Bkt Transition Quantum Vortices 2d Bose Gas Review

In the realm of condensed matter physics, the Berezinskii-Kosterlitz-Thouless (BKT) transition stands as a pivotal phenomenon, particularly when considering its influence on two-dimensional (2D) Bose gases and the emergence of quantum vortices. This transition, characterized by a change in the topological order of the system, has profound implications for the superfluid behavior and the dynamics of quantum fluids. This review delves into the intricacies of the BKT transition, focusing on the role of quantum vortices in 2D Bose gases, and provides a comprehensive understanding of this fascinating area of research.

Introduction to the BKT Transition

The Berezinskii-Kosterlitz-Thouless (BKT) transition is a unique phase transition that occurs in two-dimensional systems, distinguishing itself from conventional phase transitions marked by the spontaneous breaking of symmetry and characterized by a local order parameter. Instead, the BKT transition involves a change in the topological order of the system. This transition is named after Vadim Berezinskii, John Kosterlitz, and David Thouless, who were awarded the Nobel Prize in Physics in 2016 for their theoretical work on topological phase transitions and topological phases of matter.

At high temperatures, topological defects such as vortices exist as free entities in the system. As the temperature decreases, there's a critical point known as the BKT transition temperature (T_BKT). Below T_BKT, vortices and antivortices bind together to form vortex-antivortex pairs. This binding leads to a dramatic change in the system's properties, resulting in a phase with quasi-long-range order. Unlike true long-range order, quasi-long-range order is characterized by a power-law decay of correlations rather than an exponential decay.

Two-Dimensional Bose Gases

A Bose gas is a quantum-mechanical phase of matter composed of bosons. In three dimensions, a Bose gas can undergo Bose-Einstein condensation (BEC) at sufficiently low temperatures, where a macroscopic fraction of the bosons occupies the lowest quantum state, resulting in superfluidity. However, in two dimensions, the situation is different due to enhanced thermal fluctuations, which prevent true long-range order. Instead, a 2D Bose gas can undergo a BKT transition to a superfluid state.

The Hamiltonian for a 2D Bose gas can be written as:

H = ∫ d²r [ (ħ²/2m) |∇ψ(r)|² + Vext(r) |ψ(r)|² + (g/2) |ψ(r)|⁴ ]

where:

• ψ(r) is the Bose field operator.
• m is the mass of the bosons.
• Vext(r) is the external potential.
• g is the interaction strength between the bosons.

This Hamiltonian describes the kinetic energy, the external potential energy, and the interaction energy of the bosons. The behavior of the system is governed by the interplay between these terms, especially at low temperatures.

Quantum Vortices: Topological Defects

Quantum vortices are topological defects in a superfluid. They are characterized by a circulating flow of the superfluid around a core where the superfluid density vanishes. The circulation of the flow is quantized, meaning it is an integer multiple of h/m, where h is Planck's constant and m is the mass of the bosons. In a 2D Bose gas, vortices play a crucial role in the BKT transition.

The phase of the Bose-Einstein condensate (BEC) order parameter, ψ(r), winds by an integer multiple of 2π around the core of a vortex. This winding ensures that the superfluid velocity field, v(r) = (ħ/m)∇θ(r), where θ(r) is the phase of ψ(r), is well-defined everywhere except at the vortex core. The energy associated with a single vortex in a 2D Bose gas is proportional to ln(L/a), where L is the system size and a is the vortex core size. This logarithmic divergence means that isolated vortices have a high energy cost and are therefore suppressed at low temperatures.

The BKT Transition in 2D Bose Gases: A Detailed Examination

In a 2D Bose gas, the BKT transition occurs when the binding of vortices and antivortices leads to the onset of superfluidity. At high temperatures, vortices and antivortices exist as free excitations, disrupting the superfluid order. However, as the temperature decreases, it becomes energetically favorable for vortices and antivortices to pair up. These vortex-antivortex pairs have a much lower energy than free vortices, and their presence does not destroy the superfluid order.

The BKT transition temperature (T_BKT) is given by:

T_BKT = (πħ²/2mk_B) ρs(T_BKT)

where:

• k_B is Boltzmann's constant.
• ρs(T) is the superfluid density at temperature T.

Below T_BKT, the superfluid density jumps discontinuously to a finite value, indicating the onset of superfluidity. The jump in superfluid density is a universal feature of the BKT transition.

Superfluid Density and Vortex Unbinding

The superfluid density, ρs, plays a critical role in the BKT transition. Above T_BKT, the superfluid density is zero because the free vortices disrupt the superfluid order. As the temperature decreases and approaches T_BKT, the vortices begin to pair up, and the superfluid density increases. At T_BKT, the superfluid density jumps discontinuously to a finite value, signaling the onset of superfluidity.

The unbinding of vortex-antivortex pairs at T_BKT is what drives the transition. Above T_BKT, the thermal energy is sufficient to overcome the binding energy of the pairs, and the vortices become free to move around, disrupting the superfluid order. Below T_BKT, the thermal energy is not sufficient to break the pairs, and the system is in a superfluid state.

Correlation Functions

The BKT transition is characterized by the behavior of correlation functions. Above T_BKT, the correlation functions decay exponentially, indicating short-range order. Below T_BKT, the correlation functions decay algebraically, indicating quasi-long-range order.

The algebraic decay of the correlation functions is a hallmark of the BKT phase. It means that the correlations between particles persist over long distances, albeit with a power-law decay. This is in contrast to the exponential decay of correlations in a normal fluid, where correlations are limited to short distances.

Experimental Observations of the BKT Transition in 2D Bose Gases

The BKT transition in 2D Bose gases has been observed in a variety of experimental systems, including:

• Helium Films: Thin films of helium adsorbed on a substrate can behave as a 2D Bose gas. Experiments on helium films have provided early evidence for the BKT transition.
• Superconducting Films: Thin superconducting films can also undergo a BKT transition. In this case, the vortices are magnetic vortices, and the transition is driven by the unbinding of vortex-antivortex pairs.
• Trapped Atomic Gases: Ultracold atomic gases confined to two dimensions using optical lattices or magnetic traps provide a clean and controllable system for studying the BKT transition.

Experiments with Trapped Atomic Gases

Experiments with trapped atomic gases have provided the most detailed observations of the BKT transition in 2D Bose gases. These experiments typically involve cooling a gas of atoms, such as rubidium or sodium, to ultracold temperatures (nanokelvin range) and then confining the atoms to a two-dimensional plane using optical or magnetic traps.

These experiments have confirmed many of the theoretical predictions for the BKT transition, including:

• The jump in superfluid density at T_BKT: Experiments have measured the superfluid density as a function of temperature and have observed a clear jump at the BKT transition temperature.
• The algebraic decay of correlation functions below T_BKT: Experiments have measured the correlation functions using techniques such as matter-wave interference and have confirmed that they decay algebraically below T_BKT.
• The presence of vortex-antivortex pairs below T_BKT: Experiments have directly imaged vortices and antivortices in the 2D Bose gas and have observed that they form pairs below T_BKT.

Challenges in Experimental Observations

Despite the successes of these experiments, there are still challenges in observing the BKT transition in 2D Bose gases. One challenge is the finite size of the experimental systems. The BKT transition is a thermodynamic phenomenon that occurs in the limit of an infinite system size. In finite-size systems, the transition is broadened, and it can be difficult to distinguish between the BKT phase and a normal fluid.

Another challenge is the presence of disorder in the experimental systems. Disorder can arise from imperfections in the traps or from interactions with the environment. Disorder can also broaden the BKT transition and make it more difficult to observe.

Theoretical Models and Approaches

The BKT transition in 2D Bose gases has been studied using a variety of theoretical models and approaches, including:

• Classical Statistical Mechanics: The BKT transition can be understood using classical statistical mechanics by treating the vortices as classical particles interacting via a logarithmic potential.
• Quantum Field Theory: The BKT transition can also be studied using quantum field theory techniques such as the renormalization group.
• Numerical Simulations: Numerical simulations such as Monte Carlo simulations and path integral Monte Carlo simulations can be used to study the BKT transition in 2D Bose gases.

Renormalization Group Approach

The renormalization group (RG) approach is a powerful tool for studying phase transitions. In the context of the BKT transition, the RG approach involves integrating out the short-wavelength degrees of freedom in the system and deriving effective equations for the long-wavelength degrees of freedom.

The RG equations for the BKT transition can be written as:

d y / d l = y² - z² d z / d l = yz

where:

• y is the fugacity of the vortices.
• z is the superfluid stiffness.
• l is the length scale.

These equations describe how the fugacity and superfluid stiffness change as the length scale is increased. The RG flow has a fixed point at y = 0 and z = zc, where zc is the critical superfluid stiffness. This fixed point corresponds to the BKT transition.

Monte Carlo Simulations

Monte Carlo simulations are a powerful tool for studying the BKT transition in 2D Bose gases. These simulations involve discretizing the system onto a lattice and then using a random sampling technique to calculate the properties of the system.

Monte Carlo simulations can be used to calculate the superfluid density, the correlation functions, and the vortex density as a function of temperature. These simulations have provided valuable insights into the BKT transition and have helped to confirm the theoretical predictions.

Beyond the Basics: Advanced Topics

The study of the BKT transition in 2D Bose gases extends beyond the basic understanding. Here are some advanced topics:

Effects of Disorder

Disorder can significantly affect the BKT transition in 2D Bose gases. Disorder can arise from imperfections in the traps or from interactions with the environment. Disorder can also broaden the BKT transition and make it more difficult to observe.

In the presence of disorder, the BKT transition can be replaced by a crossover. The superfluid density no longer jumps discontinuously at T_BKT, but instead increases gradually as the temperature is lowered. The correlation functions also decay more rapidly than in the clean case.

Driven-Dissipative Systems

Driven-dissipative systems are systems that are continuously driven by an external force and dissipate energy to the environment. The BKT transition can also occur in driven-dissipative systems.

In driven-dissipative systems, the BKT transition can be affected by the driving and dissipation. The transition temperature can be shifted, and the properties of the superfluid phase can be modified.

Topological Superfluids

Topological superfluids are superfluids that have non-trivial topological properties. These properties can lead to the existence of exotic quasiparticles such as Majorana fermions.

The BKT transition can also occur in topological superfluids. In this case, the vortices can carry non-Abelian statistics, which means that the order in which they are exchanged affects the state of the system.

Future Directions and Unresolved Questions

The study of the BKT transition in 2D Bose gases is an active area of research. There are many open questions and future directions for research, including:

• Understanding the effects of disorder on the BKT transition: More research is needed to understand how disorder affects the BKT transition and how to control the effects of disorder in experimental systems.
• Studying the BKT transition in driven-dissipative systems: More research is needed to understand how driving and dissipation affect the BKT transition and how to create and control driven-dissipative superfluids.
• Exploring the BKT transition in topological superfluids: More research is needed to understand the properties of topological superfluids and how to create and manipulate topological vortices.
• Investigating the dynamics of vortex unbinding: The dynamics of vortex unbinding at the BKT transition is still not fully understood. More research is needed to develop theoretical models and experimental techniques to study this process.

Conclusion

The Berezinskii-Kosterlitz-Thouless (BKT) transition in two-dimensional (2D) Bose gases is a fascinating and complex phenomenon with profound implications for the behavior of quantum fluids. This transition, driven by the binding and unbinding of quantum vortices, leads to the emergence of superfluidity and quasi-long-range order. Through experimental observations, theoretical models, and numerical simulations, significant progress has been made in understanding the BKT transition.

The study of the BKT transition in 2D Bose gases has opened up new avenues for research in condensed matter physics and has led to the development of new technologies, such as quantum sensors and quantum computers. As research continues, we can expect to gain even deeper insights into the BKT transition and its applications in quantum technology. Further exploration into the effects of disorder, driven-dissipative systems, and topological superfluids promises to unveil even more intricate and fascinating aspects of this pivotal phase transition.