Drude-like Response Research Articles

Molecular semiconductors and the Ioffe–Regel criterion: A terahertz study on band transport in DBTTT

Terahertz electromodulation spectroscopy provides insight into the physics of charge carrier transport in molecular semiconductors. The work focuses on thin-film devices of dibenzothiopheno[6,5-b:6′,5′-f]thieno[3,2-b]thiophene. Frequency-resolved data show a Drude-like response of the hole gas in the accumulation region. The temperature dependence of the mobilities follows a T1/2 power law. This indicates that the thermal mean free path of the charge carriers is restricted by disorder. Only a fraction of approximately 5% of the injected carriers fulfills the Ioffe–Regel criterion and participates in band transport.

Probing Interface of Perovskite Oxide Using Surface-Specific Terahertz Spectroscopy

The surface/interface species in perovskite oxides play essential roles in many novel emergent physical phenomena and chemical processes. With low eigen-energies in the terahertz region, such species at buried interfaces remain poorly understood due to the lack of feasible surface-specific spectroscopic probes to resolve the resonances. Here, we show that polarized phonons and two-dimensional electron gas at the interface can be characterized using surface-specific nonlinear optical spectroscopy in the terahertz range. This technique uses intra-pulse difference frequency mixing process, which is allowed only at the surface/interface of a centrosymmetric medium. Submonolayer sensitivity can be achieved using the state-of-the-art detection scheme for the terahertz emission from the surface/interface. Through symmetry analysis and proper polarization selection, background-free Drude-like nonlinear response from the two-dimensional electron gas emerging at the LaAlO 3 /SrTiO 3 or Al 2 O 3 /SrTiO 3 interface was successfully observed. The surface/interface potential, which is a key parameter for SrTiO 3 -based interface superconductivity and photocatalysis, can now be determined optically in a nonvacuum environment via quantitative analysis on the phonon spectrum that was polarized by the surface field in the interfacial region. The interfacial species with resonant frequencies in the THz region revealed by our method provide more insights into the understanding of physical properties of complex oxides.

CMOS-compatible direct laser writing of sulfur-ultrahyperdoped silicon: Breakthrough pre-requisite for UV-THz optoelectronic nano/microintegration

Direct laser writing/ultra-hyperdoping technology was harnessed to fabricate a universal CMOS-compatible Si-based broadband (UV-THz) n-p junction detector, with its spectral response controlled by external liquid helium-nitrogen cooling. Near-far IR (2–40 μm) temperature-dependent photoconductivity (PC) was revealed in a biased n-p junction, made of n-type sulfur-ultrahyperdoped Si (uh-Si, sulfur content ∼1 at. %) sub-micron-thick layer on a surface of a p-doped Si substrate. The observed broadband photoconductivity is provided by a dense quasi-continuum series of sulfur-impurity donor states near the conduction band bottom (so-called “intermediate band”, the bandwidth ≈0.6 eV), corresponding to IR-absorbing neutral and singly-ionized substitutional atomic and cluster centers of sulfur. The “intermediate” donor band supports the gradual “blue” spectral migration of PC maximum due to thermal ionization of deeper states at the temperature increasing in the range of 5–250 K. As a result, in the temperature range of 77–300 K complementary THz (wavenumbers <100 cm−1) spectroscopy indicates the predominating Drude-like conduction-band response of electrons and their gradually raising density ∼1019–1020 cm−3. Illustrating the thermal-ionization depletion of the donor sulfur states from the THz-probed plasma side, this unveils the unprecedently high concentration of electrically-active donor sulfur centers ∼1020 cm−3 and related donor-state density ∼1020 states/eV·cm3 in the uh-Si sample. Overall, these advances enable in situ laser writing of universal liquid helium/nitrogen cooled Si nano/micro/macrodetectors with broad – near-far IR and even THz – spectral response, crucial for photovoltaics, thermal and bio-imaging.

Revealing the simultaneous increase in transient transmission and reflectivity in InN

The ultrafast transient behavior of InN under intensive laser irradiation remains unclear. The simultaneous measurements of pump–probe transient transmission and reflectivity are reported in this study. The irradiation-induced change in the dielectric constant of InN films due to the generation of thermalized carriers gives rise to complex transient behavior, and simultaneous increase in both transient transmission and reflectivity are observed at certain wavelengths. Herein, transient transmission is interpreted as the occupation probability of thermalized electrons at the probing level originating from a hot Fermi–Dirac distribution, and our calculation results are in good agreement with the experiments. Likewise, the Drude-like response due to the collective motion of thermalized carriers causes the increase in transient reflectivity, which depends on the change of dielectric constant caused by the collective motion of thermalized carriers. The ultrafast carrier dynamics is modeled by calculating the temporal evolution of the occupation probability of thermalized electrons in the conduction band. On the basis of the two-temperature model, the electron–phonon scattering time is extrapolated to be ∼300 fs in InN, which dominates the cooling of excited electrons.

Graphene oxide synthesis using modified Tour method

Graphene oxide (GO) colloidal solution has been synthesised by the modified Tour method, in which the pH of the reaction medium has been increased to 2.0–2.2 at the final stage of graphite oxidation by adding NaOH solution. The solid-phase graphene oxide consists of multilayered graphene particles with a thickness of about 7.5 nm (9–10 layers of graphene) and an average size of about 7.7 nm. Reduced graphene oxide (rGO) has been prepared by hydrazine and microwave reduction. A comparative study of the structure, morphology, and electrical transport properties of rGO samples obtained by various methods has been carried out using XRD, SAXS, Raman spectroscopy, low-temperature nitrogen adsorption, and impedance spectroscopy. Structural analysis has shown the presence of two fractions of plate-like rGO particles for each reduction method, which consist of 4–6 layers of graphene in stacks and have a lateral size in the range of 7.1–7.6 nm. The BET specific surface area of the microwave-reduced rGO was higher than that of the chemically reduced one (296 and 237 m2 g−1 respectively). The frequency dependence (10−2–105 Hz) of the AC conductivity of the GO and rGO samples has been analysed in the temperature range of 25 °C–175 °C. For the GO sample, the proton exchange conductivity mechanism dominates. A Drude-like response of electrical conductivity at frequencies above 103 Hz with transition to Johnscher’s law response at 175 °C has been observed for rGO samples obtained using both chemical and microwave routes. Changes in activation energies and relaxation times have been interpreted using a model of a thermally activated frequency-dependent electron hopping mechanism between randomly coupled conducting sp 2 rGO packages separated by disordered sp 3 rGO regions with correspondingly higher resistivity.

Casimir Puzzle and Casimir Conundrum: Discovery and Search for Resolution

This paper provides a review of the complicated problems in Lifshitz theory describing the Casimir force between real material plates composed of metals and dielectrics, including different approaches to their resolution. For both metallic plates with perfect crystal lattices and any dielectric plates, we show that the Casimir entropy calculated in the framework of Lifshitz theory violates the Nernst heat theorem when the well-approved dielectric functions are used in computations. The respective theoretical Casimir forces are excluded by the measurement data of numerous precision experiments. In the literature, this situation has been called the Casimir puzzle and the Casimir conundrum for the cases of metallic and dielectric plates, respectively. This review presents a summary of both the main theoretical and experimental findings on this subject. Next, a discussion is provided of the main approaches proposed in the literature to bring the Lifshitz theory into agreement with the measurement data and with the laws of thermodynamics. Special attention is paid to the recently suggested spatially nonlocal Drude-like response functions, which consider the relaxation properties of conduction electrons, as does the standard Drude model, but lead to the theoretical results being in agreement with both thermodynamics and the measurement data through the alternative response to quantum fluctuations of the mass shell. Further advances and trends in this field of research are discussed.

Divergence of the dielectric constant in ultrathin granular metal films near the percolation threshold

We report on the electronic and optical properties of ultrathin granular films. We demonstrate that the static dielectric constant increases with thickness in the dielectric regime and diverges at the critical thickness, as predicted by classical percolation theory. However, for thicker samples, the dc conductivity does not obey scaling laws due to the presence of tunneling conduction. In this region the dielectric constant is positive, and the electronic transport is not metallic but can be described by Jonscher’s universal power law, even though there is a Drude-like response indicating the presence of free charge carriers. Only for thicker films when the dielectric constant becomes negative is there metallic conduction.

Magnetically driven band shift and metal-insulator transition in spin-orbit-coupled Sr3(Ir1−xRux)2O7

We report a combined infrared and angle-resolved photoemission study of the electronic response of $\mathrm{S}{\mathrm{r}}_{3}{(\mathrm{I}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{R}{\mathrm{u}}_{x})}_{2}{\mathrm{O}}_{7}$ ($x=0$, 0.22, 0.34). The low-temperature optical conductivities of the three compounds exhibit the characteristic feature of the effective total angular momentum ${J}_{\mathrm{eff}}=1/2$ antiferromagnetic Mott state. As the temperature increases across the antiferromagnetic ordering temperature ${T}_{\mathrm{N}}$, the indirect gap gradually closes whereas the direct gap remains open. In the optical conductivity of $\mathrm{S}{\mathrm{r}}_{3}{(\mathrm{I}{\mathrm{r}}_{0.66}\mathrm{R}{\mathrm{u}}_{0.34})}_{2}{\mathrm{O}}_{7}$ which shows a thermally driven insulator-metal transition at ${T}_{\mathrm{N}}$, a Drude-like response from itinerant carriers is registered in the paramagnetic phase. We observe in angle-resolved photoemission data of $\mathrm{S}{\mathrm{r}}_{3}{(\mathrm{I}{\mathrm{r}}_{0.66}\mathrm{R}{\mathrm{u}}_{0.34})}_{2}{\mathrm{O}}_{7}$ that the valence band shifts continuously toward the Fermi energy with the weakening of the antiferromagnetic order and crosses the Fermi level in the paramagnetic phase. Our findings demonstrate that the temperature-induced metal-insulator transition of the $\mathrm{S}{\mathrm{r}}_{3}{(\mathrm{I}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{R}{\mathrm{u}}_{x})}_{2}{\mathrm{O}}_{7}$ system should be attributed to a magnetically driven band shift.

Broadband angle- and permittivity-insensitive nondispersive optical activity based on planar chiral metamaterials

Because of the strong inherent resonances, the giant optical activity obtained via chiral metamaterials generally suffers from high dispersion, which has been a big stumbling block to broadband applications. In this paper, we propose a type of planar chiral metamaterial consisting of interconnected metal helix slat structures with four-fold symmetry, which exhibits nonresonant Drude-like response and can therefore avoid the highly dispersive optical activity resulting from resonances. It shows that the well-designed chiral metamaterial can achieve nondispersive and pure optical activity with high transmittance in a broadband frequency range. And the optical activity of multi-layer chiral metamaterials is proportional to the layer numbers of single-layer chiral metamaterial. Most remarkably, the broadband behaviors of nondispersive optical activity and high transmission are insensitive to the incident angles of electromagnetic waves and permittivity of dielectric substrate, thereby enabling more flexibility in polarization manipulation.

Phonon and Defect Induced Transparencies in the Mid-Infrared Spectrum of Grafted Single Layer Graphene

The Drude-like response of graphene in the terahertz and infrared region of the spectrum has made it attractive for optoelectronic applications in this range, because the response can be controlled by gating and doping. [1] Graphene infrared response can further be tailored for photonics and plasmonics, as the patterned material harbors low energy plasmon modes. [2] However, the infrared spectrum of pristine single layer graphene (SLG) is monotonous; in contrast to Raman, there are no infrared-active phonon modes, while bilayer graphene displays a Fano resonance in the infrared at ~1600 cm-1. [3] In a first time, we show experimentally that grafting SLG with halogenophenyl moieties induces optical transparencies at two specific energies: 1250 and 1600 cm-1 [4], in close similarity to the bands that can also be observed in carbon nanotubes as Fano resonances. [5] Unlike bands caused by the absorption of light by vibrational modes, these antiresonances show a decrease of the absorbance, an optical transparency effect. Moreover, we show that the amplitude of the transparencies can be modulated by changing the charge carrier density through doping, and by the defect density through controlled grafting. In as second time, we will present a theory based on quantum mechanics to calculate the optical conductivity of grafted SLG. [4] The model puts into play phonon modes with momenta different from Γ that can be addressed through scattering on defects. Numerical simulations reproduce the experimental data with good agreement. The theory also captures the dependence of the signal on charge carrier density and defect density. Our findings bring a new understanding for the physics behind the infrared activity of nanostructures, while opening new capabilities for tailoring the optical spectrum of nanomaterials.

Phonon-Electron Scattering Limits Free Charge Mobility in Methylammonium Lead Iodide Perovskites.

The nature of the photoconductivity in solution-processed films of methylammonium lead iodide perovskite is investigated by determining the variation of the photoconductive response with temperature. Ultrabroadband terahertz (THz) photoconductivity spectra in the 0.3-10 THz range can be reproduced well by a simple Drude-like response at room temperature, where free charge carrier motion is characterized by an average scattering time. The scattering time determined from Drude fits in the 0.3-2THz region increases from ∼4 fs at 300 K (tetragonal phase; mobility of ∼27 cm(2) V(-1) s(-1)) to almost ∼25 fs at 77 K (orthorhombic phase, mobility of ∼150 cm(2) V(-1) s(-1)). For the tetragonal phase (temperature range 150 < T < 300 K) the scattering time shows a ∼T(-3/2) dependence, approaching the theoretical limit for pure acoustic phonon (deformation potential) scattering. Hence, electron-phonon, rather than impurity scattering, sets the upper limit on free charge transport for this perovskite.

Ultrafast charge and lattice dynamics in one-dimensional Mott insulator of CuO-chain compoundCa2CuO3investigated by femtosecond absorption spectroscopy

Charge dynamics in one-dimensional (1D) Mott insulators was investigated by femtosecond pump-probe absorption spectroscopy on ${\mathrm{Ca}}_{2}{\mathrm{CuO}}_{3}$. An irradiation of a femtosecond laser pulse gives rise to a Drude-like response due to the nature of spin-charge separation characteristic of 1D Mott insulators. The photoinduced metallic state decays with \ensuremath{\sim}30 fs, and photocarriers are localized as polarons via charge-phonon coupling which produce a broad midgap absorption. Calculations with the extended Hubbard-Holstein model suggest that the peak structure of the midgap absorption is a magnon sideband of the polaron absorption. This demonstrates that charge-spin coupling becomes effective via charge-phonon coupling in polarons.

Terahertz-infrared spectroscopy of overdoped manganites La1−xCaxMnO3

Terahertz and infrared spectra of dielectric permittivity and optical conductivity of overdoped manganites La1−xCaxMnO3 (0.5≤x≤1) have been measured at frequencies ν from 4cm−1 to 700cm−1 and at temperatures T from 5K to 300K. The samples were prepared in the form of bulk polycrystals (ceramics) and epitaxial films (free-standing and on MgO substrates). Strongly asymmetric absorption bands have been found in the compounds which are in charge-ordered state (0.5<x<0.85) and have commensurate calcium doping. The bands are located close to the position of lowest energy van Hove singularity of the folded Brillouin zone. The bands are associated with a boson peak, i.e., with the excess of lattice acoustical vibrational states over the standard Debye contribution (~ν2). In addition, Raman scattering measurements on ceramic samples have been conducted, which confirmed our assignment of the bands’ origin. We firmly identify contribution to the low energy spectra due to quasi-free charge carriers. In the paramagnetic phase all samples show a Drude-like response corresponding to the band type of charge transport. At low temperatures, for the range of dopings 0.5≤x<0.85, the conductivity spectra show signatures of variable-range hopping mechanisms of transport.

Nondispersive optical activity of meshed helical metamaterials.

Extreme optical properties can be realized by the strong resonant response of metamaterials consisting of subwavelength-scale metallic resonators. However, highly dispersive optical properties resulting from strong resonances have impeded the broadband operation required for frequency-independent optical components or devices. Here we demonstrate that strong, flat broadband optical activity with high transparency can be obtained with meshed helical metamaterials in which metallic helical structures are networked and arranged to have fourfold rotational symmetry around the propagation axis. This nondispersive optical activity originates from the Drude-like response as well as the fourfold rotational symmetry of the meshed helical metamaterials. The theoretical concept is validated in a microwave experiment in which flat broadband optical activity with a designed magnitude of 45° per layer of metamaterial is measured. The broadband capabilities of chiral metamaterials may provide opportunities in the design of various broadband optical systems and applications.

Carrier dynamics of rubrene single-crystals revealed by transient broadband terahertz spectroscopy

Carrier dynamics of an organic molecular semiconductor, rubrene, was investigated by optical-pump terahertz-probe spectroscopy from 1 to 15 THz. At 294 K, a Drude-like response due to photogenerated hole carriers is observed below 8 THz. The real part σ1(ω) of the optical conductivity is suppressed below 2 THz, indicating the presence of a localization effect. Such a spectral feature was reproduced by a Drude-Anderson model including the effect of dynamical disorder due to intermolecular vibrations. At 50 K, the spectral weight of σ1(ω) due to photocarriers shifts to lower frequency below 4 THz and the suppression of σ1(ω) is hardly observed, which we associate with a reduction of thermal molecular motions. The overall photocarrier generation and recombination dynamics is also discussed.

Optical properties of graphite oxide and reduced graphite oxide

We studied the optical properties of a graphite oxide and a reduced graphite oxide by using the optical spectroscopic technique. The graphite oxide does not show a finite dc conductivity and has several characteristic absorption modes in the mid-infrared region, caused by an epoxide functional group and hydroxyl and carboxyl moieties in the mid-infrared range. The reduced graphite oxide shows a Drude-like response in the far-infrared region and the estimated dc conductivity and electric mobility are around 200 Ω−1cm−1 and ∼100 cm2V−1s−1, respectively. We found that the optical conductivity cannot be fitted with a simple Drude model, which indicates that the charge carriers are correlated. We applied an extended Drude model and obtained the optical scattering rate and the optical effective mass. We found that the optical effective mass can carry information of both the enhanced mass by correlation and the electronic band structure.

Electron–phonon coupling in the superconducting single crystal [formula omitted

The optical properties of the superconducting single crystal Rb0.8Fe1.68Se2 with Tc≃31K have been measured over a wide frequency range in the ab plane. We found that the optical conductivity is dominated by a series of infrared-active phonon modes at low-frequency region as well as several other high-frequency bound excitations. The low-frequency optical conductivity has rather low value and shows quite small Drude-like response, indicating low carriers density in this material. Furthermore, the phonon modes increase continuously in frequency with decreasing temperature; specifically, the phonon mode around 200cm-1 shows an enhanced asymmetry effect at low temperatures, suggesting an increasing electron–phonon coupling in this system.

Temperature dependence of the optical response of perovskite-type SrIrO3 thin film

We report on the temperature-dependent optical absorption spectra of an orthorhombic SrIrO3 thin film in the energy region between 0.08 and 6 eV. The optical absorption spectra show the characteristic spectral features associated with spin-orbit coupling-induced effective total angular momentum Jeff states. A Drude-like response from the electrons in the Jeff = 1/2 bands is observed in the energy region below about 0.5 eV followed by an interband transition from the Jeff = 3/2 to the Jeff = 1/2 bands at 0.7 eV. The electron excitations between the Jeff bands exhibit a strong temperature dependence that is different from the behavior of conventional metals. The spectral weight analysis of the absorption data suggests an anomalous low-frequency electrodynamic behavior for of SrIrO3.

Anomalous domain-wall conductance in pyrochlore-typeNd2Ir2O7on the verge of the metal-insulator transition

We have investigated the charge transport and low-energy dynamics originating from the antiferromagnetic (AF) domain wall (DW) in pyrochlore-type Nd${}_{2}$Ir${}_{2}$O${}_{7}$ with the all-in all-out type AF ground state by means of the magneto-resistance measurement and the terahertz time-domain spectroscopy. The resistivity is much lower in the multi-magnetic-domain state than in the nearly single-magnetic-domain one. Furthermore, the terahertz optical conductivity spectrum unravels a Drude-like response only in the multidomain state; the Drude-like absorption is strongly suppressed with increasing field and irreversibly disappears above 5 T. These observations suggest that the AF DW is highly conductive with a minimal damping constant of 2 meV, contrary to the fully-gapped (45 meV) AF insulating state of bulk. We discuss the nature of metallic states at AF DWs and their possible relevance to the gapless edge state inherent to the Weyl semimetal.

Signature of bulk longitudinal plasmon-polaritons in the transmission/reflection spectra of one-dimensional metamaterial heterostructures

A systematic theoretical study of the transmission and reflection spectra of a 1D metamaterial heterostructure consisting of the finite repetition of a double layer of two different building blocks A (normal non-dispersive layer) and B (dispersive metamaterial) is presented, with the multi-layered system sandwiched between two semi-infinite layers of the A material. For the metamaterial slabs B we have studied two different dielectric/magnetic responses: a first one with both the electric permittivity and magnetic permeability given by a Drude-like metamaterial dispersion response, and a second one with the electric permittivity still given by a Drude-like response, whereas the magnetic permeability is given by a more realistic split-ring resonator metamaterial response. Theoretical calculations are performed within the transfer-matrix formalism, and transmission and reflection coefficients of the metamaterial heterostructure are straightforwardly evaluated. For oblique incidence, a finite projection along the growth direction of the electric or magnetic field of the incident wave associated with the TM or TE modes, respectively, leads to a coupling in each layer of the metamaterial heterostructure of the photon modes with the bulk electric or magnetic metamaterial plasmons, respectively. Such photon–plasmon coupling results in the formation of plasmon-polariton modes and gives rise to signatures of the electric or magnetic longitudinal bulk-plasmon polariton modes in the transmission and reflection spectra of the metamaterial heterostructure. Such features survive even in the case of a single AB double-layer metamaterial heterostructure, and experimental observation should be, therefore, easily achieved.