Ultracold atoms in optical lattices have great potential to contribute to a better understanding of some of the most important issues in many-body physics, such as high-temperature (high-Tc) superconductivity [1]. Thirty years ago, Anderson suggested that the Hubbard model, a simplified representation of fermions moving on a periodic lattice, may contain the essence of copper oxide superconductivity [2]. The Hubbard model describes many of the features shared by the copper oxides, including an interaction-driven Mott insulating state and an antiferromagnetic (AFM) state. Optical lattices filled with a two-spin-component Fermi gas of ultracold atoms can faithfully realise the Hubbard model with readily tunable parameters, and thus provide a platform for the systematic exploration of its phase diagram [3,4]. Realisation of strongly correlated phases, however, has been hindered by the need to cool the atoms to temperatures as low as the magnetic exchange energy, and also by the lack of reliable thermometry [5]. Here we demonstrate spin-sensitive Bragg scattering of light to measure AFM spin correlations in a realisation of the three-dimensional (3D) Hubbard model at temperatures down to 1.4 times that of the AFM phase transition. This temperature regime is beyond the range of validity of a simple high-temperature series expansion, which brings our experiment close to the limit of the capabilities of current numerical techniques. We reach these low temperatures using a unique compensated optical lattice technique [6], in which the confinement of each lattice beam is compensated by a blue-detuned laser beam. The temperature of the atoms in the lattice is deduced by comparing the light scattering to determinantal quantum Monte Carlo [7] (DQMC) and numerical linked-cluster expansion [8] (NLCE) calculations. Further refinement of the compensated lattice may produce even lower temperatures which, along with light scattering thermometry, would open avenues for achieving and characterising other novel quantum states of matter, such as the pseudogap regime of the 2D Hubbard model.A two-spin-component Fermi gas in a simple cubic optical lattice may be described by a single-band Hubbard model with nearest-neighbour tunnelling t and on-site interaction U > 0. At a density n of one atom per site, and for sufficiently large U/t there is a crossover from a 'metallic' state to a Mott insulating regime [9] as the temperature T is reduced below U . The Mott regime has been demonstrated with ultracold atoms in an optical lattice by observing the reduction of doubly occupied sites [10] and the related reduction of the global compressibility [11]. For T below the Néel ordering temperature T N , which for U t is approximately equal to the exchange energy J = 4t 2 /U , the system undergoes a phase transition to an AFM state [12]. In the context of quantum simulations, AFM phases of Ising spins have been previously engineered with bosonic atoms in an optical lattice [13] and with spin-1 2 ions [14,15]. Also, nearest-neighbour AFM correlat...
We have used the narrow 2S 1/2 → 3P 3/2 transition in the ultraviolet (uv) to laser cool and magneto-optically trap (MOT) 6 Li atoms. Laser cooling of lithium is usually performed on the 2S 1/2 → 2P 3/2 (D2) transition, and temperatures of ∼300 μK are typically achieved. The linewidth of the uv transition is seven times narrower than the D2 line, resulting in lower laser cooling temperatures. We demonstrate that a MOT operating on the uv transition reaches temperatures as low as 59 μK. Furthermore, we find that the light shift of the uv transition in an optical dipole trap at 1070 nm is small and blueshifted, facilitating efficient loading from the uv MOT. Evaporative cooling of a two spin-state mixture of 6 Li in the optical trap produces a quantum degenerate Fermi gas with 3 × 10 6 atoms in a total cycle time of only 11 s. The creation of quantum degenerate gases using all-optical techniques [1-4] offers several advantages over methods employing magnetic traps. Optical potentials can trap any ground state, allowing selection of hyperfine sublevels with favorable elastic and inelastic scattering properties. In the case of Fermi gases, the ability to trap atoms in more than one sublevel eliminates the need for sympathetic cooling with another species [5,6], greatly simplifying the experimental setup. All-optical methods also facilitate rapid evaporative cooling since magnetically tunable Feshbach resonances can be employed to achieve fast thermalization.There are, however, challenges to all-optical methods. An essential prerequisite is an optical potential whose depth is sufficiently greater than the temperature of the atoms being loaded. The usual starting point is a laser cooled atomic gas confined to a magneto-optical trap (MOT). In a twolevel picture, atoms may be cooled to the Doppler limit T D =h /(2k B ), where /(2π ) is the natural linewidth of the excited state of the cooling transition [7,8]. In many cases, however, sub-Doppler temperatures can be realized due to the occurrence of polarization gradient cooling arising from the multilevel character of real atoms [9]. Polarization gradient cooling mechanisms are effective if the linewidth of the cooling transition is small compared to the hyperfine splitting of the excited state, or if there is a large degree of magnetic degeneracy in the ground state [10]. The limit to cooling in these cases is the recoil temperature T R =h 2 k 2 /(2mk B ), where k is the wave number of the laser cooling transition and m is the mass of the atom.Polarization gradient cooling is found to be efficient for most of the alkali-metal atoms including Na, Rb, and Cs; MOTs of these species routinely attain temperatures of ∼10 μK, which is not far above T R . Unfortunately, for Li and K, the elements most often employed in Fermi-gas experiments, sub-Doppler cooling is ineffective in the presence of magnetic fields, including those required for a MOT. For Li, sub-Doppler cooling is inhibited because the hyperfine splitting of the excited state is unresolved (Fig. 1), thus limiting ...
Genotoxicity-induced hair loss from chemotherapy and radiotherapy is often encountered in cancer treatment, and there is a lack of effective treatment. In growing hair follicles (HF), quiescent stem cells (SC) are maintained in the bulge region and hair bulbs at the base contain rapidly dividing, yet genotoxicity-sensitive transit-amplifying cells (TAC) that maintain hair growth. How genotoxicity-induced HF injury is repaired remains unclear. We report here that HF mobilize ectopic progenitors from distinct TAC compartments for regeneration in adaptation to the severity of dystrophy induced by ionizing radiation (IR). Specifically, after low-dose IR, keratin5+ basal hair bulb progenitors, rather than bulge SC, were quickly activated to replenish matrix cells and regenerated all concentric layers of HF, demonstrating their plasticity. After high-dose IR, when both matrix and hair bulb cells were depleted, the surviving outer root sheath cells rapidly acquired a SC-like state and fueled HF regeneration. Their progeny then homed back to SC niche and supported new cycles of HF growth. We also revealed that IR induced HF dystrophy and hair loss and suppressed WNT signaling in a p53- and dose-dependent manner. Augmenting WNT signaling attenuated the suppressive effect of p53 and enhanced ectopic progenitor proliferation after genotoxic injury, thereby preventing both IR- and cyclophosphamide-induced alopecia. Hence, targeted activation of TAC-derived progenitor cells, rather than quiescent bulge SC, for anagen HF repair can be a potential approach to prevent hair loss from chemotherapy and radiotherapy.
We characterize the Mott insulating regime of a repulsively interacting Fermi gas of ultracold atoms in a three-dimensional optical lattice. We use in situ imaging to extract the central density of the gas and to determine its local compressibility. For intermediate to strong interactions, we observe the emergence of a plateau in the density as a function of atom number, and a reduction of the compressibility at a density of one atom per site, indicating the formation of a Mott insulator. Comparisons to state-of-the-art numerical simulations of the Hubbard model over a wide range of interactions reveal that the temperature of the gas is of the order of, or below, the tunneling energy scale. Our results hold great promise for the exploration of many-body phenomena with ultracold atoms, where the local compressibility can be a useful tool to detect signatures of different phases or phase boundaries at specific values of the filling. The Hubbard model, which describes spin-1=2 fermions in a lattice with on-site interactions, is one of the fundamental models in quantum many-body physics. It is a notable example of how strongly correlated phases emerge from simple Hamiltonians: it exhibits a Mott insulating regime, antiferromagnetism, and is widely believed to support a d-wave superfluid state in two dimensions (2D), which could explain high-temperature superconductivity as observed in the cuprates [1]. Despite intense efforts, an exact solution of the Hubbard model in more than one dimension and for arbitrary filling has evaded theoretical and computational approaches to this day. Complementing these approaches, the last decade has seen the development of ultracold atoms in optical lattices as a new and versatile platform for the study of many-body physics [2,3]. In this Letter, we study a two-spin component degenerate gas of fermions in a simple cubic lattice, a system which realizes the three-dimensional (3D) single band Hubbard model.Previous groundbreaking experiments investigated the Mott transition in trapped lattice fermions by measuring the variation of the bulk double occupancy with atom number [4][5][6] and the response of the cloud radius to changes in external confinement [7], both of which are related to the global compressibility. Several key issues, however, remain to be addressed. (i) As bulk measurements are the result of an average over both metallic and insulating phases simultaneously present in the trap, how does the local compressibility behave within the trap? (ii) How does the compressibility respond at lower temperatures, as one approaches the magnetic transition? (iii) Can more robust theoretical treatments be employed to benchmark the observed behavior?In this Letter, we address these issues, making significant progress towards understanding the physics of the fermionic Hubbard Hamiltonian through optical lattice emulation. We extract the local compressibility of the gas from a measurement of the in situ density profile, a procedure that has been previously demonstrated for a Fermi gas in ...
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