Emergence of Nontrivial Low-Energy Dirac Fermions in Antiferromagnetic EuCd2As2

Parity-time symmetry plays an essential role for the formation of Dirac states in Dirac semimetals. So far, all of the experimentally identified topologically nontrivial Dirac semimetals (DSMs) possess both parity and time reversal symmetry. The realization of magnetic topological DSMs remains a major issue in topological material research. Here, combining angle-resolved photoemission spectroscopy with density functional theory calculations, it is ascertained that band inversion induces a topologically nontrivial ground state in EuCd₂As₂. As a result, ideal magnetic Dirac fermions with simplest double cone structure near the Fermi level emerge in the antiferromagnetic (AFM) phase. The magnetic order breaks time reversal symmetry, but preserves inversion symmetry. The double degeneracy of the Dirac bands is protected by a combination of inversion, time-reversal, and an additional translation operation. Moreover, the calculations show that a deviation of the magnetic moments from the c-axis leads to the breaking of C3 rotation symmetry, and thus, a small bandgap opens at the Dirac point in the bulk. In this case, the system hosts a novel state containing three different types of topological insulator: axion insulator, AFM topological crystalline insulator (TCI), and higher order topological insulator. The results provide an enlarged platform for the quest of topological Dirac fermions in a magnetic system.     

Ultrasensitive Self-Driven Terahertz Photodetectors Based on Low-Energy Type-II Dirac Fermions and Related Van der Waals Heterojunctions

The exotic electronic properties of topological semimetals (TSs) have opened new pathways for innovative photonic and optoelectronic devices, especially in the highly pursuit terahertz (THz) band. However, in most cases Dirac fermions lay far above or below the Fermi level, thus hindering their successful exploitation for the low-energy photonics. Here, low-energy type-II Dirac fermions in kitkaite (NiTeSe) for ultrasensitive THz detection through metal-topological semimetal-metal heterostructures are exploited. Furthermore, a heterostructure combining two Dirac materials, namely, graphene and NiTeSe, is implemented for a novel photodetector exhibiting a responsivity as high as 1.22 A W⁻¹, with a response time of 0.6 µs, a noise-equivalent power of 18 pW Hz^−0.5, with outstanding stability in the ambient conditions. This work brings to fruition of Dirac fermiology in THz technology, enabling self-powered, low-power, room-temperature, and ultrafast THz detection.     

Cyclotron Resonance of Composite Fermions

The introduction of suitable fictitious entities occasionally permits to cast otherwise difficult strongly interacting many-body systems in a single particle form. We can then take the customary physical approach, using concepts and representations which formerly could only be applied to systems with weak interactions, and yet still capture the essential physics. A most notable recent example occurs in the conduction properties of a two-dimensional electron system (2DES), when exposed to a strong perpendicular magnetic field B. They are governed by electron–electron interactions, that bring about the fractional quantum hall effect (FQHE). S. Das Sarma and A. Pinczuk (eds.), Perspectives on Quantum Hall Effects (Wiley, New York, 1996). Composite fermions, that do not experience the external magnetic field but a drastically reduced effective magnetic field B*, were identified as apposite quasi-particles that simplify our understanding of the FQHE. J. K. Jain, Phys. Today, 39–45 (2000). J. K. Jain, Phys. Rev. Lett. 63, 199–202 (1989). They precess, like electrons, along circular cyclotron orbits, with a diameter determined by B* rather than B. B. I. Halperin, P. A. Lee, and N. Read, Phys. Rev. B 47, 7312–7343 (1993). O. Heinonen, (ed.), Composite Fermions (World Scientific, Singapore, 1998). R. R. Du, H. L. Stormer, D. C. Tsui, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 70, 2944–2947 (1993). R. R. Du et al., Phys. Rev. Lett. 75, 3926–3929 (1995). R. L. Willett, R. R. Ruel, K. W. West, and L. N. Pfeiffer, Phys. Rev. Lett. 71, 3846–3849 (1993). V. J. Goldman, B. Su, and J. K. Jain, Phys. Rev. Lett. 72, 2065–2068 (1994). J. H. Smet, D. Weiss, R. H. Blick, G. Lütjering, and K. von Klitzing, Phys. Rev. Lett. 77, 2272–2275 (1996). The frequency of their cyclotron motion remained hitherto enigmatic, since the effective mass is no longer related to the band mass of the original electrons and is entirely generated from electron–electron interactions. Here, we demonstrate the enhanced absorption of a microwave field that resonates with the frequency of their circular motion. From this cyclotron resonance, we derive a composite fermion effective mass that varies from 0.7 to 1.2 times the electron mass in vacuum as their density is tuned from 0.6× 10¹¹/cm² to 1.2× 10¹¹/cm².     

Stripe-Superconductor Duality and the Emergence of Nodal Fermions

Recent experiments suggest a stripe-superconductor duality to be operating for some regime of the x vs. T phase diagram of the cuprate oxide superconductors. Extending a nonperturbative approach previously applied to quasi-1d systems, we formulate this duality where special emphasis is laid on the role of global antiferromagnetic fluctuations which are enhanced in the vicinity of stripe dislocations. A condensate of such antiferromagnetic fluctuation is found to be able to support nodal fermions.     

First Sound in Mixtures of Trapped Bosons and Fermions

We study the collisional density oscillations of a mixture of bosonic and fermionic potassium atoms confined in a spherical trap at zero temperature. We find that the lowest-lying fluctuations of the whole system occupy either a non-overlapping spatial domain or overlap with opposite velocities. For moderate number of particles, we compare these results to previous calculations within a random-phase approximation.     

Photoemission method of measuring temperature

A photoemission method of measuring temperature is presented, and the range of its application is indicated. Expressions are obtained for calculating the systematic error, and a nomogram is given for determining it.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 35, No. 2, pp. 257–265, August, 1978.     

Flow-Equations for the Model of Hybridized Bosons and Fermions

Using the flow-equations method we showed analytically the occurrence of dispersion for the local bosons in a model of hybridized local bosons and fermions.     

Fermions in Two Dimensions: Scattering and Many-Body Properties

Ultracold atomic Fermi gases in two dimensions (2D) are an increasingly popular topic of research. The interaction strength between spin-up and spin-down particles in two-component Fermi gases can be tuned in experiments, allowing for a strongly interacting regime where the gas properties are yet to be fully understood. We have probed this regime for 2D Fermi gases by performing T = 0 ab initio diffusion Monte Carlo calculations. The many-body dynamics are largely dependent on the two-body interactions; therefore, we start with an in-depth look at scattering theory in 2D. We show the partial-wave expansion and its relation to the scattering length and effective range. Then, we discuss our numerical methods for determining these scattering parameters. We close out this discussion by illustrating the details of bound states in 2D. Transitioning to the many-body system, we use variationally optimized wave functions to calculate ground-state properties of the gas over a range of interaction strengths. We show results for the energy per particle and parametrize an equation of state. We then proceed to determine the chemical potential for the strongly interacting gas.     

Hard-core Bosons and Spinless Fermions Trapped on 1D Lattices

No Heading We study ground state properties and the non-equilibrium dynamics of trapped hard-core bosons on 1D lattices. We follow an exact approach based on the Jordan-Wigner transformation, which maps the hard-core boson Hamiltonian into the one of non-interacting spinless fermions. Off-diagonal one-particle correlations and the momentum distribution function of hard-core bosons and spinless fermions are contrasted in and out of equilibrium.PACS numbers: 03.75.Ss, 05.30.Fk, 71.10.Ca     

Low-Energy Electronic Collective Modes and Superconductivity in Layered Systems

The Coulomb interaction induces low-energy electronic collective modes (LEECM) in layered conductors. We study how the superconducting critical temperature T _c is affected by such collective modes within the strong-coupling theory of superconductivity. Whereas we account for both the electron–phonon and the screened Coulomb interactions, the main focus is set on the latter. The dielectric function is displayed and screening within RPA is discussed. We then show that LEECM affect the superconducting state constructively in layered structures. Their effect can be interpreted as a negative * in the Eliashberg theory. We examine in what respect the collective-mode scenario differs in layered conductors as compared to three-dimensional (3D) isotropic metals.     

Photoemission Spectroscopy of Ta2NiSe5

We report temperature-dependent angle-resolved photoemission spectroscopy measurement of Ta₂NiSe₅ which shows a semiconductor-semiconductor structural phase transition at around 330?K. Characteristically, flat band at the top of the valence band is observed, which is ascribed to the excitonic insulator effect. The top valence band shifts to higher binding energy and its bandwidth increases as the temperature decreases. As the system exceeds the transition temperature, the flat feature of the valence band weakens though the exciton fluctuations remain finite.     

Measurement of Broadband Temperature-Dependent Ultrasonic Attenuation and Dispersion Using Photoacoustics

The broadband ultrasonic characterization of biological fluids and tissues is important for the continued development and application of high-resolution ultrasound imaging modalities. Here, a photoacoustic technique for the transmission measurement of temperature-dependent ultrasonic attenuation and dispersion is described. The system uses a photoacoustic plane wave source constructed from a polymethylmethacrylate substrate with a thin optically absorbent layer. Broadband ultrasonic waves are generated by illuminating the absorbent layer with nanosecond pulses of laser light. The transmitted ultrasound waves are detected by a planar 7-μm high-finesse Fabry-Perot interferometer. Temperatureinduced thickness changes in the Fabry-Perot interferometer are tracked to monitor the sample temperature and maintain the sensor sensitivity. The measured ?6-dB bandwidth for the combined source and sensor is 1 to 35 MHz, with an attenuation corrected signal level at 100 MHz of ?10 dB. The system is demonstrated through temperature-dependent ultrasound measurements in castor oil and olive oil. Power law attenuation parameters are extracted by fitting the experimental attenuation data to a frequency power law while simultaneously fitting the dispersion data to the corresponding Kramers-Kr?nig relation. The extracted parameters are compared with other calibration measurements previously reported in the literature.     

Josephson Oscillations and Self-Trapping of Superfluid Fermions in a Double-Well Potential

We investigate the behaviour of a two-component Fermi superfluid in a double-well potential. We numerically solve the time dependent Bogoliubov-de Gennes equations and characterize the regimes of Josephson oscillations and self-trapping for different potential barriers and initial conditions. In the weak link limit the results agree with a two-mode model where the relative population and the phase difference between the two wells obey coupled nonlinear Josephson equations. A more complex dynamics is predicted for large amplitude oscillations and large tunneling.     

Photoemission Charging of a Collective of Aerosol Particles

The photoemission charging of a collective of particles at normal atmospheric pressure and at lowered pressure of the air has been investigated. The dependences of the average initial charge of a droplet, the average equilibrium photoemission loss of charge by a droplet, and the equilibrium concentration of the electrons on the air pressure are presented as the basic experimental results. The process of photoemission charging of an aerodisperse system was theoretically described by solution of the equation of charge equilibrium of the average-radius particle with space charge. Comparison of the experimental and calculated results showed their satisfactory agreement.     

Temperature-Dependent Thermal Conductivity of Undoped Polycrystalline Silicon Layers

Polycrystalline silicon is used in microelectronic and microelectromechanical devices for which thermal design is important. This work measures the in-plane thermal conductivities of free-standing undoped polycrystalline layers between 20 and 300 K. The layers have a thickness of 1 m, and the measurements are performed using steady-state Joule heating and electrical-resistance thermometry in patterned aluminum microbridges. The layer thermal conductivities are found to depend strongly on the details of the deposition process through the grain size distribution, which is investigated using atomic force microscopy and transmission electron microscopy. The room-temperature thermal conductivity of as-grown polycrystalline silicon is found to be 13.8 W·m^-1·K^-1and that of amorphous recrystallized polycrystalline silicon is 22 W·m^-1·K^-1, which is almost an order of magnitude less than that of single-crystal silicon. The maximum thermal conductivities of both samples occur at higher temperatures than in pure single-crystalline silicon layers of the same thickness. The data are interpreted using the approximate solution to the Boltzmann transport equation in the relaxation time approximation together with Matthiessen's rule. These measurements contribute to the understanding of the relative importance of phonon scattering on grain and layer boundaries in polysilicon films and provide data relevant for the design of micromachined structures.     

Studying the Transparency of Graphene for Low-Energy Electrons

The transparency of graphene membranes for electrons with energies in the range from 5 to 50 eV has been studied with a view to using graphene as an electrode stimulating field-induced emission in microand nanoelectronic devices. The behavior of electrons reflected from a membrane was analyzed with allowance for their return under the action of a retarding electric field. Low-energy electrons were represented by photoelectrons emitted from a diamond photocathode under the action of vacuum ultraviolet radiation.

     

Comparison of the NIST and PTB Air-Kerma Standards for Low-Energy X-Rays

A comparison has been made of the air-kerma standards for low-energy x rays at the National Institute of Standards and Technology (NIST) and the Physikalisch-Technische Bundesanstalt (PTB). The comparison involved a series of measurements at the PTB and the NIST using the air-kerma standards and two NIST reference-class transfer ionization chamber standards. Results are presented for the reference radiation beam qualities in the range from 25 kV to 50 kV for low energy x rays, including the techniques used for mammography dose traceability. The tungsten generated reference radiation qualities, between 25 kV and 50 kV used for this comparison, are new to NIST; therefore this comparison will serve as the preliminary comparison for NIST and a verification of the primary standard correction factors. The mammography comparison will repeat two previously unpublished comparisons between PTB and NIST. The results show the standards to be in reasonable agreement within the standard uncertainty of the comparison of about 0.4 %.     

Tunneling of Graphene Massive Dirac Fermions Through a Double Barrier

We study the tunneling of Dirac fermions in graphene through a double barrier potential. This is allowing the carriers to have an effective mass inside the barrier as generated by a lattice miss-match with the boron nitride substrate. The consequences of this gap opening on the transmission are investigated and the realization of resonant tunneling conditions is analyzed.