arXiv subjects

Maxime Markov

Publications and source records attributed to Maxime Markov.

9 recordsLinked to original sources

arXiv2024

Occurrence of the collective Ziman limit of heat transport in cubic semiconductors: scattering channels and size effects

In this work, we discuss the possibility of reaching the Ziman conditions for collective heat transport in cubic bulk semiconductors, such as Si, Ge, AlAs and AlP. In natural and enriched silicon and germanium, the collective heat transport limit is impossible to reach due to strong isotopic scattering. However, we show that in hyperenriched silicon and germanium, as well as in materials with one single stable isotope like AlAs and AlP, at low temperatures, normal scattering plays an important role, making the observation of the collective heat transport possible. We further discuss the effects of sample sizes, and analyse our results for cubic materials by comparing them to bulk bismuth, in which second sound has been detected at cryogenic temperatures. We find that collective heat transport in cubic semiconductors studied in this work is expected to occur at temperatures between 10 and 20 K.

cond-mat.mtrl-sci↗

arXiv2023

Optimal portfolio allocation with uncertain covariance matrix

In this paper, we explore the portfolio allocation problem involving an uncertain covariance matrix. We calculate the expected value of the Constant Absolute Risk Aversion (CARA) utility function, marginalized over a distribution of covariance matrices. We show that marginalization introduces a logarithmic dependence on risk, as opposed to the linear dependence assumed in the mean-variance approach. Additionally, it leads to a decrease in the allocation level for higher uncertainties. Our proposed method extends the mean-variance approach by considering the uncertainty associated with future covariance matrices and expected returns, which is important for practical applications.

q-fin.PM↗

arXiv2023

Portfolio Optimization Rules beyond the Mean-Variance Approach

In this paper, we revisit the relationship between investors' utility functions and portfolio allocation rules. We derive portfolio allocation rules for asymmetric Laplace distributed $ALD(\mu,\sigma,\kappa)$ returns and compare them with the mean-variance approach, which is based on Gaussian returns. We reveal that in the limit of small $\frac{\mu}{\sigma}$, the Markowitz contribution is accompanied by a skewness term. We also obtain the allocation rules when the expected return is a random normal variable in an average and worst-case scenarios, which allows us to take into account uncertainty of the predicted returns. An optimal worst-case scenario solution smoothly approximates between equal weights and minimum variance portfolio, presenting an attractive convex alternative to the risk parity portfolio. We address the issue of handling singular covariance matrices by imposing conditional independence structure on the precision matrix directly. Finally, utilizing a microscopic portfolio model with random drift and analytical expression for the expected utility function with log-normal distributed cross-sectional returns, we demonstrate the influence of model parameters on portfolio construction. This comprehensive approach enhances allocation weight stability, mitigates instabilities associated with the mean-variance approach, and can prove valuable for both short-term traders and long-term investors.

q-fin.PM↗

arXiv2022

The impact of big winners on passive and active equity investment strategies

We investigate the impact of big winner stocks on the performance of active and passive investment strategies using a combination of numerical and analytical techniques. Our analysis is based on historical stock price data from 2006 to 2021 for a large variety of global indexes. We show that the log-normal distribution provides a reasonable fit for total returns for the majority of world stock indexes but highlight the limitations of this model. Using an analytical expression for a finite sum of log-normal random variables, we show that the typical return of a concentrated portfolio is less than that of an equally weighted index. This finding indicates that active managers face a significant risk of underperforming due to the potential for missing out on the substantial returns generated by big winner stocks. Our results suggest that passive investing strategies, that do not involve the selection of individual stocks, are likely to be more effective in achieving long-term financial goals.

q-fin.PM↗

arXiv2020

Ferroelectricity and multiferroicity in anti-Ruddlesden-Popper structures

Combining ferroelectricity with other properties such as visible light absorption or long-range magnetic order requires the discovery of new families of ferroelectric materials. Here, through the analysis of a high-throughput database of phonon band structures, we identify a new structural family of anti-Ruddlesden-Popper phases A$_4$X$_2$O (A=Ca, Sr, Ba, Eu, X=Sb, P, As, Bi) showing ferroelectric and anti-ferroelectric behaviors. The discovered ferroelectrics belong to the new class of hyperferroelectrics which polarize even under open-circuit boundary conditions. The polar distortion involves the movement of O anions against apical A cations and is driven by geometric effects resulting from internal chemical strains. Within this new structural family, we show that Eu$_4$Sb$_2$O combines coupled ferromagnetic and ferroelectric order at the same atomic site, a very rare occurrence in materials physics.

cond-mat.mtrl-sci↗

arXiv2019

Thermoelectric properties of semimetals

Heavily doped semiconductors are by far the most studied class of materials for thermoelectric applications in the past several decades. They have Seebeck coefficient values which are 2-3 orders of magnitude higher than metals, making them attractive for thermoelectric applications. Semimetals generally demonstrate smaller Seebeck coefficient values due to lack of an energy bandgap. However, when there is a large asymmetry in their electron and hole effective masses, semimetals could have large Seebeck coefficient values. In this work, we study the band structure of a class of 18 semimetals using first principles calculations and calculate their Seebeck coefficient using the linearized Boltzmann equation within the constant relaxation time approximation. We conclude, despite the absence of the band gap, that some semimetals are good thermoelectrics with Seebeck coefficient values reaching up to 200 $μ$V/K. We analyze the metrics often used to describe thermoelectric properties of materials, and show that the effective mass ratio is a key parameter resulting in high Seebeck coefficient values in semimetals.

cond-mat.mtrl-sci↗

arXiv2018

Breakdown of Herring's processes in cubic semiconductors for sub-terahertz longitudinal acoustic phonons

In the present work we explain the anomalous behavior of the attenuation of the longitudinal acoustic phonon in GaAs as a function of the phonon energy $ω$ in the sub-THz domain. These attenuations along the [100] direction show a plateau between 0.6 and 1 GHz at low temperatures. We found an excellent agreement between measurements performed by some of us, and new \textit{ab initio} calculations of third-order anharmonic processes. The formation of the plateau is explained by the competition between different phonon-phonon scattering processes as Herring's mechanism, which dominates at low frequencies, saturates and disappears. The plateau is shown to be determined by the phononic final-state phase-space available at a given temperature. We predict that a change of scattering mechanism should also show up in the attenuation of silicon around 1.2-1.7 THz, and argue that the attenuation plateau is a general feature of cubic semiconductors.

cond-mat.mtrl-sci↗

arXiv2018

Semi-metals as potential thermoelectric materials: case of HgTe

The best thermoelectric materials are believed to be heavily doped semiconductors. The presence of a bandgap is assumed to be essential to achieve large thermoelectric power factor and figure of merit. In this work, we study HgTe as an example semimetal with competitive thermoelectric properties. We employ ab initio calculations with hybrid exchange-correlation functional to accurately describe the electronic band structure in conjunction with the Boltzmann Transport theory to investigate the electronic transport properties. We show that intrinsic HgTe, a semimetal with large disparity in its electron and hole masses, has a high thermoelectric power factor that is comparable to the best known thermoelectric materials. We also calculate the lattice thermal conductivity using first principles calculations and evaluate the overall figure of merit. Finally, we prepare semi-metallic HgTe samples and we characterize their transport properties. We show that our theoretical calculations agree well with the experimental data

cond-mat.mtrl-sci↗

arXiv2018

Hydrodynamic heat transport regime in bismuth: a theoretical viewpoint

Bismuth is one of the rare materials in which second sound has been experimentally observed. Our exact calculations of thermal transport with the Boltzmann equation predict the occurrence of this Poiseuille phonon flow between $\approx$ 1.5 K and $\approx$ 3.5 K, in sample size of 3.86 mm and 9.06 mm, in consistency with the experimental observations. Hydrodynamic heat flow characteristics are given for any temperature: heat wave propagation length, drift velocity, Knudsen number. We discuss a Gedanken-experiment allowing to assess the presence of a hydrodynamic regime in any bulk material.

cond-mat.mtrl-sci↗