Magnetic and structural changes in LaCo0.9Mn0.1O3 at high pressure.

The structural and magnetic properties of LaCo0.9Mn0.1O3 have been studied as a function of pressure by neutron powder diffraction and DC magnetometry. The material is confirmed to exhibit rhombohedral R [Formula: see text] c symmetry between ambient pressure and 6 GPa. We have determined the bulk modulus B 0 of the sample using a second-order Birch-Murnaghan equation of state which yielded: B 0 = 140(9) GPa and V [Formula: see text]. We report a non-linear increase of the Curie temperature T C from an ambient pressure value of 224.7 K to ∼236 K at a pressure of 4 GPa. Finally, we confirm the glassy-like nature of the magnetism in LaCo0.9Mn0.1O3, which is maintained throughout the pressure range explored.

Mottness at finite doping and charge-instabilities in cuprates.

The influence of the Mott physics on the doping-temperature phase diagram of copper oxides represents a major issue that is subject of intense theoretical and experimental effort. Here, we investigate the ultrafast electron dynamics in prototypical single-layer Bi-based cuprates at the energy scale of the O-2p→Cu-3d charge-transfer (CT) process. We demonstrate a clear evolution of the CT excitations from incoherent and localized, as in a Mott insulator, to coherent and delocalized, as in a conventional metal. This reorganization of the high-energy degrees of freedom occurs at the critical doping pcr ≈0.16 irrespective of the temperature, and it can be well described by dynamical mean field theory calculations. We argue that the onset of the low-temperature charge instabilities is the low-energy manifestation of the underlying Mottness that characterizes the p < pcr region of the phase diagram. This discovery sets a new framework for theories of charge order and low-temperature phases in underdoped copper oxides.

Dimensionality-Driven Metal-Insulator Transition in Spin-Orbit-Coupled SrIrO_{3}.

Upon reduction of the film thickness we observe a metal-insulator transition in epitaxially stabilized, spin-orbit-coupled SrIrO_{3} ultrathin films. By comparison of the experimental electronic dispersions with density functional theory at various levels of complexity we identify the leading microscopic mechanisms, i.e., a dimensionality-induced readjustment of octahedral rotations, magnetism, and electronic correlations. The astonishing resemblance of the band structure in the two-dimensional limit to that of bulk Sr_{2}IrO_{4} opens new avenues to unconventional superconductivity by "clean" electron doping through electric field gating.

Field-Driven Mott Gap Collapse and Resistive Switch in Correlated Insulators.

Mott insulators are "unsuccessful metals" in which Coulomb repulsion prevents charge conduction despite a metal-like concentration of conduction electrons. The possibility to unlock the frozen carriers with an electric field offers tantalizing prospects of realizing new Mott-based microelectronic devices. Here we unveil how such unlocking happens in a simple model that shows the coexistence of a stable Mott insulator and a metastable metal. Considering a slab subject to a linear potential drop, we find, by means of the dynamical mean-field theory, that the electric breakdown of the Mott insulator occurs via a first-order insulator-to-metal transition characterized by an abrupt gap collapse in sharp contrast to the standard Zener breakdown. The switch on of conduction is due to the field-driven stabilization of the metastable metallic phase. Outside the region of insulator-metal coexistence, the electric breakdown occurs through a more conventional quantum tunneling across the Hubbard bands tilted by the field. Our findings rationalize recent experimental observations and may offer a guideline for future technological research.

Immunoscore and Immunoprofiling in cancer: an update from the melanoma and immunotherapy bridge 2015.

The fifth "Melanoma Bridge Meeting" took place in Naples, December 1-5th, 2015. The main topics discussed at this meeting were: Molecular and Immuno advances, Immunotherapies and Combination Therapies, Tumor Microenvironment and Biomarkers and Immunoscore. The natural history of cancer involves interactions between the tumor and the immune system of the host. The immune infiltration at the tumor site may be indicative of host response. Significant correlations were shown between the levels of immune cell infiltration in tumors and patient's clinical outcome. Moreover, incredible progress comes from the discovery of mutation-encoded tumor neoantigens. In fact, as tumors grow, they acquire mutations that are able to influence the response of patients to immune checkpoint inhibitors. It has been demonstrated that sensitivity to PD-1 and CTLA-4 blockade in patients with advanced NSCLC and melanoma was enhanced in tumors enriched for clonal neoantigens. The road ahead is still very long, but the knowledge of the mechanisms of immune escape, the study of tumor neo-antigens as well as of tumor microenvironment and the development of new immunotherapy strategies, will make cancer a more and more treatable disease.

The strength of electron electron correlation in Cs3C60.

Cs3C60 is an antiferromagnetic insulator that under pressure (P) becomes metallic and superconducting below Tc = 38 K. The superconducting dome present in the T - P phase diagram close to a magnetic state reminds what found in superconducting cuprates and pnictides, strongly suggesting that superconductivity is not of the conventional Bardeen-Cooper-Schrieffer (BCS) type We investigate the insulator to metal transition induced by pressure in Cs3C60 by means of infrared spectroscopy supplemented by Dynamical Mean-Field Theory calculations. The insulating compound is driven towards a metallic-like behaviour, while strong correlations survive in the investigated pressure range. The metallization process is accompanied by an enhancement of the Jahn-Teller effect. This shows that electronic correlations are crucial in determining the insulating behaviour at ambient pressure and the bad metallic nature for increasing pressure. On the other hand, the relevance of the Jahn-Teller coupling in the metallic state confirms that phonon coupling survives in the presence of strong correlations.

First-order character and observable signatures of topological quantum phase transitions.

Topological quantum phase transitions are characterized by changes in global topological invariants. These invariants classify many-body systems beyond the conventional paradigm of local order parameters describing spontaneous symmetry breaking. For noninteracting electrons, it is well understood that such transitions are continuous and always accompanied by a gap closing in the energy spectrum, given that the symmetries protecting the topological phase are maintained. Here, we demonstrate that a sufficiently strong electron-electron interaction can fundamentally change the situation: we discover a topological quantum phase transition of first-order character in the genuine thermodynamic sense that occurs without a gap closing. Our theoretical study reveals the existence of a quantum critical endpoint associated with an orbital instability on the transition line between a 2D topological insulator and a trivial band insulator. Remarkably, this phenomenon entails unambiguous signatures related to the orbital occupations that can be detected experimentally.

Mesenchymal stem cells are enriched in head neck squamous cell carcinoma, correlates with tumour size and inhibit T-cell proliferation.

BACKGROUND: Cancer is a multifactorial disease not only restricted to transformed epithelium, but also involving cells of the immune system and cells of mesenchymal origin, particularly mesenchymal stem cells (MSCs). Mesenchymal stem cells contribute to blood- and lymph- neoangiogenesis, generate myofibroblasts, with pro-invasive activity and may suppress anti-tumour immunity. METHODS: In this paper, we evaluated the presence and features of MSCs isolated from human head neck squamous cell carcinoma (HNSCC). RESULTS: Fresh specimens of HNSCC showed higher proportions of CD90+ cells compared with normal tissue; these cells co-expressed CD29, CD105, and CD73, but not CD31, CD45, CD133, and human epithelial antigen similarly to bone marrow-derived MSCs (BM-MSCs). Adherent stromal cells isolated from tumour shared also differentiation potential with BM-MSCs, thus we named them as tumour-MSCs. Interestingly, tumour-MSCs showed a clear immunosuppressive activity on in vitro stimulated T lymphocytes, mainly mediated by indoelamine 2,3 dioxygenase activity, like BM-MSCs. To evaluate their possible role in tumour growth in vivo, we correlated tumour-MSC proportions with neoplasm size. Tumour-MSCs frequency directly correlated with tumour volume and inversely with the frequency of tumour-infiltrating leukocytes. CONCLUSIONS: These data support the concept that tumour-MSCs may favour tumour growth not only through their effect on stromal development, but also by inhibiting the anti-tumour immune response.

Photo-enhanced antinodal conductivity in the pseudogap state of high-Tc cuprates.

A major challenge in understanding the cuprate superconductors is to clarify the nature of the fundamental electronic correlations that lead to the pseudogap phenomenon. Here we use ultrashort light pulses to prepare a non-thermal distribution of excitations and capture novel properties that are hidden at equilibrium. Using a broadband (0.5-2 eV) probe, we are able to track the dynamics of the dielectric function and unveil an anomalous decrease in the scattering rate of the charge carriers in a pseudogap-like region of the temperature (T) and hole-doping (p) phase diagram. In this region, delimited by a well-defined T*neq(p) line, the photoexcitation process triggers the evolution of antinodal excitations from gapped (localized) to delocalized quasiparticles characterized by a longer lifetime. The novel concept of photo-enhanced antinodal conductivity is naturally explained within the single-band Hubbard model, in which the short-range Coulomb repulsion leads to a k-space differentiation between nodal quasiparticles and antinodal excitations.

Electronic correlations stabilize the antiferromagnetic Mott state in Cs3C60.

Cs(3)C(60) in the A15 structure is an antiferromagnet at ambient pressure in contrast with other superconducting trivalent fullerides. Superconductivity is recovered under pressure and reaches the highest critical temperature of the family. Comparing density-functional calculations with generalized gradient approximation to the hybrid functional of Heyd, Scuseria, and Ernzerhof, which includes a suitable component of exchange, we establish that the antiferromagnetic state of Cs(3)C(60) is not due to a Slater mechanism, and it is stabilized by electron correlation. Pressure reduces the stability of the antiferromagnetic state. Our findings corroborate previous analyses suggesting that the properties of this compound can be understood as the result of the interplay between electron correlations and Jahn-Teller electron-phonon interaction.

Pressure induced magnetic phase separation in La0.75Ca0.25MnO3 manganite.

The pressure dependence of the Curie temperature T(C)(P) in La(0.75)Ca(0.25)MnO(3) was determined by neutron diffraction up to 8 GPa, and compared with the metallization temperature T(IM)(P) (Postorino et al 2003 Phys. Rev. Lett. 91 175501). The behavior of the two temperatures appears similar over the whole pressure range, suggesting a key role of magnetic double-exchange also in the pressure regime where the superexchange interaction is dominant. The coexistence of antiferromagnetic and ferromagnetic peaks at high pressure and low temperature indicates a phase separated regime which is well reproduced with a dynamical mean-field calculation for a simplified model. A new P-T phase diagram has been proposed on the basis of the whole set of experimental data.

High-temperature optical spectral weight and fermi-liquid renormalization in bi-based cuprate superconductors.

The optical conductivity σ(ω) and the spectral weight W(T) of two superconducting cuprates at optimum doping, Bi2Sr2-xLaxCuO6 and Bi2Sr2CaCu2O8, have been first measured up to 500 K. Above 300 K, W(T) deviates from the usual T2 behavior in both compounds, even though σ(ω→0) remains larger than the Ioffe-Regel limit. The deviation is surprisingly well described by the T4 term of the Sommerfeld expansion, but its coefficients are enhanced by strong correlation, as shown by the good agreement with dynamical mean field calculations.

Kinks in the electronic specific heat.

We find that the heat capacity of a strongly correlated metal presents striking changes with respect to Landau Fermi-liquid theory. In contrast with normal metals, where the electronic specific heat is linear at low temperature (with a T3 term as a leading correction), a dynamical mean-field study of the correlated Hubbard model reveals a clear kink in the temperature dependence, marking a rapid change from a low-temperature linear behavior and a second linear regime with a reduced slope. Experiments on LiV2O4 support our findings, implying that correlated materials are more resistive to cooling at low T than expected from the intermediate temperature behavior.

Nodal-antinodal dichotomy and the two gaps of a superconducting doped Mott insulator.

We study the superconducting state of the hole-doped two-dimensional Hubbard model using cellular dynamical mean-field theory, with the Lanczos method as impurity solver. In the underdoped regime, we find a natural decomposition of the one-particle (photoemission) energy gap into two components. The gap in the nodal regions, stemming from the anomalous self-energy, decreases with decreasing doping. The antinodal gap has an additional contribution from the normal component of the self-energy, inherited from the normal-state pseudogap, and it increases as the Mott insulating phase is approached.

Electron-phonon interaction and antiferromagnetic correlations.

We study effects of the Coulomb repulsion on the electron-phonon interaction (EPI) in the Holstein-Hubbard model, using the antiferromagnetic (AF) dynamical mean-field approximation. AF correlations strongly enhance EPI effects on the electron Green's function with respect to the paramagnetic correlated system, but the net effect of the Coulomb interaction is a moderate suppression of the EPI. Doping leads to additional suppression. In contrast, the Coulomb interaction strongly suppresses EPI effects on phonons, but the suppression weakens with doping.

Dynamical breakup of the fermi surface in a doped Mott insulator.

The evolution from an anomalous metallic phase to a Mott insulator within the two-dimensional Hubbard model is investigated by means of the cellular dynamical mean-field theory. We show that approaching the density-driven Mott metal-insulator transition the Fermi surface is strongly renormalized and the quasiparticle description breaks down in a very anisotropic fashion. Regions where the quasiparticles are strongly scattered (hot spots) and regions where the scattering rate is relatively weak (cold spot) form irrespective of whether the parent insulator has antiferromagnetic long-range order, while their location is not universal and is determined by the interplay of the renormalization of the scattering rate and the Fermi surface shape.

Temperature dependence of the optical spectral weight in the cuprates: role of electron correlations.

We compare calculations based on the dynamical mean-field theory of the Hubbard model with the infrared spectral weight W(Omega,T) of La(2-x)SrxCuO4 and other cuprates. Without using fitting parameters we show that most of the anomalies found in W(Omega,T) with respect to normal metals, including the existence of two different energy scales for the doping and the T dependence of W(Omega,T), can be ascribed to strong correlation effects.

Electron-phonon interaction close to a Mott transition.

The effect of Holstein electron-phonon interaction on a Hubbard model close to a Mott-Hubbard transition at half filling is investigated by means of dynamical mean-field theory. We observe a reduction of the effective mass that we interpret in terms of a reduced effective repulsion. When the repulsion is rescaled to take into account this effect, the quasiparticle low-energy features are unaffected by the electron-phonon interaction. Phonon features are only observed within the high-energy Hubbard bands. The lack of electron-phonon fingerprints in the quasiparticle physics can be explained interpreting the quasiparticle motion in terms of rare fast processes.