Download Physics Close to Absolute Zero Temperature

THE UNIVERSITY OF HONG KONG
FACULTY OF SCIENCE
Physics Close to
Absolute Zero Temperature
Dr Shizhong Zhang
Department of Physics
The attainment of temperature
about a billionth of a degree above
absolute zero (‐273.15 Celsius) is the
culmination of century‐long pursuit
in low temperature physics, and has
spurred many activities in research
and tremendous progresses in
technology. In these extremely low
temperatures, matter behaves in a
very different way compared to
what we experience in daily life and
requires a complete readjustment of
the theoretical framework (quantum
mechanics) and new concepts for its
description. Among the most
fundamental modifications is the
fact
that
particles
possess
simultaneously the corpuscular and
wave properties, and as a result,
show the typical behavior of
diffraction and interference we
usually associated with waves.
How then a collection of these
particles behaves in extremely low
temperatures while subjecting to
mutual interactions? What are the
new quantum states that can arise?
One can think of superfluidity and
superconductivity as examples, but
clearly there could be more in store
for us as we keep pushing towards
absolute zero temperature.
Table 1: Interaction parameters describing the low energy scattering of atoms.
香 港 大 學 理 學 院
Dr Zhang’s research interests:
 Physics of Ultracold Cold Atomic Gases
 Correlated electronic systems
Dr Shizhong Zhang has been working in this general area for the past
several years, making use of a new type of system, consisting of ultracold
atomic gases confined in a magnetic or an optical trap. Unlike electrons in
solid state materials, these much larger neutral entities move with a
velocity that is much smaller than electrons, and offer new ways of
manipulation and diagnosis. In common with electrons, these atoms can
be made to interact very strongly, using a technique called Feshbach
resonance, and as a result, can be used to simulate real materials.
There are by now a dozen of different atoms that have been cooled to
these fantastically low temperatures. Despite the different atomic
weights, the inter‐atomic potentials, these gases appears to share some
universal features especially when interactions are very strong.
Table 2: Spin diffusion constants with different
modalities (longitudinal or transverse) in various
quantum liquids. The minimum are set by atomic
gases in two or three‐dimensions. ԰/m is the
natural unit for spin diffusion constant. ԰ is the
Planck constant and m is the mass of the atom in
question.
(1) Universal Thermodynamics. It turns out
that there exists a stronger version of the
laws of thermodynamics which relates
directly to the interaction parameters (see
Table 1).
The terms proportional to CV and CR are
conjugates to scattering parameters, and
they determine many other properties of
the system.
(2) Universal Quantum
Transport.
Electric
transport can be viewed
as movement of electrons
from one end of the wire
towards the other, while
suffer many collisions in
between. This gives rise to
resistance.
However,
quantum
mechanically,
electrons also behave as
waves, and hence the
associated phenomena of
diffraction
and
interference. It is these
quantum
mechanical
properties that set the
fundamental limit on how
fast an electron can move.
In the case of spin
diffusion,
the
fundamentally limits have
been
investigated
theoretically and verified
experimentally in ultracold
atomic gases; see Table 2.
To know more about Dr Zhang’s work on the properties of ultracold atomic
gases, please contact him via: [email protected]