Volume 12 Issue 1, January 2016

The redefinition of several physical base units planned for 2018 requires precise knowledge of the values of certain fundamental physical constants. Scientists are working hard to meet the deadlines for realizing the ultimate International System of Units.

The past 25 years have seen tremendous progress in thermometry across the moderate temperature range of 1 K to 1,235 K. Various primary thermometers, based on a wide range of different physics, have uncovered errors in the International Temperature Scale of 1990, and set the stage for the planned redefinition of the kelvin.

In the quest for ever-lower temperatures, making new discoveries and overcoming technical challenges go hand in hand — and push the limits of thermometry standardization.

Sustaining and measuring high temperatures in fusion plasmas is a challenging task that requires different heating systems and diagnostic tools. Information on the spatial distribution of temperature is one of the key elements for improving and controlling plasma performance.

Coherent population trapping in 'dark states', a well-known and much-used phenomenon in atomic physics, can also be observed in a superconducting qubit and a single nuclear spin in diamond.

High-temperature superconductivity in ultrathin films of iron selenide deposited on strontium titanate has been attributed to various exotic mechanisms. New experiments indicate that it may be conventional, with broader implications.

The rotational motion of liquids can induce a flow of electron spins, and could enable ultra-small spin-hydrodynamic generators that operate with liquid metals.

Magnetic skyrmion lattices can be stabilized outside the narrow region where they are thermodynamically favoured by exploiting their topological protection.

Coupling two mechanical objects becomes tricky when they are quantum and can interact only through photons. An experiment now demonstrates such an optomechanical system with two separate atomic ensembles in the same optical cavity.

The Mott insulator Sr₂IrO₄ is intensively studied because of its electronic similarity to the high-temperature cuprate superconductor La₂CuO₄. Now, spectroscopic experiments reveal evidence for a hidden order with odd-parity symmetry in this system.

Sr₂IrO₄ bears a striking electronic resemblance to the cuprate superconductors, except the iridate is an insulator. Introducing electrons into Sr₂IrO₄ leads to a d-wave gap, suggesting superconductivity or something equally exotic.

Electric-field-induced superconductivity in samples of ultrathin FeSe grown on SrTiO₃ and MgO substrates shows that the superconductivity is not an interfacial effect but is rather related to a charge imbalance of electrons and holes.

Specific heat measurements up to 35 T provide thermodynamic evidence for a magnetic-field-driven phase transition within the superconducting dome of a copper-oxide-based superconductor.

Magnetohydrodynamic generators use magnetic fields to convert the kinetic energy of conducting fluids into electricity. Fluid motion is now shown to generate spin currents, which can induce electric voltages without applying magnetic fields.

A switchable induced magnetic moment in a non-magnetic metal that is separated from a ferromagnet by a thick superconducting layer contradicts existing models.

Topological protection can stabilize states of matter, but for how long? By creating metastable magnetic skyrmion lattices, the interplay between topological and thermodynamic stability has now been probed experimentally.

Despite the simplicity of the Carnot cycle, realizing it at the microscale is complicated by the difficulty in implementing adiabatic processes. A clever solution subjects a charged particle to a noisy electrostatic force that mimics a thermal bath.

Tunable interactions in quantum many-body systems have practical applications in quantum technologies. The effective spin-dependent long-range interaction known as Rydberg dressing is now exploited to entangle a pair of ultracold neutral atoms.

Using an artificial three-level lambda system realized in a superconducting transmon qubit in a microwave cavity one can observe coherent population trapping, electromagnetically induced transparency and superluminal pulse propagation.

Heat transport is well described by the Green–Kubo formalism. Now, the formalism is combined with density-functional theory, enabling simulations of thermal conduction in systems that cannot be adequately modelled by classical interatomic potentials.

Exchange interactions are typically short-ranged as they depend on wavefunction overlap, but a long-ranged exchange is now seen in a hybrid ferromagnet–semiconductor system, which may be mediated by elliptically polarized phonons.

What happens to correlated electronic phases—superconductivity and charge density wave ordering—as a material is thinned? Experiments show that both can remain intact in just a single layer of niobium diselenide.

A study of a composite soft-matter nanomechanical system consisting of a rotating ring of optically trapped colloidal particles confining a set of untrapped colloids demonstrates the possibility of gearwheel-like torque transmission on the nanoscale.

Michael de Podesta discusses the current definition of the kelvin — and why it is worth changing one last time.

The concept of temperature pervades the laws of physics. This Focus highlights various aspects of thermometry, from the (re)definition of the kelvin to ways of measuring temperature — from pK to MK — documenting some compelling physics along the way.