A long-standing question in the field of superconductivity is whether pairing of electrons can arise in some cases as a result of magnetic interactions instead of electron-phonon-induced interactions as in the conventional Bardeen-Cooper-Schrieffer theory 1 . A major challenge to the idea of magnetically mediated superconductivity has been the dramatically different behaviour of the cerium and ytterbium heavy-fermion compounds. The cerium-based systems are often found to be superconducting 1-6 , in keeping with a magnetic pairing scenario, but corresponding ytterbium systems, or hole analogues of the cerium systems, are not. Despite searches over two decades there has been no evidence of heavy-fermion superconductivity in an ytterbium system, casting doubt on our understanding of the electron-hole parallelism between the cerium and the ytterbium compounds. Here we present the first empirical evidence that superconductivity is indeed possible in an ytterbium-based heavy-fermion system. In particular, we observe a superconducting transition at T c = 80 mK in high-purity single crystals of YbAlB 4 in the new structural β phase 7 . We also observe a novel type of non-Fermi-liquid state above T c that arises without chemical doping, in zero applied magnetic field and at ambient pressure, establishing β-YbAlB 4 as a unique system showing quantum criticality without external tuning.First we present the bulk magnetic and electronic properties of β-YbAlB 4 , a new morphology of the previously known α-YbAlB 4 (refs 7,8). Shown in Fig. 1a is the orthorhombic crystal structure of β-YbAlB 4 and the temperature dependence of the d.c. magnetic susceptibility χ = M/H. Here, M and H represent the magnetization and external field, respectively. The magnetic susceptibility shows the strong uniaxial anisotropy of an Ising system with moments aligned along the c axis. Above 100 K the c-axis susceptibility has a Curie-Weiss form χ c (T ) = C/(T − θ W ), with θ W ∼ −210 K and a Curie constant C corresponding to an effective Ising moment µ eff = g J J Z ∼ 3.1 µ B , where g J is the Landé g factor and J Z is the c-axis component of the total angular momentum. The in-plane susceptibility, on the other hand, is almost temperature independent, showing a weak peak around 200 K.Shown in Fig. 1b is the temperature dependence of the in-plane resistivity, ρ ab , along with the estimated 4f -electron contribution ρ m (defined in the figure caption), which shows a coherence peak at about 250 K. The low residual resistivity ρ ab (0) ∼ 0.4 µ cm and correspondingly high residual resistivity ratio, ρ ab (300 K)/ρ ab (0) ∼ 300, suggest that the electronic mean free path is of the order of 0.1 µm.In contrast to most other heavy-fermion compounds, the resistivity does not show a Fermi liquid (FL) regime characterized by a T 2 temperature variation (Fig. 1b). As shown in Fig. 1b insets, ρ ab is linear between 4 and 1 K and varies as T 1.5 below T 0 ∼ 1 K down to 80 mK. Below 80 mK our highest-purity samples are superconducting (Fig. 1b, insets). We shall...

A quantum phase transition is observed in a stoichiometric compound at ambient pressure and in zero magnetic field.

Frustrated magnetic materials, in which local conditions for energy minimization are incompatible because of the lattice structure, can remain disordered to the lowest temperatures. Such is the case for Ba(3)CuSb(2)O(9), which is magnetically anisotropic at the atomic scale but curiously isotropic on mesoscopic length and time scales. We find that the frustration of Wannier's Ising model on the triangular lattice is imprinted in a nanostructured honeycomb lattice of Cu(2+) ions that resists a coherent static Jahn-Teller distortion. The resulting two-dimensional random-bond spin-1/2 system on the honeycomb lattice has a broad spectrum of spin-dimer-like excitations and low-energy spin degrees of freedom that retain overall hexagonal symmetry.

An antiferroquadrupolar ordering at T(Q)=0.11  K has been found in a Pr-based superconductor PrIr(2)Zn(20). The measurements of specific heat and magnetization revealed the non-Kramers Γ(3) doublet ground state with the quadrupolar degrees of freedom. The specific heat exhibits a sharp peak at T(Q)=0.11  K. The increment of T(Q) in magnetic fields and the anisotropic B-T phase diagram are consistent with the antiferroquadrupolar ordered state below T(Q). The entropy release at T(Q) is only 20% of Rln2, suggesting that the quadrupolar fluctuations play a role in the formation of the superconducting pairs below T(c)=0.05  K.

A series of Ni(4) cubane complexes with the composition [Ni(hmp)(ROH)Cl](4) complexes 1-4 where R= -CH(3) (complex 1), -CH(2)CH(3) (complex 2), -CH(2)CH(2)(C(4)H(9)) (complex 3), -CH(2)CH(2)CH(2)(C(6)H(11)) (complex 4), hmp(-) is the anion of 2-hydroxymethylpyridine, t-Buhmp(-) is the anion of 4-tert-butyl-2-hydroxymethylpyridine, and dmb is 3,3-dimethyl-1-butanol] and [Ni(hmp)(dmb)Br](4) (complex 5) and [Ni(t-Buhmp)(dmb)Cl](4) (complex 6) were prepared. All six complexes were characterized by dc magnetic susceptibility data to be ferromagnetically coupled to give an S = 4 ground state with significant magnetoanisotropy (D approximately equal to -0.6 cm(-1)). Magnetization hysteresis measurements carried out on single crystals of complexes 1-6 establish the single-molecule magnet (SMM) behavior of these complexes. The exchange bias observed in the magnetization hysteresis loops of complexes 1 and 2 is dramatically decreased to zero in complex 3, where the bulky dmb ligand is employed. Fast tunneling of magnetization is observed for the high-symmetry (S(4) site symmetry) Ni(4) complexes in the crystal of complex 3, and the tunneling rate can even be enhanced by destroying the S(4) site symmetry, as is the case for complex 4, where there are two crystallographically different Ni(4) molecules, one with C(2) and the other with C(1) site symmetry. Magnetic ordering temperatures due to intermolecular dipolar and magnetic exchange interactions were determined by means of very low-temperature ac susceptibility measurements; complex 1 orders at 1100 mK, complex 3 at 290 mK, complex 4 at approximately 80 mK, and complex 6 at <50 mK. This confirms that bulkier ligands correspond to more isolated molecules, and therefore, magnetic ordering occurs at lower temperatures for those complexes with the bulkiest ligands.

Direct demagnetization has been made for two-dimensional solid 3He in both the paramagnetic and the antiferromagnetic phases. The lowest temperature is about 10 microK, judging from the observed magnetization for the paramagnetic solid 3He. The magnetization of the antiferromagnetic solid 3He shows a gradual increase to about 10 microK for the 4/7 phase adsorbed on both one layer of 4He and two layers of HD preplated graphite. This strongly suggests that the triangular antiferromagnet with the higher order multiple exchange has a quantum spin liquid ground state with nearly zero or extremely small spin gap less than 10 microK.

beta-YbAlB4 is the first Yb-based heavy fermion superconductor with Tc = 80 mK. Our study using high-purity single crystals reveals that strongly type-II heavy fermion superconductivity emerges from the non-Fermi-liquid state with enhanced ferromagnetic fluctuations. High sensitivity of Tc to sample purity indicates strong pair-breaking effects due to impurities, probably of nonmagnetic type, suggesting an unconventional character of the superconductivity.

α-YbAlB4 is the locally isostructural polymorph of β-YbAlB4 , the first example of an Yb-based heavy fermion superconductor which exhibits pronounced non-Fermi-liquid behavior above Tc. Interestingly, both α-YbAlB4 and β-YbAlB4 have strongly intermediate valence. Our single crystal study of the specific heat, magnetization and resistivity has confirmed the Fermi liquid ground state of α-YbAlB4 in contrast with the quantum criticality observed in β-YbAlB4 . Both systems exhibit Kondo lattice behavior with the characteristic temperature scale T * ∼ 8 K in addition to a valence fluctuation scale ∼ 200 K. Below T * , α-YbAlB4 forms a heavy Fermi liquid state with an electronic specific heat coefficient γ ∼ 130 mJ/mol K 2 and a large Wilson ratio more than 7, which indicates ferromagnetic correlation between Yb moments. A large anisotropy in the resistivity suggests that the hybridization between 4f and conduction electrons is much stronger in the ab-plane than along the c-axis. The strongly anisotropic hybridization as well as the large Wilson ratio is the key to understand the unusual Kondo lattice behavior and heavy fermion formation in mixed valent compounds.