239 research outputs found
Observation of two-orbital spin-exchange interactions with ultracold SU(N)-symmetric fermions
No full textSpin-exchanging interactions govern the properties of strongly correlated electron systems such as many magnetic materials. When orbital degrees of freedom are present, spin exchange between different orbitals often dominates, leading to the Kondo effect, heavy fermion behaviour or magnetic ordering. Ultracold ytterbium or alkaline-earth ensembles have attracted much recent interest as model systems for these effects, with two (meta-) stable electronic configurations representing independent orbitals. We report the observation of spin-exchanging contact interactions in a two-orbital SU(N)-symmetric quantum gas realized with fermionic Yb-173. We find strong inter-orbital spin exchange by spectroscopic characterization of all interaction channels and demonstrate SU(N = 6) symmetry within our measurement precision. The spin-exchange process is also directly observed through the dynamic equilibration of spin imbalances between ensembles in separate orbitals. The realization of an SU(N)-symmetric two-orbital Hubbard Hamiltonian opens the route to quantum simulations with extended symmetries and with orbital magnetic interactions, such as the Kondo lattice model
Direct Probing of the Mott Crossover in the SU(N) Fermi-Hubbard Model
No full textWe report on a detailed experimental investigation of the equation of state (EoS) of the three-dimensional Fermi-Hubbard model (FHM) in its generalized SU(N)-symmetric form, using a degenerate ytterbium gas in an optical lattice. In its more common spin-1/2 form, the FHM is a central model of condensed-matter physics. The generalization to N>2 was first used to describe multi-orbital materials and is expected to exhibit novel many-body phases in a complex phase diagram. By realizing and locally probing the SU(N) FHM with ultracold atoms, we obtain model-free access to thermodynamic quantities. The measurement of the EoS and the local compressibility allows us to characterize the crossover from a compressible metal to an incompressible Mott insulator. We reach specific entropies above Néel order but below that of uncorrelated spins. Having access to the EoS of such a system represents an important step towards probing predicted novel SU(N) phases
Observation of two-orbital spin-exchange interactions with ultracold SU(N)-symmetric fermions (vol 10, pg 779, 2014)
No full textKinetics of Intramolecular Contact Formation in a Denatured Protein
No full textQuenching of the triplet state of tryptophan by cysteine has provided a
new tool for measuring the rate of forming a specific intramolecular contact
in disordered polypeptides. Here, we use this technique to investigate
contact formation in the denatured state of CspTm, a small cold-shock
protein from Thermotoga maritima, engineered to contain a single tryptophan
residue (W29) and a single cysteine residue at the C terminus
(C67). At all concentrations of denaturant, the decay rate of the W29
triplet of the unfolded protein is more than tenfold faster than the rate
observed for the native protein (10^4 1/s). Experiments on the unfolded
protein without the added C-terminal cysteine residue show that this faster
rate results entirely from contact quenching by C67. The quenching
rate in the unfolded state by C67 increases at concentrations of denaturant
that favor folding, indicating a compaction of the unfolded protein as
observed previously in single-molecule Forster resonance energy transfer
(FRET) experiments
Kinetics of Intramolecular Contact Formation in Unfolded Proteins
No full textQuenching of the triplet state of tryptophan by cysteine after nanosecond UV excitation has provided a new tool for measuring the rate of forming a specific intramolecular contact in disordered polypeptides. Here, we use this technique to measure the kinetics of loop formation in two unfolded single domain proteins: CspTm-C, a 67 residues cold-shock protein from Thermotoga maritima, engineered to contain a single tryptophan (W29) and a single cysteine at the C terminus; C-HP-35, a 36 residues variant of the headpiece subdomain of the F-actin-binding protein villin, containing a single tryptophan (W24) and engineered to contain a cysteine residue at the N terminus.
We investigate the kinetic of intramolecular loop formation between tryptophan and cysteine for different denaturant concentrations. For both proteins, the measured triplet decay rates in the unfolded state are tenfold faster than the rates observed in the native state. Experiments on the unfolded proteins without the added cysteine residue show that the faster rate results entirely from contact quenching with cysteine. Differently, the triplet decay rate measured in the native state is equal or slower than the decay rate of free tryptophan in solution, since contact formation with cysteine is prevented in the native conformation of both proteins
Observation of an Orbital Interaction-Induced Feshbach Resonance in <sup>173</sup>Yb
No full textWe report on the experimental observation of a novel interorbital Feshbach resonance in ultracold Yb-173 atoms. This opens up the possibility of tuning the interactions between the S-1(0) and P-3(0) metastable state, both possessing zero total electronic angular momentum. The resonance is observed at experimentally accessible magnetic field strengths and occurs universally for all hyperfine state combinations. We characterize the resonance in the bulk via interorbital cross thermalization as well as in a three-dimensional lattice using high-resolution clock-line spectroscopy. Our measurements are well described by a generalized two-channel model of the orbital-exchange interactions
Probing the SU(N) Fermi-Hubbard model with ytterbium atoms in an optical lattice
No full textThis thesis reports on the experimental realization of the 3D SU(N) Fermi-Hubbard model and the direct probing of the equation of state with an ultracold quantum gas of fermionic ytterbium in an optical lattice. Ultracold atoms in optical lattices constitute a flexible and highly tunable system to investigate Hamiltonians of condensed matter physics such as the Hubbard model. In particular, ytterbium atoms are ideal candidates for the realization of the Fermi-Hubbard model with SU(N)-symmetry due to a high decoupling of the nuclear spin from the electronic configuration. As a consequence of this enlarged symmetry, thermodynamic properties of the atomic sample depend on N, the number of spin components in the quantum gas, and novel, exotic phases are predicted to emerge at low temperatures.
By locally probing a quantum gas of 173Yb in a 3D optical lattice, we determine the equation of state of the SU(6) and SU(3) Fermi-Hubbard model. The measurement of the equation of state allows us to obtain direct, model-independent access to the thermodynamic quantities of the lattice gas. In this way, we can characterize the crossover from a Fermi liquid to an SU(N) Mott insulator when tuning the interaction strength, and can probe the compressibility of the quantum gas in different interaction regimes. Moreover, we find a low specific entropy of the SU(6) gas below that of
uncorrelated spins, indicating the presence of partial spin correlations in the atomic sample. The ability to access the equation of state of such high spin systems, as well as the low obtained entropy, represent an important step towards the realization of SU(N) spin Hamiltonians and the characterization of novel SU(N) phases.Diese Doktorarbeit beschreibt die experimentelle Umsetzung des 3D SU(N) Fermi-Hubbard Modells und die direkte Messung der Zustandsgleichung mit Hilfe eines ultrakalten Quantengases von fermionischen Ytterbium-Atomen in einem optischen Gitter. Ultrakalte, neutrale Atome in optischen Gittern stellen ein gut kontrollierbares und hochflexibles System dar um Modelle aus der Festkörperphysik, wie z.B. das Hubbard Modell, zu untersuchen. Insbesondere erlauben Ytterbium-Atome, diese Modelle mit SU(N) Symmetrie zu realisieren, da bei ihnen der Kernspin nahezu vollständig von der elektronischen Konfiguration der Atome entkoppelt ist. Als Folge dieser erweiterten Symmetrie hängen die thermodynamischen Größen von N – der Anzahl der Spinkomponenten im Quantengas – ab, und man erwartet neuartige Phasenzustände dieser Systeme bei niedrigen Temperaturen.
Durch Messen der lokalen Eigenschaften eines 173Yb Quantengases, erhalten wir die Zustandsgleichung des SU(6) und SU(3) Fermi-Hubbard Modells. Die Zustandsgleichung erlaubt es uns, direkten, modellunabhängigen Zugang zu den thermodynamischen Größen des Gases im Gitter zu erlangen. Hiermit ist es möglich, durch Ändern der Wechselwirkungsstärke den Übergang von einer Fermi-Flüssigkeit zu einem SU(N) Mott-Isolator zu beobachten, sowie die Kompressibilität des Gases für unterschiedlich starke Wechselwirkungen zu ermitteln. In dem Experiment beobachten wir eine niedrige spezifische Entropie des SU(6) Gases, niedriger als die von unkorrelierten Spins, was auf partielle Spinkorrelationen im Quantengas hinweist. Die Möglichkeit, die Zustandsgleichung solcher Systeme mit hohem Spin direkt zu bestimmen, sowie die niedrige Entropie die erzielt wurde, stellen einen wichtigen Schritt für die Realisierung von SU(N) Spin-Hamiltonoperatoren dar, sowie für die Charakterisierung von neuartigen SU(N) Phasenzuständen
Unfolding and refolding of cytochrome c driven by the interaction with lipid micelles
No full textBinding of native cyt c to L-PG micelles leads to a partially unfolded conformation of cyt c. This micelle-bound state has no stable tertiary structure, but remains as -helical as native cyt c in solution. In contrast, binding of the acid-unfolded cyt c to L-PG micelles induces folding of the polypeptide, resulting in a similar helical state to that originated from the binding of native cyt c to L-PG micelles. Far-ultraviolet (UV) circular dichroism (CD) spectra showed that this common micelle-associated helical state (HL) has a native-like -helix content, but is highly expanded without a tightly packed hydrophobic core, as revealed by tryptophan fluorescence, near-UV, and Soret CD spectroscopy. The kinetics of the interaction of native and acid-unfolded cyt c was investigated by stopped-flow tryptophan fluorescence. Formation of HL from the native state requires the disruption of the tightly packed hydrophobic core in the native protein. This micelle-induced unfolding of cyt c occurs at a rate 0.1 s1, which is remarkably faster in the lipid environment compared with the expected rate of unfolding in solution. Refolding of acid-unfolded cyt c with L-PG micelles involves an early highly helical collapsed state formed during the burst phase (<3 ms), and the observed main kinetic event reports on the opening of this early compact intermediate prior to insertion into the lipid micelle
A stereo electron spectrometer for carrier-envelope phase measurements of few-cycle laser pulses
No full textThe characterisation of single ultrashort laser pulses as needed for spectroscopy requires the precise knowledge of the offset between the electric field and the pulse envelope, the so called carrier envelope phase. Within the framework of this diploma thesis a so called stereo-ATI spectrometer was constructed that is able to detect this phase. In contrast to the f-2f self-referencing technique, this type of spectrometer is able to determine absolute phases for single shots. In addition, this apparatus allows to detect drifts in the laser system during sophisticated, long-time measurements with the Reaction Microscope. A laser pulse in focused on a gas target and the left-right distribution of photoelectrons, which are emitted along the laser polarisation axis, is measured. The carrier envelope phase can be determined by comparing the left-right asymmetries of photoelectrons at different electron energy regions. It is shown that this method works down to the limit of single pulses. The comparison with the standard f-2f technique is done. Since the stereo-ATI technique represents a non invasive method to detect drifts in our laser laser system the stability of the f-2f interferometer for long time measurement is examined
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