led to a great interest in QCPs are the very unusual properties that are observed near a QCP, such as unconventional superconductivity (SC), non-Fermi-liquid or anomalous critical behaviour 3-6 . Quantum fluctuations are therefore suspected to cause these anomalous properties, but most aspects are far from being understood and are still the subject of active discussions. An issue that has attracted great interest is the appearance of unconventional SC observed at magnetic QCPs, that is where a magnetic ordered state is suppressed. Such unconventional SC states have been reported for very different kinds of compounds, for example heavy-fermion systems 3,7 , cuprates 8,9 and iron pnictides 10,11 . There is some evidence that backs up the hypothesis that the binding of electrons into the superconducting Cooper pairs is not mediated by phononic excitations as in classical superconductors, but by magnetic excitations connected to the disappearing magnetic order 12,13 . The fact that SC is observed only in the vicinity of the QCP and presents a strong dependence on the tuning parameter supports this hypothesis. It often results in a dome-like shape of the phase diagram of tuning parameter versus temperature.However, QCPs are not restricted to magnetic systems; they can be associated with any continuous phase transition. While searching for appropriate non-magnetic systems that show a QCP with associated SC, we looked at compounds that show a charge density wave (CDW) type of structural transition. CDW systems present an instability of the electronic states close to the Fermi level F , which results in a modulation of the electronic charges below a transition temperature T CDW (ref. 14). The modulation periodicity is usually associated with a nesting vector of the Fermi surface. Therefore, a gap opens in the electronic density of states (DOS) at F , N ( F ), below T CDW . In one-dimensional (1D) systems, for which such a transition was initially proposed, this effect changes the character of the system from metallic for T > T CDW to insulating for T < T CDW . In 2D or 3D systems, the gap opens only on a part of the Fermi surface and, therefore, the conductivity remains metallic below T CDW . The opening of the gap does, however, lead to a very characteristic upturn of the resistivity ρ(T ) at T CDW .Some CDW transitions can be tuned to a T = 0 critical point as well [15][16][17][18][19][20][21] . The appearance or a strengthening of a superconducting state is frequently found near the vanishing point of the CDW state, similar to magnetic QCPs. Yet, in this case, it can easily be explained within the standard BCS-based theories: the gap associated with the CDW closes at the critical point, resulting in an increase of N ( F ), which, in turn, leads to an increase of the SC transition temperature T c . As a consequence, the dependence of T c on the tuning parameter is usually rather weak in the non-CDW regime beyond the critical point 16,17,[20][21][22][23] . There is currently no clear evidence that quantum critical fluc...

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Molecular quantum magnetism involving an isolated spin state is of particular interest due to the characteristic quantum phenomena underlying spin qubits or molecular spintronics for quantum information devices, as demonstrated in magnetic metal–organic molecular systems, the so-called molecular magnets. Here we report the molecular quantum magnetism realized in an inorganic solid Ba3Yb2Zn5O11 with spin–orbit coupled pseudospin-½ Yb3+ ions. The magnetization represents the magnetic quantum values of an isolated Yb4 tetrahedron with a total (pseudo)spin 0, 1 and 2. Inelastic neutron scattering results reveal that a large Dzyaloshinsky–Moriya interaction originating from strong spin–orbit coupling of Yb 4f is a key ingredient to explain magnetic excitations of the molecular magnet states. The Dzyaloshinsky–Moriya interaction allows a non-adiabatic quantum transition between avoided crossing energy levels, and also results in unexpected magnetic behaviours in conventional molecular magnets.

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Adiabatic demagnetization is currently gaining strong interest in searching for alternatives to 3He-based refrigeration techniques for achieving temperatures below 2 K. The main reasons for that are the recent shortage and high price of the rare helium isotope 3He. Here we report the discovery of a large magnetocaloric effect in the intermetallic compound YbPt2Sn, which allows adiabatic demagnetization cooling from 2 K down to 0.2 K. We demonstrate this with a home-made refrigerator. Other materials, for example, paramagnetic salts, are commonly used for the same purpose but none of them is metallic, a severe limitation for low-temperature applications. YbPt2Sn is a good metal with an extremely rare weak magnetic coupling between the Yb atoms, which prevents them from ordering above 0.25 K, leaving enough entropy free for use in adiabatic demagnetization cooling. The large volumetric entropy capacity of YbPt2Sn guarantees also a good cooling power.

The f-electron compound CeAuSb2, which crystallizes in the ZrCuSi2-type tetragonal structure, orders antiferromagnetically between 5 and 6.8 K, where the antiferromagnetic transition temperature TN depends on the occupancy of the Au site. Here we report the electrical resistivity and heat capacity of a high-quality crystal CeAuSb2 with TN of 6.8 K, the highest for this compound. The magnetic transition temperature is initially suppressed with pressure, but is intercepted by a new magnetic state above 2.1 GPa. The new phase shows a dome shape with pressure and coexists with another phase at pressures higher than 4.7 GPa. The electrical resistivity shows a T 2 Fermi liquids behavior in the complex magnetic state, and the residual resistivity and the T 2 resistivity coefficient increases with pressure, suggesting the possibility of a magnetic quantum critical point at a higher pressure.

We report on measurements of the in-plane magnetic penetration depth λ ab in the infinite-layer electron-doped high-temperature cuprate superconductor Sr0.9La0.1CuO2 by means of muon-spin rotation. The observed temperature and magnetic field dependences of λ ab are consistent with the presence of a substantial s−wave component in the superconducting order parameter in good agreement with the results of tunneling, specific heat, and small-angle neutron scattering experiments.

We present low-temperature specific heat of the electron-doped Ba(Fe0.9Co0.1)2As2, which does not show any indication of an upturn down to 400 mK, the lowest measuring temperature. The lack of a Schottky-like feature at low temperatures or in magnetic fields up to 9 Tesla enables us to identify enhanced low-temperature quasiparticle excitations and to study anisotropy in the linear term of the specific heat. Our results can not be explained by a single or multiple isotropic superconducting gap, but are consistent with multi-gap superconductivity with nodes on at least one Fermi surface sheet.