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Dissipative dynamics and novel quantum phases in strongly correlated cold-atom mixtures.

机译:高度相关的冷原子混合物中的耗散动力学和新型量子相。

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摘要

We study the static and dynamical properties of a number of strongly correlated quantum many-body systems, that can be experimentally realized using cold-atoms. In the first part of the thesis, we investigate various quantum spin systems that interact with their environment, which we model as a bath of harmonic oscillators. Coupling to the bosonic bath modes induces a phonon-mediated ferromagnetic interaction between the spins. It also introduces decoherence and dissipation as a result of spin-bath entanglement. We extensively study the effect of dissipation on a single spin, two spins and the quantum Ising model, focusing on universal properties. Static properties become universal close to a quantum phase transition, where dissipation profoundly affects the scaling behavior. Universal dynamics occurs in the scaling limit, where the bandwidth of the bath oc becomes large.;For a single spin, we study the famous Landau-Zener level crossing problem in the presence of dissipation. Interaction with the bath leads to universal decay from the upper to the lower spin state, even far away from the resonance. The timescale to reach the final Landau-Zener spin transition probability is determined by the large parameter oc. To address this strongly driven non-equilibrium problem, we devise a novel non-perturbative stochastic Schrodinger equation method, based on a real-time functional integral description. This approach is particularly well-suited to study time-dependent bias fields, both at zero and finite temperature.;We also investigate a system of two Ising-coupled quantum spins, that are embedded in a common bosonic bath. To study the ground state phases for an Ohmic and a sub-Ohmic bath, we employ a combination of non-perturbative analytical and numerical renormalization group (NRG) methods. We discuss a number of different non-equilibrium situations, mainly using the time-dependent NRG. Most interestingly, spin oscillations may synchronize due to the proximity of a common bath, even at weak spin-bath coupling. We point out similarities and differences to the dynamics of the single-spin-boson model, and provide quantitative limits on the applicability of the perturbative Bloch-Redfield approach.;The quantum Ising model exhibits a quantum phase transition from a paramagnetic to a ferromagnetic state. The universality class of the transition is known to change in the presence of Ohmic dissipation resulting in different critical exponents. We suggest an experimental cold-atom setup comprising a bosonic mixture in state-selective trapping potentials where critical exponents can be measured using common time-of-flight imaging techniques.;In the second part of the thesis, we study how novel quantum phases may emerge in cold-atom gases subject to an optical lattice. We show that the intriguing supersolid phase, that combines density wave with superfluid order, arises at low temperatures in Bose-Fermi mixtures on 2D lattices in the presence of a nested Fermi surface. Nesting induces a density wave instability for the fermions. The density wave order is imprinted on the superfluid bosons via the boson-fermion interaction. At weak interaction, supersolidity competes with phase separation and we calculate the low temperature phase diagram for two concrete examples 87Rb40K and 23 Na6Li in various lattice geometries. We identify the 2D square lattice to be the optimal geometry to experimentally realize and detect a supersolid phase.
机译:我们研究了许多强相关的量子多体系统的静态和动态特性,这些特性可以使用冷原子通过实验实现。在论文的第一部分,我们研究了与环境相互作用的各种量子自旋系统,我们将其建模为谐波振荡器的浴池。耦合到玻色浴模式会引起自旋之间的声子介导的铁磁相互作用。由于自旋浴的缠结,它还会引入退相干和耗散。我们广泛研究了耗散对单个自旋,两个自旋和量子伊辛模型的影响,重点是通用特性。静态特性在量子相变附近变得很普遍,而耗散会严重影响缩放行为。普遍动力学发生在水垢oc的带宽变大的定标极限中;对于一次自旋,我们研究了在存在耗散的情况下著名的Landau-Zener平交问题。与镀液的相互作用导致从上旋态到下旋态的普遍衰减,甚至远离共振。达到最终Landau-Zener自旋转变概率的时标由大参数oc确定。为了解决这个强烈驱动的非平衡问题,我们基于实时函数积分描述设计了一种新颖的非扰动随机薛定inger方程方法。这种方法特别适合研究零温度和有限温度下随时间变化的偏置场。我们还研究了两个伊辛耦合量子自旋系统,它们嵌入一个普通的玻色子浴中。为了研究欧姆和亚欧姆浴的基态相,我们采用了非扰动分析和数值归一化组(NRG)方法的组合。我们讨论了许多不同的非平衡情况,主要是使用时间相关的NRG。最有趣的是,即使在弱自旋浴耦合的情况下,由于公共浴池的接近,自旋振荡也可能同步。我们指出了与单自旋玻色子模型动力学的异同,并为微扰Bloch-Redfield方法的适用性提供了定量限制。;量子伊辛模型展现了从顺磁态到铁磁态的量子相变。 。已知过渡的通用性类别会在存在欧姆耗散的情况下发生变化,从而导致不同的临界指数。我们提出了一种实验性的冷原子装置,该装置包括处于状态选择俘获势中的玻色子混合物,可以使用常见的飞行时间成像技术来测量关键指数。;在论文的第二部分,我们研究了新的量子相如何出现在受光晶格影响的冷原子气体中。我们表明,结合了密度波和超流体级的有趣的超固相,是在存在嵌套费米表面的2D晶格中的Bose-Fermi混合物中在低温下产生的。嵌套引起费米子的密度波不稳定。通过玻色子-费米子相互作用将密度波阶印在​​超流体玻色子上。在弱相互作用下,超固相与相分离竞争,我们针对两个不同晶格几何形状的具体实例87Rb40K和23 Na6Li计算了低温相图。我们确定二维方格是通过实验实现和检测超固相的最佳几何形状。

著录项

  • 作者

    Orth, Peter Philipp.;

  • 作者单位

    Yale University.;

  • 授予单位 Yale University.;
  • 学科 Physics Low Temperature.;Physics Condensed Matter.;Physics Atomic.;Physics Theory.
  • 学位 Ph.D.
  • 年度 2011
  • 页码 343 p.
  • 总页数 343
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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