arXiv subjects

Matteo Masino

Publications and source records attributed to Matteo Masino.

12 recordsLinked to original sources

arXiv2025

Electrons and phonons in pentacene, insights from comparison between experiment and simulations

We have performed a comprehensive computational study of the vibrational properties and electron-phonon couplings in the three known polymorphs of pentacene. Vibrational patterns and electron-phonon interactions were calculated at several q-points of the Brillouin zone, allowing for a detailed mapping of the phonon landscape and the associated coupling mechanisms relevant to charge transport. Using a pool of post-processing tools, we analyze the different phonon dispersions. Thus, we shed light on how low-frequency phonons modulate the transport differently in the polymorphs via their distinct electron-phonon coupling (EPC) signatures in reciprocal space. In fact, we show that distinct phonons dominate in high-temperature and thin-film polymorphs with respect to the high mobility low-temperature polymorph, and that these lead to different decoherence/localization trends. We describe the microscopic origin of the mobility in bulk polymorph, demonstrating that polymorphism not only modulates the transfer integral but also the phonon pattern. For the first time we show that different EPC patterns lead to very different mobility values even in similar structures. Also, we explain how phonon confinement is responsible for the increased mobility observed in 2D phase. Finally, we address the problem of possible coexistence of multiple polymorphs within a single specimen, frequently encountered in organic crystals due to their subtle energy landscape and processing conditions. In this context, we consider the implications of polymorph intergrowth, structural defects and disorder.

cond-mat.mtrl-sci↗

arXiv2024

Electron-phonon coupling and mobility modeling in organic semiconductors: method and application to tetracene polymorphs

We have developed a first-principles method to calculate the electron-phonon coupling for specific modes and q-points in the Brillouin Zone for crystalline organic semiconductors. Using the obtained coupling strengths, we propose an approach to compute the temperature-dependent mobilities of electrons and holes. This methodology is applied to both bulk and thin-film polymorphs of tetracene. To validate our treatment of the electronic structures and vibrational properties, we calculate the Raman spectra in the lattice-phonon region and compare them with experimental data. We then compare the computed mobilities with available data for single crystals, finding good agreement within the experimental range, especially when accounting for possible charged impurities. Finally, we discuss the observed differences between the polymorphs.

cond-mat.mtrl-sci↗

arXiv2021

Temperature induced Neutral to Ionic phase Transition of the charge transfer crystal Tetrathiafulvalene-Fluoranil

The Temperature Induced Neutral to Ionic phase Transition (TI-NIT) is a rare phenomenon occurring in mixed stack charge transfer (CT) crystals made up of alternating $π$-electron Donor (D) and Acceptor (A) molecules. We were able to grow crystals of Tetrathiafulvalene-Fluoranil (TTF-FA), and to show that it undergoes TI-NIT like the prototype CT crystal TTF-Chloranil. We characterized both room and low T phases through IR and Raman spectroscopy and XRD, demonstrating that while TTF-FA is quasi-neutral at room T, its ionicity jumps from 0.15 to 0.7 at low T, therefore crossing the Neutral-Ionic borderline. The transition, occuring around 150K, is first order, with large thermal hysteresis and accompanied by crystal cracking. In the high T phase D and A molecules lie on inversion center, i.e. the stacks are regular, whereas the low T phase is characterized by the loss of the inversion symmetry along the stack as the stacks are strongly dimerized and by the doubling of the unit cell.

cond-mat.mtrl-sci↗

arXiv2021

Tetramethylbenzidine-TetrafluoroTCNQ: A narrow-gap semiconducting salt with room temperature relaxor ferroelectric behavior

We present an extension and revision of the spectroscopic and structural data of the mixed stack charge transfer (CT) crystal 3,3$^\prime$,5,5$^\prime$-tetramethylbenzidine--tetrafluoro-tetracyanoquinodimethane (TMB-TCNQF4), associated with new electric and dielectric measurements. Refinement of syncrotron structural data at low temperature has led to revise the previously reported [Phys. Rev. Mat. 2, 024602 (2018)] $C2/m$ structure. The revised structure is $P2_1/m$, with two dimerized stacks per unit cell, and is consistent with the vibrational data. However, polarized Raman data in the low-frequency region also indicate that by increasing temperature above 200 K the structure presents an increasing degree of disorder mainly along the stack axis. X-ray diffraction data at room temperature have confirmed that the correct structure is $P2_1/m$ -- no phase transitions -- but did not allow to definitely substantiate the presence of disorder. On the other hand, dielectric measurement have evidenced a typical relaxor ferroelectric behavior already at room temperature, with a peak in real part of dielectric constant $\epsilon'(T,\nu)$ around 200 K and 0.1 Hz. The relaxor behavior is explained in terms of the presence of spin solitons separating domains of opposite polarity that yield to ferroelectric nanodomains. TMB-TCNQF4 is confirmed to be a narrow gap band semiconductor ($E_a \sim 0.3$ eV) with room temperature conductivity of $\sim 10^{-4}~ \Omega^{-1}$ cm$^{-1}$.

cond-mat.mtrl-sci↗

arXiv2014

Charge-order fluctuations and superconductivity in two-dimensional organic metals

We report comprehensive Raman and infrared investigations of charge-order (CO) fluctuations in the organic metal $\beta^{\prime\prime}$-(BEDT-TTF)$_2$SF$_5$CHFSO$_3$ and superconductor $\beta^{\prime\prime}$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$. The charge-sensitive vibrational bands have been analyzed through an extension of the well-known Kubo model for the spectral signatures of an equilibrium between two states. At room temperature, both salts exhibit charge fluctuations between two differently charged molecular states with an exchange frequency of about $6\times10^{11} {\rm s}^{-1}$. The exchange rate of the metallic salt remains roughly constant down to 10 K, while in the superconductor the exchange velocity starts to decrease below 200 K, and a "frozen" charge-ordered state emerges, and coexists with the charge-order fluctuation state down to the superconducting temperature. These findings are confronted with other existing spectroscopic experiments, and a tentative phase diagram is proposed for the $\beta^{\prime\prime}$ BEDT-TTF quarter-filled salts.

cond-mat.supr-con↗

arXiv2007

Intermediate regime in Tetrathiafulvalene-Chloranil (TTF-CA) pressure-induced neutral-ionic transition

We report a detailed spectroscopic study of the pressure induced neutral-ionic phase transition (NIT) of the mixed-stack charge-transfer (CT) crystal tetrathiafulvalene-chloranil (TTF-CA). We show that the pressure induced phase transition is still first-order and involves the presence of an intermediate disordered phase, defined by the coexistence of two species of different ionicity. Further application of pressure gradually converts this phase into an homogeneous ferroelectric phase with a single ionicity. In addition, we detect strong pretransitional phenomena which anticipate the intermediate phase and are indicative of a precursor dynamic regime dominated by fluctuations.

cond-mat.mtrl-sci↗

arXiv2007

DMTTF-CA revisited: temperature-induced valence and structural instability

We report a detailed spectroscopic investigation of temperature-induced valence and structural instability of the mixed-stack organic charge-transfer (CT) crystal 4,4'-dimethyltetrathiafulvalene-chloranil (DMTTF-CA). DMTTF-CA is a derivative of tetrathiafulvalene-chloranil (TTF-CA), the first CT crystal exhibiting the neutral-ionic transition by lowering temperature. We confirm that DMTTF-CA undergoes a continuous variation of the ionicity on going from room temperature down to $\sim$ 20 K, but remains on the neutral side throughout. The stack dimerization and cell doubling, occurring at 65 K, appear to be the driving forces of the transition and of the valence instability. In a small temperature interval just below the phase transition we detect the coexistence of molecular species with slightly different ionicities. The Peierls mode(s) precursors of the stack dimerization are identified.

cond-mat.mtrl-sci↗

arXiv2004

Phonons and structures of tetracene polymorphs at low temperature and high pressure

Crystals of tetracene have been studied by means of lattice phonon Raman spectroscopy as a function of temperature and pressure. Two different phases (polymorphs I and II) have been obtained, depending on sample preparation and history. Polymorph I is the most frequently grown phase, stable at ambient conditions. A pressure induced phase transition, observed above 1 GPa, leads to polymorph II, which is also obtained at temperatures below 140 K. Polymorph II can also be maintained at ambient conditions. We have calculated the crystallographic structures and phonon frequencies as a function of temperature, starting from the configurations of the energy minima found by exploring the potential energy surface of crystalline tetracene. The spectra calculated for the first and second deepest minima match satisfactorily those measured for polymorphs I and II, respectively. All published x-ray structures, once assigned to the appropriate polymorph, are also reproduced.

cond-mat.mtrl-sci↗

arXiv2003

Polymorphism, phonon dynamics and carrier-phonon coupling in pentacene

The crystal structure and phonon dynamics of pentacene is computed with the Quasi Harmonic Lattice Dynamics (QHLD) method, based on atom-atom potential. We show that two crystalline phases of pentacene exist, rather similar in thermodynamic stability and in molecular density. The two phases can be easily distinguished by Raman spectroscopy in the 10-100 cm-1 spectral region. We have not found any temperature induced phase transition, whereas a sluggish phase change to the denser phase is induced by pressure. The bandwidths of the two phases are slightly different. The charge carrier coupling to low-frequency phonons is calculated.

cond-mat.mtrl-sci↗

arXiv2002

BEDT-TTF organic superconductors: the entangled role of phonons

We calculate the lattice phonons and the electron-phonon coupling of the organic superconductor κ-(BEDT-TTF)_2 I_3, reproducing all available experimental data connected to phonon dynamics. Low-frequency intra-molecular vibrations are strongly mixed to lattice phonons. Both acoustic and optical phonons are appreciably coupled to electrons through the modulation of the hopping integrals (e-LP coupling). By comparing the results relevant to superconducting κ- and β-(BEDT-TTF)_2 I_3, we show that electron-phonon coupling is fundamental to the pairing mechanism. Both e-LP and electron-molecular vibration (e-MV) coupling are essential to reproduce the critical temperatures. The e-LP coupling is stronger, but e-MV is instrumental to increase the average phonon frequency.

cond-mat.supr-con↗

arXiv2001

Pressure driven neutral-ionic transition in ClMePD-DMeDCNQI

Application of about 0.8 GPa pressure is sufficient to induce the neutral-ionic transition in the mixed stack charge-transfer crystal 2-chloro-5-methyl-$p$-phenylenediamine--2,5-dimethyl-dicyanoquinonediimine ({\CD}). The ionicity increases continuously from $\sim$ 0.35 at ambient conditions to $\sim$ 0.65 when the pressure is raised up to 2 GPa. Moreover, stack dimerization begins well before the crossing of the neutral-ionic interface. The evolution of the transition is similar to what observed in the temperature induced phase change in the same compound. (cond-mat/0101179) A distinguishing feature is represented by the simultaneous presence of domains of molecules with slightly different ionicities across the transition pressure. A comparison of the present example of pressure driven neutral-ionic transition with the well studied cases of tetrathiafulvalene--chloranil and of tetrathiafulvalene--2,5-dichloro-p-benzoquinone puts in evidence the remarkably different evolution of the three transitions.

cond-mat.mtrl-sci↗

arXiv2001

A new type of neutral-ionic interface in mixed-stack organic charge-tranfer crystals: Temperature induced ionicity change in ClMePD-DMeDCNQI

Raman and polarized infrared spectra of the mixed stack charge transfer crystal 2-chloro-5methyl-p-phenylendiamine- -2,5-dimethyl-dicyanoquinonediimine (ClMePD-DMeDCNQI) are reported as a function of temperature. A detailed spectral interpretation allows us to gain new insight into the temperature induced neutral-ionic transition in this compound. In particular, the crossing of the neutral-ionic borderline appears to be quite different from that of the few known temperature induced neutral-ionic phase transitions. First of all, the ionicity change is continuous. Furthermore, the onset of stack dimerization precedes, rather than accompanies, the neutral-ionic crossing. The (second order) phase transition is then driven by the dimerization, but the extent of dimerization is in turn affected by the ionicity change.

cond-mat.mtrl-sci↗