物理量和单位讲座 第二讲 电学和磁学的量和单位

物理学中的电学和磁学是电工、电力、电子、电信、计算机等许多科学技术的基础．电学和磁学的量和单位在这些科学技术领域中被广泛地使用着．因此,电学和磁学的量和单位的名称必须统一,定义必须严格,单位的量值必须精确,切忌混淆．在各学科互相渗透、国内外学术交流和贸易往来日益频繁的今天,量和单位的统一更显得迫切和重要．科学的量和单位与日常生活用语往往混合使用．但前者要求严格而后者要求则不十分严格,可以适当?...     

声波传播和分子运动、声强和响度

一、声波传播和分子运动在讲授波动时,常在图中画出許多貭点,这些貭点在波的传播过程中,发生和波传播方向垂直(横波)或一致(纵波)的振动,这样就比較容易形象地描繪出机械波传播和质点振动的物理图景。問題在于波在物貭中传播时实际情况是怎样的呢?特別是原子或分子运动情况又是怎样的呢?看来在固体(特別是晶体),液体和气体中是不一样的。在晶体中,原子在空間中是有序排列的,取一維最簡单的情况看,原子排列如图1所示(即一維布拉非格子)。     

左手定则和右手定则的正确认识和运用

左手定则和右手定则的正确认识和运用金演宁（浙江省医药学校宁波３１５０１０）１弄清左手定则和右手定则适用范围，正确选择解题方法左手定则是判定磁场力方向的，它适用于磁场对场内通电导线（电流）或运动电荷产生作用力的情形；右手定则是判定感应电动势（或感应电流...     

物理量和单位讲座 第三讲 光学和原子物理的量和单位

一、国家标准ＧＢ３１０２文件规定的有关量随着现代科学技术和经济的发展,光学和原子物理的量和单位的名称及符号的统一成为必然趋势．国际电工委员会,国际照明委员会等编纂的国际电工辞典,早已对有关量和单位给出了规范化的术语、符号和定义［１］．国际标准化组织制定了相应标准［２,３］．我国本着“向国际标准靠拢”的原则,颁布了国家标准［４,５］．ＧＢ３１０２．６《光及有关电磁辐射的量和单位》规定了如下光学?...     

物理量和单位讲座 第四讲 电离辐射和核物理的量和单位

电离辐射和核物理的量和单位在我国的国家标准中分列为两类,即ＣＢ３１０２．９原子物理学和核物理学的量和单位及ＧＢ３１０２．１０核反应和电离辐射的量和单位．两类标准中共列出１１０项物理量和相应的单位．这些量和单位的建立时间至今也不过８０年,如１９０８年法国维拉提出了“ｅ”单位,用测量空气中电离的方法来确定Ｘ射线的量．这些量的演变和发展过程比较复杂,这一方面说明问题本身的复杂性,另一方面也说明,随?...     

大西洋和太平洋中的环流:昨天和今天

全球气候变化是当今公众所关心的一个热门话题 .《物理》曾在 2 0 0 0年第 7期报道了“记录古气候的同位素温度计” .现在 ,在这篇短文中 ,所要介绍的是有关海洋子午圈环流影响气候的两项研究成果 .其一 ,德国的Ｇａｎｏｐｏｌｓｋｉ等对北大西洋古气候变化完成了随机共振 (ｓｔｏｃｈａｓｔｉｃｒｅｓｏｎａｎｃｅ)计算机模拟 ;其二 ,美国的ＭｃＰｈａｄｅｎ等对今天太平洋洋流的实验记录作出了归纳分析 .大气与海洋的相互作用对气候的变迁有重要影响 .如所周知 ,源于墨西哥湾的北大西洋湾流 ,水温很高 ,势力也很强 .表层海水在海风的驱动…     

科学和艺术

科学与完美是不是一致的？科学家们是否有权使用完美这个词？至少，对于物理学家和数学家来说答案是肯定的“是”．2002年，温伯格（Steven Weinberg）写道：“近代物理学的一些重要方程式是科学知识的永久部分，甚至比它们较早出现的美丽大教堂还要经久不衰．”1960年代，狄拉克（Paul A．M．Dirac）有句名言：“完整的方程式之完美较之它们适合实验结果更为重要．”1965年诺贝尔物理学奖获得者费因曼（Richard P．Feynman）也坚信他提出的一项理论，即使该理论显得与实验数据有矛盾．1957年，他写道：“理论是优雅和完美的，重大事件是短暂的闪光．”     

书评和书讯

科学出版社物理类新书推荐书名作(译)者定价出版日期发行号亚稳金属材料胡壮麒140估计2006年10月暂无半导体异质结物理(第二版)虞丽生$52.002006年5月O-2443高等原子分子物理学(第二版)徐克尊$54.002006年9月O-2488辐射和光场的量子统计理论曹昌祺$60.002006年3月O-2463实验物理中的概率和统计(第二版)朱永生$72.002006年3月O-2464物理学中的群论(第二版)马中骐$68.002006年2月O-2421微分几何入门与广义相对论(上册,第二版)梁灿彬,周彬$59.002005年12月O-2363液晶光学与液晶显示王新久$59.002005年12月O-2424量子信息物理原理张永德$59.0…     

光合作用体系捕光复合物线性和非线性光谱的理论研究：修正的Redfield方法的验证

使用数值精确的级联方程方法对其计算可靠性进行了验证．目前已知,修正的Redfield方法所用的微扰和马尔科夫近似,在中间耦合区对激发态能量转移会给出不正确的描述．因此研究其在计算各 种光谱信号时是否仍然有效有重要意义．利用不同参数下的二聚体模型和Fenna-Matthews-Olson复合物模型,使用级联方程方法和修正的Redfield方法,计算并比较了它们的吸收谱、发射谱和二维电子相干光谱．研究发现在较大的参数范围内两种方法给出一致的结果,这两种方法的对比也增进了对光合作用捕光复合物的2D光谱中量子拍频信号来源的认识．     

Spectroscopy of a driven solid-state qubit coupled to a structured environment

We study the asymptotic dynamics of a driven spin-boson system where the environment is formed by a broadened localized mode. Upon exploiting an exact mapping, an equivalent formulation of the problem in terms of a quantum two-state system (qubit) coupled to a harmonic oscillator which is itself Ohmically damped, is found. We calculate the asymptotic population difference of the two states in two complementary parameter regimes. For weak damping and low temperature, a perturbative Floquet-Born-Markovian master equation for the qubit-oscillator system can be solved. We find multi-photon resonances corresponding to transitions in the coupled quantum system and calculate their line-shape analytically. In the complementary parameter regime of strong damping and/or high temperatures, non-perturbative real-time path integral techniques yield analytic results for the resonance line shape. In both regimes, we find very good agreement with exact results obtained from a numerical real-time path-integral approach. Finally, we show for the case of strong detuning between qubit and oscillator that the width of the n-photon resonance scales with the nth Bessel function of the driving strength in the weak-damping regime.     

Numerical renormalization group for bosonic systems and application to the sub-ohmic spin-boson model

We describe the generalization of Wilson's numerical renormalization group method to quantum impurity models with a bosonic bath, providing a general nonperturbative approach to bosonic impurity models which can access exponentially small energies and temperatures. As an application, we consider the spin-boson model, describing a two-level system coupled to a bosonic bath with power-law spectral density, J(omega) proportional to omega(s). We find clear evidence for a line of continuous quantum phase transitions for sub-Ohmic bath exponents 0相似文献   

Divergence-free WKB method

A new semiclassical approach to the linear and nonlinear one-dimensional Schr?dinger equation is presented. For both equations our zeroth-order solutions include nonperturbative quantum corrections to the WKB solution and the Thomas-Fermi solution, thereby allowing us to make uniformly converging perturbative expansions of the wave functions. Our method leads to a new quantization condition that yields exact eigenenergies for the harmonic-oscillator and Morse potentials.     

Dynamics of non-Markovianity in the presence of a driving field

We investigate a two-level system in a cavity QED by considering the effects of amplitude damping, phase damping and driving field. We have studied the non-Markovianity in resonance and non-resonance limits in the presence of these effects using Breuer–Laine–Piilo (BLP) non-Markovianity measure (N_BLP). The evolution of the system is derived using the time convolutionless (TCL) master equation. In some conditions, it is shown that in the presence of a driving field, the N_BLP increases in the resonance and non-resonance limits. We have also found the exact solution of the master equation in order to investigate the effect of temperature- and environment-excited states. We have shown that the behaviour of non-Markovianity is very different from what one can see from the TCL approach. We have also presented some explanation about the behaviour of non-Markovianity in the exact solution using quantum discord (QD).     

Decoherence in quantum mechanics

We study decoherence in a simple quantum mechanical model using two approaches. Firstly, we follow the conventional approach to decoherence where one is interested in solving the reduced density matrix from the perturbative master equation. Secondly, we consider our novel correlator approach to decoherence where entropy is generated by neglecting observationally inaccessible correlators. We show that both methods can accurately predict decoherence time scales. However, the perturbative master equation generically suffers from instabilities which prevents us to reliably calculate the system’s total entropy increase. We also discuss the relevance of the results in our quantum mechanical model for interacting field theories.     

Nonequilibrium dynamics of correlated electron transfer in molecular chains

The relaxation dynamics of correlated electron transport along molecular chains is studied based on a substantially improved numerically exact path integral Monte Carlo approach. As an archetypical model, we consider a Hubbard chain containing two interacting electrons coupled to a bosonic bath. For this generalization of the ubiquitous spin-boson model, non-Boltzmann equilibrium distributions are found for many-body states. By mapping the multiparticle dynamics onto an isomorphic single particle motion, this phenomenon is shown to be sensitive to particle statistics and, due to its robustness, allows for new control schemes in designed quantum aggregates.     

Quantum kinetics and thermalization in a particle bath model

We study the dynamics of relaxation and thermalization in an exactly solvable model of a particle interacting with a harmonic oscillator bath. Our goal is to understand the effects of non-Markovian processes on the relaxational dynamics and to compare the exact evolution of the distribution function with approximate Markovian and non-Markovian quantum kinetics. There are two different cases that are studied in detail: (i) a quasiparticle (resonance) when the renormalized frequency of the particle is above the frequency threshold of the bath and (ii) a stable renormalized "particle" state below this threshold. The time evolution of the occupation number for the particle is evaluated exactly using different approaches that yield to complementary insights. The exact solution allows us to investigate the concept of the formation time of a quasiparticle and to study the difference between the relaxation of the distribution of bare particles and that of quasiparticles. For the case of quasiparticles, the exact occupation number asymptotically tends to a statistical equilibrium distribution that differs from a simple Bose-Einstein form as a result of off-shell processes whereas in the stable particle case, the distribution of particles does not thermalize with the bath. We derive a non-Markovian quantum kinetic equation which resums the perturbative series and includes off-shell effects. A Markovian approximation that includes off-shell contributions and the usual Boltzmann equation (energy conserving) are obtained from the quantum kinetic equation in the limit of wide separation of time scales upon different coarse-graining assumptions. The relaxational dynamics predicted by the non-Markovian, Markovian, and Boltzmann approximations are compared to the exact result. The Boltzmann approach is seen to fail in the case of wide resonances and when threshold and renormalization effects are important.