Zero-Point Vacancies in Quantum Solids

A Jastrow wave function (JWF) and a shadow wave function (SWF) describe a quantum solid with Bose–Einstein condensate; i.e. a supersolid. It is known that both JWF and SWF describe a quantum solid with also a finite equilibrium concentration of vacancies x _v . We outline a route for estimating x _v by exploiting the existing formal equivalence between the absolute square of the ground state wave function and the Boltzmann weight of a classical solid. We compute x _v for the quantum solids described by JWF and SWF employing very accurate numerical techniques. For JWF we find a very small value for the zero point vacancy concentration, x _v =(1.4±0.1)×10^?6. For SWF, which presently gives the best variational energy of solid ⁴He, we find the significantly larger value x _v =(1.4±0.1)×10^?3 at a density close to melting. We also study two and three vacancies with SWF. We find that there is a strong short range attraction but the vacancies do not form a bound state, at variance with the exact finite temperature PIMC results.     

Properties of Metastable Quantum Solids

We consider the properties of quantum solids at pressures below the pressure range in which the solid is the stable phase. We estimate the spinodal pressure and determine how this pressure varies as a function of the de Boer parameter.     

Resonance Phenomena in Macroscopic Quantum Tunneling

We consider two types of resonance phenomena in Josephson junctions, both related to the existence of highly resolved levels of a quantum particle in a potential well. The existence of the levels is manifested in the most striking way in a resonance decrease of the lifetime of the metastable state under the action of an alternating external current. A strong resonance should take place at the frequency equal to the level spacing. The second type of resonance phenomena takes place at such values of the bias current I _x for which two levels in adjacent wells happen to cross. The shape and size of peaks depend strongly on the parameters of the junctions, as well as on the temperature and number of resonance levels.     

From Quantum Disorder to Quantum Chaos

We study the statistics of wave functions in a ballistic chaotic system. The statistical ensemble is generated by adding weak smooth random potential, which allows us to apply the ballistic -model approach. We analyze conditions of applicability of the -model, emphasizing the role played by the single-particle mean free path and the Lyapunov exponent due to the random potential. In particular, we present a resolution of the puzzle of repetitions of periodic orbits counted differently by the -model and by the trace formula.     

Pseudohalide‐Exchanged Quantum Dot Solids Achieve Record Quantum Efficiency in Infrared Photovoltaics

Application of pseudohalogens in colloidal quantum dot (CQD) solar‐cell active layers increases the solar‐cell performance by reducing the trap densities and implementing thick CQD films. Pseudohalogens are polyatomic analogs of halogens, whose chemistry allows them to substitute halogen atoms by strong chemical interactions with the CQD surfaces. The pseudohalide thiocyanate anion is used to achieve a hybrid surface passivation. A fourfold reduced trap state density than in a control is observed by using a suite of field‐effect transistor studies. This translates directly into the thickest CQD active layer ever reported, enabled by enhanced transport lengths in this new class of materials, and leads to the highest external quantum efficiency, 80% at the excitonic peak, compared with previous reports of CQD solar cells.     

Recent Progress and New Challenges in Quantum Fluids and Solids

The exact expression for the average kinetic energy of an inhomogeneous Bose gas in the ground state is obtained as a functional of the inhomogeneous density of the Bose–Einstein condensate. The result is based on existence of the off-diagonal long-range order in the single-particle density matrix for systems with a Bose–Einstein condensate. This makes it possible to avoid the use of anomalous averages. On this basis, the explicit expressions for the ground-state energy and the local pressure of an inhomogeneous Bose gas are derived within the self-consistent Hartree–Fock approximation.     

Macroscopic Quantum Phenomena in High Critical Temperature Superconducting Josephson Junctions

YBa₂Cu₃O_7?δ grain boundary bi-epitaxial Josepshon junctions (JJs) allow a very clear demonstration of Josephson current variation with the misorientation angle, consistent with the d-wave symmetry of the superconducting order parameter in cuprate, high temperature superconductors. Our bi-epitaxial junctions show a strong suppression of the first harmonic, I ₁ sin ø, of the current phase relation when tunneling from a lobe into a node of the superconducting gap function. In these configurations, the contribution of the second harmonic, I ₂ sin 2ø, becomes of the same magnitude as the first one, giving rise to a characteristic two-well Josephson potential as a function of phase ø instead of the usual single well. This characteristic intrinsic property has suggested proposals of a new class of qu-bit named “quiet” because of the existence a spontaneously degenerate fundamental state without the need of applying an external field. Our experiments probe the macroscopic quantum properties in a d-wave Josephson junction by measuring macroscopic quantum tunneling and energy level quantization. The switching current out of the zero voltage state is measured as a function of temperature down to 20 mK. The temperature variation of the width of an ensemble of switching events goes over from one, which is characteristic of a thermal activation of phase fluctuations to a temperature independent width which is a token of quantum tunneling of the phase. The transition regime is affected by the two-well potential in a 45^° misorientation junction as the second harmonic term gives rise to additional thermal transitions. The difference between quantized energy levels in the harmonic potential was determined by microwave spectroscopy. From the broadening of energy levels, it was possible to extract a Q-value of about 40 for the phase oscillations. The relatively high Q indicates quantum coherence over a sizeable time in d-wave junctions and gives hopes for a realization of a “quiet” high-T _c qu-bit. The contributions of V. L. Ginzburg to several different fields of physics are impressive and long standing. In superconductivity the Ginzburg–Landau theory, for instance, still represents a very powerful approach to model a huge number of different physical systems. High Temperature Superconductors (HTS) have strongly influenced research of the last 20 years and their d-wave order parameter symmetry represents one of the most intriguing features from both the fundamental point of view and some types of innovative long-term applications.     

Quantum Conductance Probing of Oxygen Vacancies in SrTiO3 Epitaxial Thin Film using Graphene

Quantum Hall conductance in monolayer graphene on an epitaxial SrTiO₃ (STO) thin film is studied to understand the role of oxygen vacancies in determining the dielectric properties of STO. As the gate‐voltage sweep range is gradually increased in the device, systematic generation and annihilation of oxygen vacancies, evidenced from the hysteretic conductance behavior in the graphene, are observed. Furthermore, based on the experimentally observed linear scaling relation between the effective capacitance and the voltage sweep range, a simple model is constructed to manifest the relationship among the dielectric properties of STO with oxygen vacancies. The inherent quantum Hall conductance in graphene can be considered as a sensitive, robust, and noninvasive probe for understanding the electronic and ionic phenomena in complex transition‐metal oxides without impairing the oxide layer underneath.     

From Thermodynamically Driven Phase Transitions to Quantum Critical Phenomena

In this short review some basic concepts of phase transitions at non-zero and zero temperature are highlighted. For that the critical behavior of classical, thermodynamically driven magnetic phase transitions is exemplified for conventional universality classes as well as for systems reflecting chiral order. Finally the quantum critical phenomena for two magnetic-field-driven phase transitions are presented.     

Study and Development of Photovoltaic Structures Based on Quantum Dot Solids of PbS with Various Ligands

The photoconductivity of condensates of lead-sulfide quantum dots (QDs)—QD solids—with various organic ligands is studied. It is demonstrated that the QD solid photoconductivity increases exponentially with a reduction in length of ligand molecules and does not depend on their chemical structures, since it is governed by hopping transport of charge carriers. In contrast, the photocurrent in photovoltaic ITO/PEDOT: PSS/PbS/ZnO/Al elements depends on the ligand structure, since this structure sets the positions of QD energy levels and thus affects the efficiency of charge carrier transfer to electrodes. The difference between mechanisms of generation of photoconductivity and photovoltaic currents is discussed.     

Disorder, Supersolidity, and Quantum Plasticity in Solid Helium 4

Several years after Kim and Chan’s discovery of an anomaly in the rotation properties of solid helium (Kim and Chan in Nature 427:225, 2004; Science 305:1941, 2004), the interpretation of the observed phenomena as a manifestation of supersolidity remains controversial. J.?Beamish and his collaborators have shown that the rotation anomaly is accompanied by an elastic anomaly (Day and Beamish in Nature 450:853, 2007; Day et?al. in Phys. Rev. Lett. 104:075302, 2010; Syshchenko et?al. in Phys. Rev. Lett. 104:195301, 2010): when the rotational inertia apparently increases, the shear modulus decreases. This softening is due to the appearance, in the solid, of a large reversible plasticity that is a consequence of the evaporation of ³He impurities from dislocations that become mobile. This plasticity is called “quantum plasticity” because the dislocations move by quantum tunneling in the low temperature limit. Since the main evidence for supersolidity comes from torsional oscillator (TO) experiments, and since the TO period depends on both the inertia and the stiffness of solid ⁴He, it is not totally clear if supersolidity really induces a change in inertia or if it is the disappearance of quantum plasticity that mimics supersolidity in TO experiments. In order to distinguish between supersolidity and quantum plasticity, we have studied the rotational and the acoustic properties of solid ⁴He samples with a variable amount of disorder and of ³He impurities. Of particular interest is the comparison of single crystals to polycrystals but the whole problem is not yet solved. This short review article is an opportunity to discuss several questions regarding the exact role of disorder in supersolidity and in quantum plasticity.     

Non-Fermi Behavior Near a Quantum Phase Transition Driven by Disorder

We showed that in a d-wave two-dimensional superconductor the disorder given by non-magnetic impurities at low temperature leads to a non-Fermi behavior for the normal state. The transition is similar to the superconductor–insulator transition in a model with a dissipative term.     

Chain Vacancies in 2D Crystals

Defects in bulk crystals can be classified into vacancies, interstitials, grain boundaries, stacking faults, dislocations, and so forth. In particular, the vacancy in semiconductors is a primary defect that governs electrical transport. Concentration of vacancies depends mainly on the growth conditions. Individual vacancies instead of aggregated vacancies are usually energetically more favorable at room temperature because of the entropy contribution. This phenomenon is not guaranteed in van der Waals 2D materials due to the reduced dimensionality (reduced entropy). Here, it is reported that the 1D connected/aggregated vacancies are energetically stable at room temperature. Transmission electron microscopy observations demonstrate the preferential alignment direction of the vacancy chains varies in different 2D crystals: MoS₂ and WS₂ prefer direction, while MoTe₂ prefers direction. This difference is mainly caused by the different strain effect near the chalcogen vacancies. Black phosphorous also exhibits directional double‐chain vacancies along 〈01〉 direction. Density functional theory calculations predict that the chain vacancies act as extended gap (conductive) states. The observation of the chain vacancies in 2D crystals directly explains the origin of n‐type behavior in MoTe₂ devices in recent experiments and offers new opportunities for electronic structure engineering with various 2D materials.