Unusual Quantum States Research Articles

ELECTRONIC AND MAGNETIC PROPERTIES OF ALLOYS BASED ON THE DIRAC SEMIMETAL Cd<sub>3</sub>As<sub>2</sub> DOPED BY Mn ATOMS WITH THE VARIABLE CONCENTRATIONS

Theoretical studies predict that the low magnetic doping of the Dirac semimetals (DS) leads to the appearance in them of unusual quantum states and properties: the states of Weil semimetals, axionic insulator, topological superconductor and so on. However the specific materials in which these phenomena can be observed, as well as the characteristic concentrations of magnetic atoms are still unknown. In the present work, an ab initio study of the electronic and magnetic properties of the DS Cd3As2 doped isoelectronically with Mn atoms at concentrations of 4, 6, and 8% was performed. When analyzing the results, the main attention is paid to breaking spatial and time reversal symmetry in alloys, the behavior of the electronic structure near the top of the Dirac cone, and the processes of spin ordering in Mn atoms. The results obtained are compared with earlier theoretical and experimental studies, and on their basis a detailed picture of the effect of isoelectronic magnetic doping on the properties of the DS Cd3As2 is given.

Pressure-induced superconductivity in the kagome single-crystal Pd3P2S8

The kagome lattice offers unique opportunities for the exploration of unusual quantum states of correlated electrons. Here, we report on the observation of superconductivity in a kagome single-crystal ${\mathrm{Pd}}_{3}{\mathrm{P}}_{2}{\mathrm{S}}_{8}$ when a semiconducting-to-metallic transition is driven by pressure. High-pressure resistance measurements show that the metallization and superconductivity are simultaneously observed at about 11 GPa. With increasing pressure, the superconducting critical temperature ${T}_{c}$ is monotonously enhanced from 2.6 K to a maximum 7.7 K at $\ensuremath{\sim}52\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Interestingly, superconductivity is retained when the pressure is fully released. Synchrotron x-ray diffraction and Raman experiments consistently evidence that the emergence of superconductivity is accompanied with an amorphization, and the retainability of superconductivity upon decompression can be attributed to the irreversibility of the amorphization.

TEM at millikelvin temperatures: Observing and utilizing superconducting qubits

We present a case for developing a millikelvin-temperature transmission electron microscope (TEM). We start by reviewing known reasons for such development, then present new possibilities that have been opened up by recent progress in superconducting quantum circuitry, and finally report on our ongoing experimental effort. Specifically, we first review possibilities to observe a quantum mechanically superposed electromagnetic field around a superconducting qubit. This is followed by a new idea on TEM observation of microwave photons in an unusual quantum state in a resonator. We then proceed to review potential applications of these phenomena, which include low dose electron microscopy beyond the standard quantum limit. Finally, anticipated engineering challenges, as well as the authors' current ongoing experimental effort towards building a millikelvin TEM are described. In addition, we provide a brief introduction to superconducting circuitry in the Appendix for the interested reader who is not familiar with the subject.

Classical and quantum dynamics of a trapped ion coupled to a charged nanowire

We study theoretically the mechanical drive of a trapped ultracold ion by a charged nanowire through their mutual Coulomb interaction. We characterize the perturbation of the trapping potential for the ion by the nanowire and discuss the parameters determining the dynamics of the ion under the action of the nanooscillator. We explore the classical dynamics as well as motional quantum states of the ion which can be generated and manipulated with the resonant drive of the nanowire and the effects of anharmonicities of the ion-trap potential on the system. Our modelling indicates that unusual quantum states of the ion motion can be generated with this approach and that sympathetic cooling and quantum entanglement can be realised when both subsystems operate in the quantum regime. The present ion-mechanical hybrid system might prove interesting as a new quantum device, for quantum sensing experiments, for spectroscopy and for mass spectrometry.

Thermodynamics of a two-dimensional dipolar Bose gas with correlated disorder in the roton regime

We study the impact of a weak random potential with a Gaussian correlation function on the thermodynamics of a two-dimensional dipolar bosonic gas. Analytical expressions for the quantum depletion, anomalous density, the ground state energy, the equation of state and the sound velocity are derived in the roton regime within the framework of the Bogoliubov theory. Surprisingly, we find that the condensate depletion and the anomalous density are comparable. The structure factor and the superfluid fraction are also obtained analytically and numerically. We show that these quantities acquire dramatically modified profiles when the roton is close to zero yielding the transition to an unusual quantum state.

Surface hall effect and nonlocal transport in SmB₆: evidence for surface conduction.

A topological insulator (TI) is an unusual quantum state in which the insulating bulk is topologically distinct from vacuum, resulting in a unique metallic surface that is robust against time-reversal invariant perturbations. The surface transport, however, remains difficult to isolate from the bulk conduction in most existing TI crystals (particularly Bi2Se3, Bi2Te3 and Sb2Te3) due to impurity caused bulk conduction. We report in large crystals of topological Kondo insulator (TKI) candidate material SmB6 the thickness-independent surface Hall effects and non-local transport, which persist after various surface perturbations. These results serve as proof that at low temperatures SmB6 has a metallic surface that surrounds an insulating bulk, paving the way for transport studies of the surface state in this proposed TKI material.

Experimental realization of a topological crystalline insulator in SnTe

A topological insulator is an unusual quantum state of matter, characterized by the appearance, at its edges or on its surface, of a gapless metallic state that is protected by time-reversal symmetry1,2. The discovery of topological insulators has stimulated the search for other topological states protected by other symmetries3,4,5,6,7, such as the recently predicted8 topological crystalline insulator (TCI) in which the metallic surface states are protected by the mirror symmetry of the crystal. Here we present experimental evidence for the TCI phase in tin telluride (SnTe), which has been predicted to be a TCI (ref. 9). Our angle-resolved photoemission spectra show the signature of a metallic Dirac-cone surface band, with its Dirac point slightly away from the edge of the surface Brillouin zone in SnTe. Such a gapless surface state is absent in a cousin material, lead telluride, in line with the theoretical prediction.

Energy density-flux correlations in an unusual quantum state and in the vacuum

In this paper we consider the question of the degree to which negative and positive energy are intertwined. We examine in more detail a previously studied quantum state of the massless minimally coupled scalar field, which we call a ''Helfer state.'' This is a state in which the energy density can be made arbitrarily negative over an arbitrarily large region of space, but only at one instant in time. In the Helfer state, the negative energy density is accompanied by rapidly time-varying energy fluxes. It is the latter feature which allows the quantum inequalities, bounds which restrict the magnitude and duration of negative energy, to hold for this class of states. An observer who initially passes through the negative energy region will quickly encounter fluxes of positive energy which subsequently enter the region. We examine in detail the correlation between the energy density and flux in the Helfer state in terms of their expectation values. We then study the correlation function between energy density and flux in the Minkowski vacuum state, for a massless minimally coupled scalar field in both two and four dimensions. In this latter analysis we examine correlation functions rather than expectation values. Remarkably, we see qualitativelymore » similar behavior to that in the Helfer state. More specifically, an initial negative energy vacuum fluctuation in some region of space is correlated with a subsequent flux fluctuation of positive energy into the region. We speculate that the mechanism which ensures that the quantum inequalities hold in the Helfer state, as well as in other quantum states associated with negative energy, is, at least in some sense, already encoded in the fluctuations of the vacuum.« less

Two-Step Restoration of SU(2) Symmetry in a Frustrated Ring-Exchange Magnet

We demonstrate the existence of a spin-nematic, moment-free phase in a quantum four-spin ring-exchange model on the square lattice. This unusual quantum state is created by the interplay of frustration and quantum fluctuations that lead to a partial restoration of SU(2) symmetry when going from a four-sublattice orthogonal biaxial Néel order to this exotic uniaxial magnet. A further increase of frustration drives a transition to a fully gapped SU(2) symmetric valence bond crystal.

Unusual quantum states: non–locality, entropy, Maxwell's demon and fractals

This paper analyses the mathematical properties of some unusual quantum states that are constructed by inserting an impenetrable barrier into a chamber confining a single particle. If the barrier is inserted at a fixed node of the wave function, then the energy of the system is unchanged. After barrier insertion, a measurement is made on one side of the chamber to determine if the particle is physically present. The measurement causes the wave function to collapse, and the energy contained in the subchamber where the particle is not present transfers instantaneously to the other subchamber where the particle now exists. This thought experiment constitutes an elementary example of an Einstein–Podolsky–Rosen experiment based on energy conservation rather than momentum or angular–momentum conservation. A more interesting situation arises when one inserts the barrier at a point that is not a fixed node of the wave function because this process changes the energy of the system; the faster the barrier is inserted, the greater the change in the energy. At the point of a sudden insertion the energy density becomes infinite; this energy instantly propagates across the subchamber and causes the wave function to become fractal. If an energy measurement is carried out on such a fractal wave function, the resulting mixed state has finite non–zero entropy. Fractal mixed states having unbounded entropy are also constructed and their properties are discussed. For a finite time insertion of the barrier, Landauer's principle is shown to be insufficient to resolve the apparent violation of the second law of thermodynamics that arises when a Maxwell demon is present. This problem is resolved by calculating the energy required to insert the barrier.

Three‐Body Approximation for the Investigation of Efimov States in Molecular Systems

Using the three‐body approximation with the well‐known interatomic potential, the Efimov states in molecular systems are calculated. A mechanism of appearance and dissappearance of the Efimov states in the helium trimer in the three‐body approximation is considered when the interatomic‐interaction force is varied. Geometrical structure of these unusual quantum states are presented.

Cold-collision properties derived from frequency shifts in a cesium fountain.

We interpret recently measured collisional frequency shifts in a cesium atomic fountain using coupled-channel theory and a simple picture of hyperfine-coupled long-range molecular states. We pre­ dict the existence of CS2 singlet and triplet states very close to threshold with an associated strong influence on ultracold collisions and large negative values for the Cs+Cs scattering lengths. Our results are important for future precision experiments using Cs fountains, and may also have important conse­ quences for attempts to realize a Bose condensate in an ultracold atomic Cs vapor. PACS number(s): 32.80.Pj, 42.50.Vk The advances of techniques for using lasers to trap and manipulate atoms have, in the past few years, made it possible to cool atoms to temperatures near absolute zero and to examine unusual quantum states of matter under these circumstances. One of the most spectacular possi­ bilities now within sight is the observation of Bose­ Einstein condensation (BEC) in a weakly interacting va­ por of atoms, which is predicted to take place at such low temperatures and high densities that the atomic de Bro­ glie wavelength becomes comparable to the average dis­ tance between the atoms. Some groups are trying to achieve this phase transition in atomic hydrogen gas in a magnetic trap [1,2]. Others are attempting the same in a laser-cooled sample of alkali-metal atoms such as Cs [3] and Li [4]. Collisions between atoms playa crucial role in this en­ deavor. In two recent papers [5], the conditions for BEC of a cold Cs vapor were examined using the properties of Cs+ Cs ground-state collisions. Due to insufficient knowledge of the interatomic potentials it was only possi­ ble to reach rough conclusions. This situation has changed drastically with a recent cesium-fountain clock experiment in which two of the present authors measured large frequency shifts due to ultracold collisions of Cs atoms during their ballistic flight in the atomic fountain [6]. While these frequency shifts will be largely eliminat­ ed in future clocks [6], here we use the measured shifts to obtain information on the interaction potentials relevant for BEC. We find strong evidence for the existence of singlet and triplet states· close to the continuum with an associated large influence on ultracold collisions. The frequency shifts enable us to analyze collisions at the coldest temperatures (T~ 1.5 J.LK) at which atomic col­ lisions have ever been studied. We find the zero-energy elastic cross section for triplet scattering to be between

Quantitative Laser-Induced Fluorescence Measurements of Reactive Species: Spectroscopy and Collision Dynamics of SiCl

AbstractLaser-induced fluorescence (LIF) is an ideal technique to determine the gas phase concentration of the chemically reactive radical species in processing plasmas. Quantitative species concentration measurements require spectroscopic and collision dynamics data. Experiments to obtain such data for the B2 and Σ+ B′2 Δ states of SiCl are described. Using LIF, the transition strengths, radiative lifetimes, and collisional removal rates are determined. Collisional transfer between the two excited electronic states, B′→B, shows a very unusual quantum state specificity for the final vibrational levels which is quite different for each of the rare gas collision partners (He, Ne, Ar). Such energy transfer makes the B′2 Δ state unsuitable for quantitative LIF diagnostics; however, the B2Σ+ state appears to be an ideal excited state for LIF diagnostic measurements in silicon etching plasmas.