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Astronomers are ready to search for the fingerprints of life in faraway planetary atmospheres. But first, they need to know where to look — and that means figuring out which planets are likely to have atmospheres in the first place. The post The Road Map to Alien Life Passes Through the ‘Cosmic Shoreline’ first appeared on Quanta Magazine

One nice bit of condensed matter/nanoscale physics news:  This year's Wolf Prize in Physics has gone to three outstanding scientists, Jim Eisenstein, Moty Heiblum, and Jainendra Jain, each of whom have done very impactful work involving 2D electron gases - systems of electrons confined to move only in two dimensions by the electronic structure and alignment of energy bands at interfaces between semiconductors.  Of particular relevance to these folks are the particularly clean 2D electron gases at the interfaces between GaAs and AlGaAs, or in GaAs quantum wells embedded in AlGaAs. A thread that connects all three of these scientists is the fractional quantum Hall effect in these 2D systems.  Electrons confined to move in 2D, in the presence of a magnetic field perpendicular to the plane of motion, form a remarkable system.  The quantum wavefunction of an electron in this situation changes as the magnetic induction \(B\) is increased.  The energy levels of such an electron are given by \((n+1/2)\hbar \omega_{c}\), where \(\omega_c \equiv eB/m*\) is the cyclotron frequency.  These energy levels are called Landau Levels.  The ratio between the 2D density of electrons and the density of magnetic flux in fundamental units (\(B/(h/e)\)) is called the "filling factor", \(\nu\), and when this is an integer, the Hall conductance is quantized in fundamental units - see here.   Figure 4 from this article by Jain, with \(R_{xx}(B)\) data from here.  Notice how the data around \(B=0\) looks a lot like the data around \(\nu = 1/2\), which looks a lot like the data around \(\nu=1/4\).  A remarkable thing happens when \(\nu = 1/2\) - see the figure above.  There is no quantum Hall effect there; in fact, if you look at the longitudinal resistance \(R_{xx}\) as a function of \(B\) near \(\nu = 1/2\), it looks remarkably like \(R_{xx}(B)\) near \(B = 0\).  At this half-integer filling factor, the 2D electrons plus the magnetic flux "bundle together", leading to a state with new low-energy excitations called composite fermions that act like they are in zero magnetic field.  In many ways the FQHE looks like the integer quantum Hall effect for these composite fermions, though the situation is more complicated than that.  Jainendra Jain did foundational work on the theory of composite fermions, among many other things. Jim Eisenstein has done a lot of great experimental work involving composite fermions and even-denominator FQH states.  My postdoctoral mentor, Bob Willett, and he are first two authors on the paper where an unusual quantum Hall state was discovered at \(\nu = 5/2\), a state still under active investigation for potential topological quantum computing applications.   One particularly surprising result from Eisenstein's group was the discovery that some "high" Landau level even-denominator fillings (\(\nu = 9/2, 11/2\)) showed enormously anisotropic resistances, with big differences between \(R_{xx}\) and \(R_{yy}\), an example of the onset of a "stripe" phase of alternating fillings.   Another very exciting result from Eisenstein's group used 2D electron gases in close proximity parallel layers and in high magnetic fields, as well as 2D electron gases near 2D hole gases.  Both can allow the formation of excitons, bound states of electrons and holes, but with the electrons and holes in neighboring layers so that they could not annihilate each other.  Moreover, a Bose-Einstein condensation of those excitons is possible leading to remarkable superflow of excitons and resonant tunneling between the layers.  This review article is a great discussion of all of this. Moty Heiblum's group at the Weizmann Institute has been one of the world-leading groups investigating "mesoscopic" physics of confined electrons in the past 30+ years.  They have performed some truly elegant experiments using 2D electron gases as their platform.  A favorite of mine (mentioned in my textbook) is this one, in which they make a loop-shaped interferometer for electrons which shows oscillations in the conductance as they thread magnetic flux through the loop; they then use a nearby quantum point contact as a charge sensor near one arm of the interferometer, a which-path detector that tunably suppresses the quantum interference.  His group also did foundational work on the use of shot noise as a tool to examine the nature and transport of charge carriers in condensed matter systems (an idea that I found inspiring).  Their results showing that the quasiparticles in the fractional quantum Hall regime can have fractional charges are remarkable.  More recently, they have shown how subtle these measurements really can be, in 2D electron systems that can support neutral excitations as well as charged ones. All in all, this is a great recognition of outstanding scientists for a large volume of important, influential work. (On a separate note:  I will be attending 3+ days of the APS meeting next week.  I'll try to do my usual brief highlight posts, time permitting.  If people have suggestions of cool content, please let me know.)

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