonal Laves phase compound NbFe2 sits on the edge of a magnetic instability. By varying the composition, Nb$_{1-y}$Fe$_{2+y}$ encompasses two ferromagnetic states, a spin density wave state, and a quantum critical point (y = -0.04). Density functional calculations, using the generalized gradient approximation, found the electronic structure. An analysis of electronic structure calculations will be presented, illuminating the magnetic behavior and susceptibility at low temperature.   

Biologically inspired MEMS based directional microphone

A novel MEMS microphone is presented which mimics the aural system of the \textit{Ormia ochracea} fly and its extraordinary directional sensitivity. To overcome the minimal separation between its ears, a flexible hinge mechanically couples the fly's two tympanic membranes. By comparing the frequency response of these two structures, the interaural differences are amplified and sound source information is processed with unparalleled speed and accuracy. The presented device is 2mm x 1mm x 10$\mu$m SOI, hinged at the middle and attached to the substrate using two narrow legs, allowing both rocking and bending modes. Along the edges of the membrane, two sets of interdigitated comb fingers are connected to an Irvine Sensors capacitive readout chip to allow electronic measurement of the displacement. Also presented are results of extensive finite element modeling performed using COMSOL Multiphysics, which are in close agreement with experimental data.   

Environment-invariant measure of distance between evolutions of an open quantum system

The problem of quantifying the difference between evolutions of an open quantum system is important in quantum control, especially in control of quantum information processing. Motivated by this problem, we develop a measure for evaluating the distance between unitary evolution operators of a composite quantum system that consists of the sub-system of interest and environment. The main characteristic of this measure is the invariance with respect to the effect of the evolution operator on the environment, which follows from an equivalence relation that exists between unitary operators acting on the composite system, when the effect on only the sub-system of interest is considered. The invariance to the environment's transformation makes it possible to quantitatively compare the evolution of an open quantum system and its closed counterpart. The distance measure also determines the fidelity bounds of a general quantum channel with respect to a unitary target transformation. As an example, the measure is used in numerical simulations to evaluate fidelities of optimally controlled quantum gate operations, in the presence of an environment.