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We studied and improved the performance of a terahertz (THz) direct detector based on the superconducting NbN hot electron bolometer (HEB) integrated with microwave readout for imaging arrays and other applications. A 6 nm thick NbN microbridge at the feed point of a twin-slot antenna is embedded into a high-Q quarter-wavelength coplanar waveguide (CPW) resonator made from a 350 nm thick NbN film which is weakly coupled to a CPW feedline. THz direct detection was performed at a signal frequency of 0.66 THz at a bath temperature of 4.2 K. When operating at a low input microwave power level, the device acting as a microwave kinetic inductance detector with a kinetic inductance fraction of 0.45 changes the surface impedance of the resonator in response to incident THz radiation. The estimated optical noise equivalent power (NEP) is ∼0.23 pW · Hz−0.5 which shows about one order of magnitude improvement compared to that of the traditional voltage-biased NbN HEBs. The improved performance of the THz direct detectors with lower NEP and easy readout makes them promising in imaging with a large number of the array pixels. [ABSTRACT FROM AUTHOR]

This paper studies the suppression of superconducting transition temperature (Tc) of ultrathin NbN film. We fabricated epitaxial NbN superconducting thin films of thicknesses ranging from 2.5 to 100 nm on single crystal MgO (100) substrates by dc magnetron sputtering. We performed structure analyses and measured their electric and far infrared properties. The experimental results were compared with several mechanisms of the suppression of superconductivity proposed in the literature, including the weak localization effect, the proximity effect, and quantum size effect (electron wave leakage model). We found that the electron wave leakage model matches best to the experimental data. [ABSTRACT FROM AUTHOR]

We report on Bi2Sr2CaCu2O8 (BSCCO) intrinsic Josephson junction stacks with improved cooling, allowing for a remarkable increase in emission frequency compared to the previous designs. We started with a BSCCO stack embedded between two gold layers. When mounted in the standard way to a single substrate, the stack emits in the range of 0.43-0.82 THz. We then glued a second, thermally anchored substrate onto the sample surface. The maximum voltage of this better cooled and dimension-unchanged sample was increased and, accordingly, both the emission frequencies and the tunable frequency range were significantly increased up to 1.05 THz and to 0.71 THz, respectively. This double sided cooling may also be useful for other "hot" devices, e.g., quantum cascade lasers. [ABSTRACT FROM AUTHOR]

Copyrolysis of two waste wood samples with block polypropylene were investigated for the reaction characteristics and properties of the pyrolytic oil. The thermal decomposition of the mixture led to an increase in the temperature range for the decomposition of polypropylene, which implied an interaction in the mixture. New tar compounds were also identified from the copyrolysis using pyrolysis gas chromatography/mass spectrometry. The oil produced from the mixtures in a batch type reactor had an increase in the mass yield, with higher C and H, and lower moisture contents. [ABSTRACT FROM AUTHOR]

Lattice mismatch between NbN and silicon (Si) reduces the superconducting properties of NbN film on Si substrate, and this in turn affects the performance of devices such as the hot electron bolometer (HEB) and superconducting nanowire single photon detector (SNSPD). We have found that the superconducting properties of NbN film on Si will be significantly improved by a Nb5N6 buffer layer. The strain of the NbN film was optimized by varying the thickness of the buffer layer. With 30 nm thick Nb5N6, the zero resistance superconducting transition temperature (TC0) of a 6 nm thick NbN film on Si is up to 13.5 K and the critical current density (JC) of the film is more than 107 A cm−2. All the details of preparation, improvement and characteristics of this film are also presented. [ABSTRACT FROM AUTHOR]

We have combined a stand-alone Bi2Sr2CaCu2O8 intrinsic Josephson junction stack, emitting terahertz radiation, with a YBa2Cu3O7 grain boundary Josephson junction acting as detector. The detector is mounted on a lens, positioned 1.2 cm away from the emitter on a similar lens. With the emitter radiating at 0.5 THz, we observed up to 7 Shapiro steps on the current-voltage characteristic of the detector. The ac current induced in this junction was 0.9 mA, and the dissipated power was 1.8 μW. The setup, although far from being optimized, may be considered as a first step towards an integrated high-Tc receiver. [ABSTRACT FROM AUTHOR]

Complex conductivity of FeSe0.5Te0.5 superconducting films has been investigated using backward-wave-oscillator spectroscopy in the terahertz frequency range. The normal state conductivity corresponds well to the lowfrequency limit of a Drude metal. In the superconducting state, the real part of the complex conductivity is not completely suppressed, indicating the existence of a strongly anisotropic gap. Weak features in the conductivity are consistent with the superconducting gap value close to 12 cm-1(1.5 meV). The experimental results are discussed within the framework of a two-gap model based on the s± symmetry of the order parameter. [ABSTRACT FROM AUTHOR]

Superconducting receivers based on large hot electron bolometer (HEB) bridges were constructed and they show low noise temperatures. Their stability has been characterized by the Allan variance measurements with either far-infrared laser or Gunn oscillator plus multiplier chain as the local oscillator. An Allan time of about 1 s has been obtained. Also demonstrated is the effectiveness of a stabilization scheme in which a radiation at a frequency much lower than the gap frequency of niobium nitride is used for improvement. [ABSTRACT FROM AUTHOR]

The terahertz dielectric response of LuFe2O4 is investigated by terahertz time-domain spectroscopy over a temperature range of 6–290 K. It is revealed that besides the central mode associated with the charge-ordered state, a soft TO1 mode at below ∼240 K is identified indicating the existence of displacing ferroelectricity, in addition to the charge-ordering-induced ferroelectricity at below 320 K. The anomaly of the soft mode at ∼180 K reflects the magnetoelectric correlation between the soft TO1 mode and the spin/charge fluctuations revealed recently. Finally, the magnetic property at below ∼240 K is discussed. [ABSTRACT FROM AUTHOR]

We report on terahertz (THz) surface impedance measurement of an epitaxial MgB2 thin film using time domain THz spectroscopy. We show that the surface resistance of the MgB2 film is much lower than that of YBa2Cu3O7-δ and copper in the THz range. A linear dependence of the surface reactance on frequency is observed, yielding a penetration depth of about 100 nm at low temperatures. The measurements agree qualitatively with calculations based on impurity scattering in the Born limit. Our results clearly indicate that MgB2 thin films have a great potential for THz electronic applications. [ABSTRACT FROM AUTHOR]

Superconductor is a compelling plasmonic medium at terahertz frequencies owing to its intrinsic low Ohmic loss and good tuning property. However, the microscopic physics of the interaction between terahertz wave and superconducting plasmonic structures is still unknown. In this paper, we conducted experiments of the enhanced terahertz transmission through a series of superconducting NbN subwavelength hole arrays, and employed microscopic hybrid wave model in theoretical analysis of the role of hybrid waves in the enhanced transmission. The theoretical calculation provided a good match of experimental data. In particular, we obtained the following results. When the width of the holes is far below wavelength, the enhanced transmission is mainly caused by localized resonance around individual holes. On the contrary, when the holes are large, hybrid waves scattered by the array of holes dominate the extraordinary transmission. The surface plasmon polaritions are proved to be launched on the surface of superconducting film and the excitation efficiency increases when the temperature approaches critical temperature and the working frequency goes near energy gap frequency. This work will enrich our knowledge on the microscopic physics of extraordinary optical transmission at terahertz frequencies and contribute to developing terahertz plasmonic devices. [ABSTRACT FROM AUTHOR]

A mean-field theory based on a modified transverse-field Ising model including the multifold interactions in Sr1-xCaxTiO3 over a broad composition range from x = 0.0 to 0.4 is developed and the full-scale phase diagram consistent with experiments is evaluated. In this theory, both the ferroelectric distortion and antiferrodistortive effects induced by Ca2+ substitution of Sr2+ are taken into account. It is revealed that the competition between those two types of lattice distortion effects plus the quantum fluctuations represent the core physics underlying the whole set of characteristics in Sr1-xCaxTiO3 up to x = 0.4. The quantum fluctuations dominant state (quantum paraelectrics state), ferroelectrically distorted state, and antiferrodistortive state are successfully predicted, upon increasing substitution level x. It is also predicted that the antiferrodistortive state in the high substitution level is the origin of antiferroelectric order which eventually suppresses the ferroelectric state. [ABSTRACT FROM AUTHOR]

The terahertz dielectric response of LuFe2O4 is investigated by terahertz time-domain spectroscopy over a temperature range of 6–290 K. It is revealed that besides the central mode associated with the charge-ordered state, a soft TO1 mode at below ∼240 K is identified indicating the existence of displacing ferroelectricity, in addition to the charge-ordering-induced ferroelectricity at below 320 K. The anomaly of the soft mode at ∼180 K reflects the magnetoelectric correlation between the soft TO1 mode and the spin/charge fluctuations revealed recently. Finally, the magnetic property at below ∼240 K is discussed. [ABSTRACT FROM AUTHOR]

This paper reports a type of low Ohmic loss terahertz (THz) metamaterials made from low-temperature superconducting niobium nitride (NbN) films. Its resonance properties are studied by THz time domain spectroscopy. Our experiments show that its unloaded quality factor reaches as high as 178 at 8 K with the resonance frequency at around 0.58 THz, which is about 24 times that of gold metamaterial at the same temperature. The unloaded quality factor keeps at a high level, above 90, even when the resonance frequency increases to 1.02 THz, which is close to the gap frequency of NbN film. All these experimental observations fit well into the framework of Bardeen-Copper-Schrieffer theory and equivalent circuit model. These new metamaterials offer an efficient way to the design and implementation of high performance THz electronic devices.

The temperature dependence of the real part of the microwave complex conductivity at 17.9 GHz obtained from surface impedance measurements of two c-axis oriented MgB2 thin films reveals a pronounced maximum at a temperature around 0.6 times the critical temperature. Calculations in the frame of a two-band model based on Bardeen-Cooper-Schrieffer (BCS) theory suggest that this maximum corresponds to an anomalous coherence peak resembling the two-gap nature of MgB2. Our model assumes there is no interband impurity scattering and a weak interband pairing interaction, as suggested by band structure calculations. In addition, the observation of a coherence peak indicates that the pi band is in the dirty limit and dominates the total conductivity of our films.