Richard Klemm

University of Central Florida | UCF · Department of Physics

Highly sensitive, broadly tunable detectors are needed for future sensing applications and quantum information systems. A promising material for these challenges comprises stacked graphene sheets having a “magic” twist angle between their in-plane symmetry axes. This material displays superconductivity with a ~ 2 K transition temperature. We invest...

The quantum spin Hall effect has been observed in topological insulators and long quantum wells using spin-orbit coupling as the probe, but it has not yet been observed in a metal. An experiment is proposed to measure the different Type-II quantum spin Hall effect of an electron or hole in a two-dimensional (2D) metal by using the previously unexplore...

We employ the Schr{\"o}dinger-Dirac method generalized to an ellipsoidal effective mass anisotropy in order to treat the spin and orbital effective mass anisotropies self consistently, which is important when Pauli-limiting effects on the upper critical field characteristic of singlet superconductivity are present. By employing the Klemm-Clem trans...

We employ the Schr{\"o}dinger-Dirac method generalized to an ellipsoidal effective mass anisotropy in order to treat the spin and orbital effective mass anisotropies self consistently, which is important when Pauli-limiting effects on the upper critical field characteristic of singlet superconductivity are present. By employing the Klemm-Clem trans...

The quantum spin Hall effect has been observed in topological insulators using spin-orbit coupling as the probe, but it has not yet been observed in a metal. An experiment is proposed to measure the quantum spin Hall effect of an electron or hole in a two-dimensional (2D) metal by using the previously unexplored but relativistically generated 2D qu...

For a spinless quantum particle in a one-dimensional box or an electromagnetic wave in a one-dimensional cavity, the respective Dirichlet and Neumann boundary conditions both lead to nondegenerate wave functions. However, in two dimensions, the symmetry of the box or microstrip antenna is an important feature, the details of which have often been o...

For a spinless quantum particle in a one-dimensional box or an electromagnetic wave in a one-dimensional cavity, the respective Dirichlet and Neumann boundary conditions both lead to non-degenerate wave functions. However, in two spatial dimensions, the symmetry of the box or microstrip antenna is an important feature that has often been overlooked...

To construct a microscopic theory of electrons and holes in anisotropic conductors that self-consistently treats their band effective mass anisotropy with their interactions with applied electric and magnetic fields, the Dirac equation is extended for an electron or hole in an orthorhombically-anisotropic conduction band. Its covariance is establis...

Current compact emitter and receiver technologies are generally inefficient and impractical at terahertz (THz) frequencies between 0.1 and 10 THz. Hence, a gap exists between mature microwave and developed optical technologies. On-chip, integrated broadly tunable and powerful quantum sources that coherently radiate THz waves between 0.1 and 11 THz...

Single crystals of FeSe1−xTex were grown for 0≤x≤0.41 by a flux method and at x=0.61 by chemical vapor transport in order to study the Te substitution effects on the quality of the single crystals and on their superconducting characteristics. From the in-plane resistivity data of the grown crystals, we found that the superconducting transition temp...

When an electron or hole is in a conduction band of a crystal, it can be very different from 2, depending upon the crystalline anisotropy and the direction of the applied magnetic induction ${\bf B}$. In fact, it can even be 0! To demonstrate this quantitatively, the Dirac equation is extended for a relativistic electron or hole in an orthorhombica...

From the Dirac equation of an electron in an anisotropic conduction band, the anisotropy of its motion dramatically affects its interaction with applied electric and magnetic fields. The quantum spin Hall effect (QSHE) is observable in two-dimensional metals without spin-orbit coupling. The dimensionality of the Zeeman interaction plays an importan...

The Dirac equation is extended for a relativistic electron in an orthorhombically-anisotropic conduction band. Its covariance is established with general proper and improper Lorentz transformations. In the non-relativistic limit, the kinetic and Zeeman energy terms of the Hamiltonian are both determined by the same three effective masses, and the q...

We discuss the cavity modes and radiation pattern in solid state terahertz (THz) sources based on layered high-temperature superconducting Bi2Sr2CaCu2O8+δ (Bi-2212). We experimentally measure the emission spectra using the Fourier transform infrared spectrometer in order to elucidate the radiation mechanism of the mesa samples. Moreover, we experim...

We studied the etching of small crystals of the high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) with various dilute compositions of hydrochloric and nitric acids. A particular mixture of those acids was chosen to simultaneously fabricate multiple rectangular stand-alone Bi2212 mesa structures from a large, doubly-cleaved and doubly metallic-coated...

Using our recent design of thermally managed sandwich device structures, we studied the radiation frequency characteristics of three such devices of the same rectangular dimensions made from the same single crystal of the high-Tc superconductor Bi2Sr2CaCu2O8+δ, and all three devices exhibit similar characteristics. Their observed radiation intensit...

A method is proposed to extend the zero-temperature Hall-Klemm microscopic theory of the Knight shift K in an anisotropic and correlated, multi-band metal to calculate K ( T ) at finite temperatures T both above and into its superconducting state. The transverse part of the magnetic induction B ( t ) = B 0 + B 1 ( t ) causes adiabatic changes suita...

We show for high-symmetry disk, square, or equilateral triangular thin microstrip antennas of any composition respectively obeying C∞v, C4v, and C3v point group symmetries, that the transverse magnetic electromagnetic cavity mode wave functions are restricted in form to those that are one-dimensional representations of those point groups. Plots of...

Joule heating is the central issue in order to develop high-power and high-performance terahertz (THz) emission from mesa devices employing the intrinsic Josephson junctions in a layered high transition-temperature Tc superconductor. Here, we describe a convenient local thermal measurement technique using charge-coupled-device-based thermoreflectan...

We show for high-symmetry disk, square, or equilateral triangular thin microstrip antennas of any composition respectively obeying C ∞v , C 4v , and C 3v point group symmetries, that the transverse magnetic electromagnetic cavity mode wave functions are restricted in form to those that are one-dimensional representations of those point groups. Plot...

A terahertz (THz) wave emitter using the stack of intrinsic Josephson junctions present in the high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) has been developed. By applying a dc voltage V across the stack, the ac-Josephson effect converts this to an ac-current that emits photons at the Josephson frequency proportional to V. The Bi2212 device also...

We show for thin square microstrip antennas that the transverse magnetic electromagnetic cavity modes are greatly restricted in number due to the point group symmetry of a square. For the ten lowest frequency emissions, we present plots of the orthonormal wave functions and of the angular distributions of the emission power obtained from the unifor...

We study the transverse magnetic (TM) electromagnetic cavity mode wave functions for an ideal equilateral triangular microstrip antenna (MSA) exhibiting $C_{3v}$ point group symmetry. When the $C_{3v}$ operations are imposed upon the antenna, the TM($m,n$) modes with wave vectors $\alpha \sqrt[ ]{m^2 + nm + n^2}$ are much less dense than commonly t...

We study the transverse magnetic (TM) electromagnetic cavity mode wave functions for an ideal equilateral triangular microstrip antenna (MSA) exhibiting C 3v point group symmetry. When the C 3v operations are imposed upon the antenna, the TM(m,n) modes with wave vectors are much less dense than commonly thought. The R 3 operations restrict the inte...

Operating high-T c superconducting intrinsic Josephson junction emitters at 77 K, coherent and continuous THz EMWs with an intensity of several tens of μW can be generated, which provides us with sufficient power for the simple imaging operation. The frequency range may be extended widely as high as 2.4 THz from 0.4 THz. An example is given.

Excessive Joule heating of conventional rectangular mesa devices of the high-transition-temperature superconductor Bi2Sr2CaCu2O leads to hot spots, in which the local temperature . Similar devices without hot spots are known to obey the ac-Josephson relation, emitting sub-terahertz (THz) waves at frequencies , where V is the applied dc voltage or e...

Using recently-developed THz emitters constructed from single crystals of the high-T-c superconductor Bi2Sr2CaCu2O8+delta, we performed three prototype tests of the devices to demonstrate their unique characteristic properties for various practical applications. The first is a compact and simple transmission type of THz imaging system using a Stirl...

We present a microscopic model of nuclear magnetic resonance in metals. The spin-1/2 local nucleus and its surrounding orbital electrons interact with the arbitrary constant and perpendicular time-oscillatory magnetic inductions and with each other via an anisotropic hyperfine interaction. An Anderson-like Hamiltonian describes the excitations of t...

We present a microscopic model of nuclear magnetic resonance in metals. The spin-1/2 local nucleus and its surrounding orbital electrons interact with the arbitrary constant B0 > and perpendicular time-oscillatory magnetic inductions B1(t) > and with each other via an anisotropic hyperfine interaction. An Anderson-like Hamiltonian describ...

A liquid helium-free, compact and continuous sub-terahertz radiation system operating at 77 K has been developed using a rectangular mesa device made from a high -superconducting Bi2Sr2CaCu2O single crystal, based on a different design of a stand-alone mesa sandwich structure to reduce the dc-current Joule heating effects. The mesa was thermally co...

Journal of Physics Condensed Matter Letter in press

A superconducting THz emitter immersed in liquid nitrogen

The intrinsic Josephson junctions (IJJs) in the high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) are shown to have great potential for the construction of an oscillator emitting in the terahertz-frequency f regime. However, earlier devices with Bi2212 substrates exhibit strong self-heating effects detrimental to their operation and limiting the maxi...

Strong, monochromatic, coherent and continuous terahertz (THz) radiation was generated from the intrinsic Josephson junctions in a cylindrical stand-alone mesa sandwich structure fabricated from a single crystal of the high-temperature superconductor Bi2Sr2CaCu2 O 8 + δ . By varying the base temperature and the dc bias current-voltage characteri...

The influence of electrode position on the current-voltage characteristics (IVC) and broadly tunable terahertz (THz) radiation emitted from two pentagonal mesa structures consisting of the intrinsic Josephson junctions in a high-T c superconducting device is investigated.

We examined the effect of the bias current position on the radiation of intense and coherent terahertz electromagnetic waves from the intrinsic Josephson junctions (IJJs) in regular pentagonal mesas of single crystalline high- superconducting Bi2Sr2CaCu2O8+δ. The emission from Sample 1, a mesa with a current bias feed on its middle, is strong and s...

To obtain higher power P and frequency f emissions from the intrinsic Josephson junctions in a high-T c superconducting Bi2Sr2CaCu2O8+ δ single crystal, we embedded a rectangular stand-alone mesa of that material in a sandwich structure to allow for efficient heat exhaust. By varying the current-voltage (I-V) bias conditions and the bath temperatur...

Transition metal dichalcogenides (TMDs) are quasi-two-dimensional layered compounds exhibiting strongly competing charge-density wave (CDW) and superconducting (SC) order parameters (OPs). The weak van der Waals interlayer bonding between hexagonal layers of octahedral or trigonal prismatic TMD building blocks allows for many polytypes. The non-sup...

Simultaneous measurements of spectroscopic terahertz emissions from and SiC photoluminescent local temperature T(r) distributions of high transition temperature Tc superconducting Bi2Sr2CaCu2O8+ δ rectangular mesa devices were made. A local region with T(r) > Tc known as a hot spot can emerge with current bias changes. When the hot spot position...

High-power, continuous, broadly tunable THz radiation from 0.29 to 1.06 THz, was obtained from the outer current-voltage characteristic (IVC) branch of a single stand-alone mesa of the high-transition temperature Tc superconductor Bi2Sr2CaCu2O8+ δ . The particular metallic film structures placed both beneath and atop the mesas resulted in more eff...

Recent developments of coherent terahertz (THz) oscillators based on the intrinsic Josephson junctions (IJJs) in mesas of the high temperature superconductor Bi2Sr2CaCu2O8+δ are reviewed. Experimental and theoretical studies of the emission from equilateral, right-angled isosceles, and acute isosceles triangular mesas are compared with those obtain...

We calculate the temperature $T$ and angular $(\theta,\phi)$ dependence of the upper critical induction $B_{c2}(\theta,\phi,T)$ for parallel-spin superconductors with an axially symmetric $p$-wave pairing interaction pinned to the lattice and a dominant ellipsoidal Fermi surface (FS). For all FS anisotropies, the chiral Scharnberg-Klemm state $B_{c...

We calculate the angular and temperature $T$ dependencies of the upper critical field $H_{c2}(\theta,\phi,T)$ for the $C_{4v}$ point group helical $p$-wave states, assuming a single uniaxial ellipsoidal Fermi surface, Pauli limiting, and strong spin-orbit coupling that locks the spin-triplet $\vec{\bf d}$-vectors onto the layers. Good fits to the S...

Terahertz (THz) electromagnetic radiation emitted from single and series-connected rectangular mesa devices of high-Tc superconducting Bi2Sr2CaCu2O8+δ is investigated spectroscopically during simultaneous temperature distribution observations using a microcrystalline SiC photoluminescence technique. In single mesas, a hot-spot region with its tempe...

From the photoluminescence of SiC microcrystals uniformly covering a rectangular mesa of the high transition temperature Tc superconductor Bi2Sr2CaCu2O8+δ, the local surface temperature T (r) was directly measured during simultaneous sub-THz emission from the N ̃103 intrinsic Josephson junctions (IJJs) in the mesa. At high bias currents I and low b...

A computed tomography (CT) imaging system using monochromatic sub-terahertz coherent electromagnetic waves generated from a device constructed from the intrinsic Josephson junctions in a single crystalline mesa structure of the high-Tc superconductor Bi2Sr2CaCu2O8+δ was developed and tested on three samples: Standing metallic rods supported by styr...

We consider a model for ferromagnetic superconductors with two general ellipsoidal Fermi surfaces (FSs), one for each spin projection, $\sigma=\{\uparrow,\downarrow\}$, each with three distinct single particle effective masses, $\{m_{i\sigma}\}$. We study this double ellipsoidal FS model in the presence of an arbitrarily oriented magnetic induction...

Using a modified mesa structure of high-Tc superconducting Bi2Sr2CaCu2O8+δ with a thin underlaying base superconductor (∼3 μm), the effective working temperature of the continuous and monochromatic terahertz emitter is extended up to 70 K, and the maximum power of ∼30 μW at 0.44 THz is achieved at the relatively high temperature of T b = 55 K in a...

In order to understand the radiation observed from the intrinsic Josephson junctions in triangular Bi2Sr2CaCu2O8+δ mesas, we calculate the transverse magnetic (TM) electromagnetic modes for thin equilateral cavities. A new set of distinct but degenerate TM modes coexists with the known modes of Helszajn and James, but are expected to lead to distin...

We report on intense and coherent terahertz (THz) electromagnetic (EM) waves emitted from equilateral triangular mesa structures of the intrinsic Josephson junctions (IJJs) in single crystalline high-Tc superconducting Bi2Sr2CaCu2O8+δ. The focused ion beam milling technique is used for mesa fabrication. THz radiation is observed when the emission f...

From the photoluminescence of SiC microcrystals uniformly covering a rectangular mesa of the high transition temperature $T_c$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, the local surface temperature $T({\bm r})$ was directly measured during simultaneous sub-THz emission from the $N\sim10^3$ intrinsic Josephson junctions (IJJs) in the mesa....

Mesa structures of the high-temperature superconductor Bi2Sr2CaCu2O8+δ have been prepared in a variety of ways. Groove mesas have so far been made in rectangular, square, circular, triangular, and pentagonal shapes. There are distinct differences in the properties of the radiation that depend strongly on the type and shape of the mesas. Two types o...

In 2007, the first observation of the coherent terahertz (THz) electromagnetic (EM) waves from a mesa structures of intrinsic Josephson junctions (IJJs) in high temperature superconductor Bi2Sr2CaCu2O8+δ (Bi-2212) is reported [1]. The ac-Josephson effect as well as the cavity resonance conditions is considered as the principle mechanism of the THz...

In order to determine if the mesa geometry might affect the properties of the coherent terahertz (THz) radiation emitted from the intrinsic Josephson junctions in mesas constructed from single crystals of the high-temperature superconductor, Bi2Sr2CaCu2O8+δ, we studied triangular mesas. For equilateral triangular mesas, the observed emission was fo...

Tunable terahertz (THz) radiation emitted from the triangular mesa structures consisting of the intrinsic Josephson junctions in single-crystalline high-Tc superconducting Bi2Sr2CaCu 2O8+δ is investigated using angular distribution measurements of the radiation power and Fourier transform infrared spectroscopy.

We extended the Scharnberg-Klemm theory of Hc2(T) in p-wave superconductors with broken symmetry to cases of partially broken symmetry in an orthorhombic crystal, as is appropriate for the more exotic ferromagnetic superconductor UCoGe in strong magnetic fields. For some partially broken symmetry cases, Hc2(T) can mimic upward curvature in two crys...

Coherent and continuous radiation sources of the electromagnetic (EM) waves at terahertz (1 THz = 1012 c/s) frequencies using a mesa structure fabricated from high temperature superconducting Bi2Sr2CaCu2O8+δ single crystals are described with a special emphasis on the physics of the radiation mechanism and the applications. After the intensive stud...

The leading contributions to the c-axis conductivity of layered superconductors arising from superconducting fluctuations of the order parameter are discussed for arbitrary intralayer scattering. The contributions from fluctuations of the normal quasiparticle density of states are shown to be opposite in sign to the Aslamazov-Larkin and Maki-Thomps...

In superconducting ferromagnets for which the Curie temperature Tm exceeds the superconducting transition temperature Tc , it was suggested that ferromagnetic spin fluctuations could lead to superconductivity with p -wave spin-triplet Cooper pairing. Using the Stoner model of itinerant ferromagnetism, we study the feedback effect of the p -wave sup...

We calculate the angular dependence of the power of stimulated terahertz amplified radiation (STAR) emitted from a dc voltage applied across a stack of intrinsic Josephson junctions. During coherent emission, we assume that a spatially uniform ac Josephson current density in the stack acts as a surface electric current density antenna source, and t...

We consider the Ginzburg-Landau free energy of the anisotropic mass form in the presence of a magnetic field of arbitrary fixed direction. It is shown that the free energy may be transformed into the isotropic Ginzburg-Landau form with a kappa that depends upon the direction of the magnetic induction B--> relative to the crystal lattice. The lower...

The angular dependence of the radiation-zone output power and electric polarization of stimulated terahertz amplified radiation (STAR) emitted from a dc voltage applied across cylindrical and rectangular stacks of intrinsic Josephson junctions is calculated. The boundary conditions are obtained from Love's equivalence principles. During coherent em...

The upper critical field Hc 2 in layered superconductors is calculated from a microscopic theory in which the electrons are assumed to propagate freely within the individual layers subject to scattering off impurities and to propagate via tunneling between the layers. For the magnetic field parallel to the layers, there is a temperature T*<Tc below...