局域化与非均匀超流性：氟化石墨烯、六方氮化硼和石墨烯上的He-4亚单层
Localization versus inhomogeneous superfluidity: Submonolayer He-4 on fluorographene, hexagonal boron nitride, and graphene
摘要：
我们研究了一个亚单层的He-4吸附在氟石墨烯（GF）和六方氮化硼（hBN）在低覆盖率。吸附势已计算从头算与合适的密度泛函理论，包括色散力。用路径积分Monte Carlo方法计算了有限温度下He-4原子的吸附性质，用变分路径积分方法计算了T=0 K时He-4原子的吸附性质。从这两种方法中我们都发现He-4在GF上的最低能态是超流体。由于吸附势的很大的ε，这种超流具有很强的空间各向异性，最大和最小面密度之比约为6，最低T时的超流分数约为55%，转变到正常态的温度在0.5-1 K范围内。因此，GF提供了一个平台，研究一个强相互作用的高度各向异性玻色超流体的性质。在较大的覆盖度，He-4有一个过渡到一个有序的相称的状态与占领的1/6的吸附位点。该相在0.5 ~ 1 K之间是稳定的。该系统具有类似于石墨上的He-4的三角序。He-4在hBN上的最低能态是一个有序的公度态，占据了1/3的吸附位，具有三角对称性。无序状态在较低覆盖率下作为亚稳态存在。在存在电场的情况下，吸附电势的峰值略微增加，但最高达1 V/Ang的量级。影响很小，并且不改变He-4在GF和hBN上的相的稳定性。我们已经证实，在石墨烯的情况下，这种电场也不会改变相称的sqrt{3}*sqrt{3}R30相的稳定性。
英文摘要：
We study a sub monolayer He-4 adsorbed on fluorographene (GF) and on hexagonal boron nitride (hBN) at low coverage. The adsorption potentials have been computed ab-initio with a suitable density functional theory including dispersion forces. The properties of the adsorbed He-4 atoms have been computed at finite temperature with path integral Monte Carlo and at T=0 K with variational path integral. From both methods we find that the lowest energy state of He-4 on GF is a superfluid. Due to the very large corrugation of the adsorption potential this superfluid has a very strong spatial anisotropy, the ratio between the largest and smallest areal density being about 6, the superfluid fraction at the lowest T is about 55%, and the temperature of the transition to the normal state is in the range 0.5-1 K. Thus, GF offers a platform for studying the properties of a strongly interacting highly anisotropic bosonic superfluid. At a larger coverage He-4 has a transition to an ordered commensurate state with occupation of 1/6 of the adsorption sites. This phase is stable up to a transition temperature located between 0.5 and 1~K. The system has a triangular order similar to that of He-4 on graphite. The lowest energy state of He-4 on hBN is an ordered commensurate state with occupation of 1/3 of the adsorption sites and triangular symmetry. A disordered state is present at lower coverage as a metastable state. In the presence of an electric field the corrugation of the adsorption potential is slightly increased but up to a magnitude of 1 V/Ang. the effect is small and does not change the stability of the phases of He-4 on GF and hBN. We have verified that also in the case of graphene such electric field does not modify the stability of the commensurate sqrt{3}*sqrt{3}R30 phase.
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